Unit 1 - Chemistry - Chapter 1 PDF

U N I T Energy and Matter in Chemical Change H ow far and how fast can you run? The answer depends in part on chemical reactions. The scientist in the small photo is testing a runner’s fitness. The scientist monitors the breath that...

U N I T Energy and Matter in Chemical Change H ow far and how fast can you run? The answer depends in part on chemical reactions. The scientist in the small photo is testing a runner’s fitness. The scientist monitors the breath that the runner exhales to determine the quantity of oxygen he consumes. This test works because the reactions that provide the energy the runner needs to keep moving consume oxygen from the air and stored glucose from his cells. The harder the athlete works, the more oxygen he consumes. Whether they are developing a fitness test or a large-scale process for a chemical plant, scientists and engineers apply theories about the nature of matter. Modern chemical theories have developed over the past four hundred years. The roots of these chemical theories, however, go much farther back in time. The observations, ideas, and discoveries of philosophers, alchemists, craftspeople, and countless others were crucial to the early development of chemical theories. In this unit, you will learn how theories of matter developed. You will apply these theories to your own observations of matter and chemical reactions. By studying matter and its interactions, you will equip yourself to make decisions and think critically about the benefits and risks of the chemistry that surrounds you. 2 Chapter 1 Atoms, Elements, and Compounds 4 Chapter 2 Names, Formulas, and Properties 40 Chapter 3 Chemical Reactions 82  1 C H A P T E R Atoms, Elements, How can you determine how to work safely with different substances? How did early scientists form hypotheses and predictions about particles that they could not see or manipulate? How are elements and compounds constructed? How do elements combine? 4 MHR Unit 1 Energy and Matter in Chemical Change and Compounds T he desk you sit at, the clothes you wear, the food you eat, and the Today, technological inventions such as the scanning tunnelling water you drink all fit under the microscope (STM) allow researchers category of matter. Matter can also to “see” atoms. Their observations be invisible, such as the air you breathe. confirm many of the predictions Matter comes in a great variety of made by scientists several hundred forms and can be found everywhere years earlier. The photograph on you look, all over Earth and the left, for example, shows an STM throughout the universe. image of iron atoms in a corral You could spend all day making pattern on a copper surface. observations about matter and asking The STM could not have been questions such as the following: invented without atomic theories that What is the composition of matter? were developed based on indirect Where does it come from? What is evidence. Today, however, instruments its underlying structure? Why does such as the STM provide direct matter behave the way it does? evidence of atoms and their structure. Throughout history, scientists have These instruments help researchers asked these same questions, trying to make further direct and indirect to explain the nature of matter. observations, and to proceed toward The idea that matter is made of ao obetter ki understanding of the nature n L g tiny particles has been around for of matter. head A thousands of years. It began in ancient Greece as a philosophical discussion. o oki n How has scientific L g In the past several centuries, however, inquiry into chemica l head and physical propert A ies of substances the particulate nature of matter helped researchers develop useful chem products? How can ical has become an important topic scientific theories ex certain substances pla in wh y of scientific inquiry. Early chemists have certain useful the Unit 1 Design Yo prop ert ies? In ur Own Investigation and physicists experimented, Antacids on page 13 : Analyzing 2 you will design an to investigate and co experiment observed, hypothesized, predicted, mpare different antac your working groups ids. Form and experimented again as they early and begin to thi nk about the investigation as you study this chap developed theories about the tiny ter. particles that make up matter. Chapter 1 Atoms, Elements, Solar Energy and Compounds and Climates MHR 5  1.1 Investigating and Working with Chemicals As a distinct science, chemistry is fairly young. In fact, some people suggest that chemistry, as it is known and practised today, did not emerge until the late 1700s. This does not mean, however, that people only learned how to use chemicals a few hundred years ago. Since prehistoric times, people have used chemical substances and chemical processes to meet their needs. North American Aboriginal Peoples and Chemistry For thousands of years, Aboriginal peoples have used the chemical properties of substances and their interactions to make clothing, preserve food, treat illness, build tools, and adorn objects with colour. Figure 1.1 shows just a few examples of the many ways in which chemicals played a role in the lives of early North American Aboriginal peoples. To tan leather, Aboriginal peoples from various parts of North America used different techniques. For example, some peoples used a mixture of ashes and water to pre-soak the hides. Substances in the ashes helped to break down the tough layer of mucous that coats the hide. Animal brains were applied to the hide, creating soft, water-resistant buckskin, shown here. Substances in the brains, including emulsified fats, penetrated the hide to change its properties. This process is called brain-tanning. A Dyes for decorating fabric, wood, and other materials were obtained from local plants. Delphinium plants, for example, produce a blue- green dye. The roots of bloodroot plants produce an orange dye. A modern example of designs B created using dyed porcupine quills is shown here. D To treat illnesses and alleviate pain, substances were extracted from local plants. For example, wild ginger was Figure 1.1 Chemicals, C used by many Aboriginal peoples to treat chemical processes, a number of ailments including coughs, and chemical techniques Many Aboriginal peoples who travelled by water melted pine colds, and flu. A tea made from the juice are involved in each of or spruce gum and mixed it with animal fat. They used the of juniper berries, shown here, could be these examples. resulting sticky resin to build waterproof canoes. applied to the skin to soothe insect bites. 6 MHR Unit 1 Energy and Matter in Chemical Change  Many ancient techniques and remedies work just as well as modern ones. Some Aboriginal people and others interested in traditional methods still use them today. For example, many people prefer hides that have been tanned using traditional methods. These hides are said to be softer and stronger than hides tanned using mass-production methods. As well, ancient means of healing and herbal medicine have become more popular. Safety in the Chemistry Laboratory Native medicines were, and still are, gathered, prepared, and administered by people trained to do so safely. Even household substances, such as cleaning products, can be dangerous if handled improperly. Therefore, everyone who works with chemicals must know how to handle them safely and responsibly. Handing chemicals may require the use of protective clothing, as shown in Figure 1.2. To find out about how to handle a chemical safely, consult its Material Safety Data Sheet (MSDS). An MSDS includes important information about a chemical. This includes physical properties (such as melting point, boiling Figure 1.2 Researchers test point, and odour) and chemical hazards. Instructions explain how to handle, protective equipment in the store, and dispose of the chemical, as well as the procedure to follow in case Defence Research Development of an accident. A sample MSDS is shown in Figure 1.3. In the activity on Canada (DRDC) laboratories at page 9, you will investigate a chemical and create your own MSDS. Suffield, Alberta. MATERIAL SAFETY DATA SHEET EMERGENCY OVERVIEW Figure 1.3 What safety precautions would you need to take when working with chlorine? Chapter 1 Atoms, Elements, and Compounds MHR 7  In Canada, managing hazardous materials is covered by WHMIS, the Workplace Hazardous Materials Information System. WHMIS legislation Class A: Class B: ensures that workers are informed about, and trained to handle, the hazards they Compressed Gas Flammable and Combustible Material may encounter. WHMIS informs workers about the chemicals in three ways. 1. Controlled products must have informative labels, in both English and French, on their containers. Class C: Class D1: Oxidizing Material Poisonous and Infectious Material Causing 2. Each controlled product must have a Material Safety Data Sheet (MSDS). Immediate and Serious Toxic Effects 3. Workers who handle chemicals must complete an education program provided through their employer. Examine Figure 1.4 to review WHMIS symbols and their meanings. WHMIS Class D2: Class D3: Poisonous and Infectious Biohazardous Infectious symbols identify eight classes of hazards labelled A, B, C, D1, D2, D3, E, and F. Material Causing Other Toxic Effects Material A substance may be associated with more than one hazard. You can learn more about the WHMIS symbols and lab safety at the beginning of your textbook. Class E: Class F: Corrosive Material Dangerously Reactive Figure 1.4 These WHMIS symbols are used throughout Canada to identify Material hazards associated with materials used in all workplaces, including schools. Pyrotechnician Fireworks! Dazzling colour, ear-splitting sound, fantastic bursts, sprays, and explosions of light — all brought to you by chemists known as pyrotechnicians. As the science of fireworks developed, chemists learned that the standard mixture of gunpowder used in fireworks could be processed to produce different effects. It could be compressed into different shapes that would vary the speed of its ignition. A common shape is the marble-sized ball, called a star. For a display, many stars are placed in a container called a shell. A lifting charge in the base of the shell is ignited electronically. When the shell is in position high in the air, a secondary burst ignites the stars. Pyrotechnicians use their chemical background to mix fireworks. Most fireworks are mixed by hand to avoid the possibility of a stray spark from metal machinery igniting the gunpowder. One dazzling aspect of fireworks is their colour. Not until the nineteenth century did pyrotechnicians figure out how to produce the vibrant colours we see today. By replacing an ingredient in gunpowder with one that raised the temperature of combustion from 1700°C to 2000˚C, they made it possible to use a new set of chemicals. They learned that adding compounds with certain metals produced vivid colours. Copper compounds produced blue, strontium compounds produced red, and barium compounds produced green. Today, pyrotechnicians can produce all the colours of the rainbow by carefully selecting the compounds. For sound, other mixtures of chemicals are used to produce “screeches,” “whistles,” and “bangs.” The number one concern of pyrotechnicians is safety. The chemicals they work with are highly explosive and very powerful. In Canada, the Explosives Research Division of Natural Resources Canada offers a course leading to the title of Display Supervisor. Supervisors are allowed to fire shells of different sizes, depending on Today, professional firework displays are their level of expertise. Team up with a partner to find out what you can about fireworks electronically controlled by computers. safety in Canada. What kinds of rules exist? How often are the rules updated? Music is often an integral part of the displays. 8 MHR Unit 1 Energy and Matter in Chemical Change  Find Out Understanding an MSDS Certain information must be included on 4. Most MSDS searches are by chemical name. a Material Safety Data Sheet, but there is no A search for a specific name often turns up set format. In this activity, you will use print and several possibilities, however, depending on Internet resources to find information you would how the chemical is sold. For example, sodium need to develop a chemical MSDS. hydroxide is sold in solid form as pellets, at different levels of purity, and in various solutions. Look up your chemical in the form specified by your teacher. 5. As you proceed, keep a list of any words that are unfamiliar to you. 6. Record the appropriate information about your chemical on your form. Here are some things to keep in mind: Note any unusual information. (E.g., does the chemical decompose before it boils?) For chemical stability and reactivity, note any dangerous decomposition products. List any warnings about contact with other chemicals. List any potential health hazards, such Procedure Performing and Recording as exposure to the skin or eyes. Is the 1. Design your own form to record the following chemical poisonous? MSDS information: Is special handling or storage required, chemical name other than a cool, dry, ventilated area? chemical formula What procedures should be followed physical properties (e.g., appearance, if there is an accidental leak or spill? odour, melting point, and boiling point) chemical stability and reactivity What Did You Find Out? Analyzing and Interpreting (e.g., explosiveness, flammability) potential health effects 1. From the MSDS you prepared, create a handling and storage 1-page safety sheet that could be placed in a disposal storeroom, near containers of your chemical. Use point form and plain English. Include 2. Use the Internet Connect on page 11 information about dangers, handling, and to learn about the Workplace Hazardous storage, as well as procedures to be Materials Information System (WHMIS) and followed in case of an accidental spill. Material Safety Data Sheets. Bookmark at least one site that has MSDS information. 2. Employers must provide education programs for people who use hazardous chemicals. What 3. Your teacher will give you the name and additional information would you expect to formula of a chemical. Write the formula learn from an education program? Use your on your form, as well as the name of list of unfamiliar words to help you answer the chemical. the question. Chapter 1 Atoms, Elements, and Compounds MHR 9  Classifying Matter Ancient Greek philosophers In previous science courses, you learned to classify matter in a variety of useful believed that all matter was ways. Figure 1.5 shows a system that chemists use to classify matter. Answer composed of four "elements" called earth, water, fire, and air. the Practice Problems that follow to make sure that you understand this system. Ancient Western medicine was also based upon this idea. The MATTER body was thought to contain anything with mass and volume four substances called the four humours, each corresponding may be solid, liquid, or gas to a different “element.” They were: black bile (earth), phlegm (water), yellow bile (fire), and MIXTURE PURE SUBSTANCE blood (air). A person’s tempera- ment and health were thought to combinations of matter that can matter that has a definite composition depend on the balance of these be separated by physical means four humours. According to this do not have definite composition theory, illness resulted from an imbalance of the humours. Physicians treated illness by HETEROGENEOUS HOMOGENEOUS ELEMENT COMPOUND attempting to restore the balance MIXTURE MIXTURE of the humours. For example, cannot be two or more (MECHANICAL (SOLUTION) chemically elements that deliberately bleeding a patient MIXTURE) (bloodletting) was thought to different broken down are chemically cure fever and headache, illnesses different components into simpler combined components are not visible substances can be associated with the characteristics of the mixture composition separated of air and fire. The theory of the are visible four humours persisted until the is constant chemically composition throughout into simpler late 16th century. is variable the mixture substances throughout the mixture Figure 1.5 Try to use this system to classify examples of matter that you see around you right now. Practice Problems 1. State whether each of the following is a pure substance or a mixture. (a) seawater (c) sodium chloride (non-iodized table salt) (b) iron (d) bronze 2. State whether each of the following mixtures is homogeneous or heterogeneous. (a) sugar dissolved in water (c) cranberry juice (b) oil-and-vinegar salad dressing (d) steel 3. State whether each of the following pure substances is an element or a compound. (a) copper, Cu (c) water, H2O (b) oxygen, O2 (d) methane, CH4 4. Classify each of the following substances. (a) graphite, C (c) motor oil (b) clear shampoo (d) sodium hydrogencarbonate, NaHCO3 (baking soda) 10 MHR Unit 1 Energy and Matter in Chemical Change Section 1.1 Summary In this section you learned about the importance of safety for everyone who works with chemical substances and products. It does not matter whether these substances are cleaning solutions, herbal medicines, or fireworks. You are now ready to take a closer look at the structure of matter in the next section. Check Your Understanding 1. Name three ways in which WHMIS ensures workers have the information they need about the substances they use. 2. What does MSDS stand for? What information would you expect to find on an MSDS? 3. For each of the following WHMIS symbols, what precautions would you take if you were using a product with the symbol on its packaging? (a) flammable and combustible material (b) corrosive material (c) poisonous and infectious material causing other toxic effects 4. Apply Using the Internet, investigate each of the eight WHMIS symbols. Create a table that shows the following for each symbol: two risk factors associated with the class of chemical, and two precautions that you could take to minimize the risk involved. 5. Thinking Critically You can work with dangerous substances safely if you take the appropriate precautions. On the other hand, substances that most people consider harmless may be dangerous under some circumstances. Water, for example, is probably the most familiar substance on Earth, and is necessary to all life. However, water can be fatal by inhalation (drowning). In addition, heating water in an enclosed container can cause an explosion. (a) Name another familiar substance that most people consider harmless. (b) Classify the substance according to Figure 1.5 on page 10. (c) Under which circumstances might this substance be dangerous? (d) How would you avoid these circumstances? www.mcgrawhill.ca/links/sciencefocus10 To find out more about the Workplace Hazardous Materials Information System (WHMIS) and Material Safety Data Sheets (MSDS), go to the web site above. Write a short report, or design your own web site, to explain the purpose and history of WHMIS legislation and the information required on a Material Safety Data Sheet. Chapter 1 Atoms, Elements, and Compounds MHR 11  1.2 Developing Atomic Theories Figure 1.6 shows a high-energy particle accelerator. Such an instrument helps modern scientists probe inside tiny particles of matter by splitting them apart. Using this tool, scientists can even create new particles that do not exist in nature. Work of this kind is based on modern theories that explain the composition of matter. These theories, in turn, are rooted in the work and the ideas of scientists from centuries ago. These scientists used techniques and tools that are “low-tech” by today’s standards. Even so, early chemists devised atomic therories that are still useful today. Early Observations During the 1600s and 1700s, scientists improved laboratory techniques for isolating pure substances and analyzing their properties. The scientists gathered a great deal of information about specific substances and the ways that they interact. In some cases, scientists observed an action or condition so consistently that they were convinced it would always happen. When scientists are convinced of the regularity of certain Figure 1.6 Linear accelerators fire observations, they generalize their observations as scientific laws. Several of fast-moving particles into a target surrounded by sensitive detectors. these early laws could be explained by the hypothesis that matter is made up of tiny particles. In this section, you will discover how that hypothesis developed to become modern atomic theory. Dalton’s Atom John Dalton (1766–1844) was an English scholar and teacher. He published a comprehensive atomic theory in 1808. The heart of Dalton’s theory was that every substance is made up of indivisible atoms. Further, the key difference between atoms of different elements is their mass. Dalton’s theory is summarized below. Note how well it explained many observations and laws. For example, by the late 1700s, scientists knew that when substances react, the total mass of the substances before and after the reaction is always the same. If matter is made up of indestructible particles, this law makes sense. Particles Figure 1.7 According to are only rearranged during reactions. They are not destroyed or created. Dalton, the atom was a solid, Today, chemists still use many parts of this theory to explain the behaviour uniform sphere. of matter. Figure 1.7 shows how Dalton might have pictured the atom. Dalton’s Atomic Theory All matter is made up of small particles called atoms. Atoms cannot be created, destroyed, or divided into smaller particles. All atoms of the same element are identical in mass and size, but they are different in mass and size from the atoms of other elements. 12 MHR Unit 1 Energy and Matter in Chemical Change  Compounds are formed when atoms of different elements hydrogen water combine in fixed (definite) proportions. The tiniest particles of any compound always contain the same types and relative nitrogen ammonia numbers of atoms. Chemical reactions change the way atoms are grouped, but the atoms themselves are not changed in reactions. carbon methane Many of Dalton’s conclusions were based on assumptions. If the assumption was wrong, it led to an incorrect conclusion. oxygen carbon dioxide For example, Dalton assumed that atoms combined in the simplest possible way. He knew that water contained hydrogen sulfur sulfur trioxide and oxygen, so he proposed that the formula for water was OH. He assigned hydrogen a mass of one unit. Then, Dalton used measurements made by French chemist Joseph Proust (1754–1826). According Figure 1.8 Dalton represented to Proust’s measurements, water contains eight times more oxygen by mass elements and molecules than hydrogen. Based on Dalton’s assumption, an atom of oxygen would then differently than modern chemists. have a mass of eight units. Dalton developed a system of symbols, shown in How are these substances represented today? Figure 1.8, to keep track of his assumptions about how atoms combined. Dalton’s assumptions about the composition of water, and several other compounds, were inaccurate. As a result, some of his calculations of relative masses were also inaccurate. However, Dalton used his theory to predict John Dalton was colour-blind different ways in which a given pair of elements might combine. He proposed, to red, a condition that made it for example, that there should be several different compounds containing difficult for him to describe and nitrogen and oxygen. These included NO, N2O, and NO2. When his prediction identify chemicals by sight. In 1794, Dalton wrote about was verified experimentally, doubts about Dalton’s atomic theory gave way to colour-blindness. His paper was widespread acceptance. the earliest scientific description of this condition. Colour-blindness is still sometimes referred to Electricity and the Atom as Daltonism. Sometimes science fosters technological discoveries, and sometimes new technology stimulates scientific discoveries. A technological achievement that helped scientists improve on Dalton’s theory was the refinement of the gas discharge tube. A gas discharge tube is a sealed glass vessel that contains a gas at low pressure. As electricity flows through the gas, a “ray” is formed across the length of the tube, and light is produced. The rays produced in gas discharge tubes are called cathode rays. In 1855, Heinrich Geissler (1814–1879), a German glass-blower and mechanic, improved the gas discharge Some ancient Greek philosophers tube. Figure 1.9 shows a modern version of a Geissler gas discharge tube. speculated that the universe must be composed of small particles that could not be broken down. They used the Greek word atomos (“indivisible”) to describe these particles, which were thought to be separated by empty space. Ancient Greek thinkers arrived at their ideas by a series of logical arguments. They did not use experimental investigation to test or develop Figure 1.9 High-voltage their ideas. Would you consider electricity flowing through the methods of the ancient Greek a low-pressure gas produces philosophers to be scientific? the glowing line in the middle Explain why or why not. of this gas discharge tube. Chapter 1 Atoms, Elements, and Compounds MHR 13  Evidence of Electrons Several experiments with gas discharge tubes led researchers to infer that matter contains tiny particles that have negative charges. This inference may seem obvious today. At the end of the nineteenth century, however, many scientists were reluctant to abandon the central theme of Dalton’s useful atomic theory. They did not want to believe that Dalton’s indivisible atoms might actually be made up of even smaller particles. Find Out Develop a Theory As you are discovering, scientists developed 4. Seal your box, and exchange boxes with theories about the structure of the atom without your partner. ever seeing the atom. They used models to represent their theories visually. In this activity, 5. Perform simple tests to determine what is you will construct a mystery box and develop inside your partner’s box. You may not open your own model to show what is inside it. Then the box. Make a table like the one below to you will challenge a partner to collect evidence record the tests you performed and what you and develop a theory about what is inside. can infer about the internal structure of the box. Give your table a title. Materials Tests Conducted Observations and Inferences Made cardboard box, the size of a shoe box on Box Evidence Collected Based on Evidence objects to place inside box adhesive tape thin, stiff wire 6. Put your inferences together to develop a Procedure Performing and Recording theory of the internal structure of the box. Then draw a model of the inside of the box. 1. Design a mystery box. Keep in mind that: a simple but creative mystery box is better than one that is too complicated What Did You Find Out? Analyzing and Interpreting your box may not contain any liquid that 1. Compare your model with your partner’s. could spill or any substance that could How similar are they? Which inferences could decompose, such as food account for the differences between them? your design must allow for simple tests or 2. Which test yielded the most useful evidence? experiments, such as probing with a thin wire or shaking 3. Having seen your partner’s model of his or her box, what test did you not carry out that 2. Construct your box. You can might have yielded useful results? put in one or two objects that can move and make noise when the box is tilted 4. Suppose you are granted access to an X-ray tape a few objects to the inside of your box machine to conduct further tests on the box. What are your hypotheses for the X-ray 3. Draw a model of the inside of your box. Your machine experiment? What predictions model must be based on the inferences you will you make before you begin? think your partner can make about it. Do not show your model to your partner. 14 MHR Unit 1 Energy and Matter in Chemical Change  In 1894, English physicist J. J. Thomson (1856–1940) used a new version of the gas discharge tube to obtain direct evidence that cathode rays were actually J. J. Thomson once said, “At a stream of negatively charged particles. Thomson’s modified gas discharge tube first there were very few who used charged plates to bend cathode rays around a curved path. He knew that believed in the existence of these bodies smaller than atoms. by measuring the radius of their path, he could calculate information about I was even told long afterwards the mass and charge of the particles. Figure 1.10 shows a simplified view of by a distinguished physicist Thomson’s experiment. who had been present at my lecture at the Royal Institution Thomson was indeed able to work out a quantitative relationship between that he thought I had been the charge and the mass of the negatively charged particles. He showed that ‘pulling their legs.’” either they had far more charge than any other particle then known, or they were much less massive than an atom. Later experiments confirmed the second option. These stable, negatively charged particles are now known as electrons. 1 Each electron has less than ! ! the mass of a single hydrogen atom. 2000 A anode (") with slit in centre B anode (") with slit in centre (!) cathode (!) cathode to vacuum pump to vacuum pump source of source of high-energy high-energy electricity electricity This diagram represents Thomson’s apparatus. Thomson knew When the current is switched on, the cathode rays travel from that cathode rays exited from the cathode, and were absorbed the cathode to the anode. A narrow portion of the cathode ray at the anode. passes through the slit in the centre of the anode. negative plate C (!) www.mcgrawhill.ca/ links/sciencefocus10 (") A “neon” sign is a modern version of a gas discharge tube. The colour of the sign depends on the positive plate gas inside the tube. Find out which colour is given off by a tube that contains neon gas. What gas glows violet-blue? source of to vacuum pump When was the neon tube invented? Where did the first high-energy neon sign appear? Answer these questions and learn electricity more about neon signs by going to the web site above. Click on Web Links to find out When a positively charged plate and a negatively charged where to go next. plate are placed near the tube, the ray is attracted to the positive plate. Figure 1.10 Based on his experiments with cathode ray tubes, Thomson concluded that matter contained tiny, charged particles. How would diagram (C) change if the positions of the positive plate and the negative plate were reversed? Chapter 1 Atoms, Elements, and Compounds MHR 15  Electrons and the Atom Thomson is sometimes called Electrons were much less massive than atoms. Electrons also appeared to be “the father of the electron.” present in all samples of matter. These discoveries suggested to scientists like When he published his results in 1897, however, he referred to the J. J. Thomson that every atom contained electrons. Electrons are negatively cathode ray particles as corpus- charged, however, and samples of matter normally have no overall charge. cles. It was not Thomson, but Therefore, each atom would also have to contain a source of positive charge. another scientist, G. Johnstone Stoney, who invented the name Yet, it was not clear where the positive charge was. Thomson at first supported “electron” in 1891 to describe a model first suggested in 1902 by another English physicist with the same a unit of charge in electrolysis last name. This person was William Thomson, better known as Lord Kelvin experiments. A third scientist, (1824–1907). According to Kelvin’s model, atoms consisted of electrons George Fitzgerald, argued that this electron and Thomson’s embedded in a spherical cloud of positive charge. corpuscle were really the Figure 1.11 shows a diagram representing the Kelvin/Thomson model. same thing. This model eventually became known simply as the Thomson model. electrons positively charged sphere Figure 1.11 The Thomson atomic model of 1903. Thomson viewed the atom as a positively charged sphere embedded with sufficient numbers of electrons to balance the total charge. The electrons in this model are like raisins in a plum pudding or raisin bun. Ernest Rutherford won the 1908 Thus, Thomson’s theory has been called the “plum-pudding” or “raisin-bun” Nobel Prize in Chemistry for theory. This model (also called the Thomson atom) could not account for finding that radioactive elements a phenomenon that Thomson himself was studying. Radioactive elements actually gave off three different types of emissions. These had only recently been isolated in pure form. They appeared to be constantly emissions are now called emitting fast-moving, positively charged particles. These particles are called alpha particles, electrons, and alpha particles and have about 7200 times the mass of an electron. The Thomson gamma rays. He studied with atom contained nothing similar to alpha particles, and gave no clues about how J. J. Thomson at Cambridge, then taught at McGill University they might be formed. in Montreal from 1898 until 1907. Then he returned to Rutherford’s Experiment England to develop his own research laboratory at the In 1909, New Zealand-born physicist Ernest Rutherford (1871–1937) designed University of Manchester. an elegant experiment to probe the structure of atoms. As shown in Figure 1.12 on the next page, Rutherford’s apparatus directed a stream of alpha particles from a shielded sample of radioactive polonium toward a very thin gold foil. Collisions with gold atoms, or parts of gold atoms, in the foil were expected to cause the alpha particles to change direction slightly and hit different parts of a fluorescent screen placed near the foil. Rutherford observed this deflection. He observed something else, as well. A small number of alpha particles bounced back from the gold foil. Rutherford did not expect this result. 16 MHR Unit 1 Energy and Matter in Chemical Change  5. some alpha particles were 6. a few alpha particles deflected off bounced backward course (deflection exaggerated here) 1. polonium source (emits alpha particles) 2. fluorescent screen (lights up when struck 3. very thin by an alpha particle) gold foil 4. most alpha particles went straight through the foil Figure 1.12 Rutherford realized that alpha particles, much more massive than electrons, would not be significantly deflected when they passed by, or even collided with, electrons in the gold In 1904, a Japanese scientist foil. “Clouds” of positive charge, as proposed in the Thomson model, could cause slight changes called Hantaro Nagaoka proposed of direction. Rutherford reasoned, however, that the alpha particles would interact with many an atomic model that was similar atoms on their way through the foil. Thus, the effects of many random deflections would tend to Rutherford’s. Nagaoka’s model to cancel one another, and the overall deflection would not be very great. described a disk-shaped atom with negatively charged atoms orbiting a positively charged Based on his observations, Rutherford developed a new atomic theory. nucleus. When Rutherford wrote about his atomic model in 1911, This theory included both electrons and positively-charged particles. The he noted that his results would theory also explained the surprising alpha particle rebounds that he had be the same if Nagaoka’s model observed. You can follow Rutherford’s logic by examining Figure 1.13. were correct. Question: How could the foil Tentative repel the answer: The dense, strongly atoms in the foil positive alpha must possess particles so something that But: This dense dramatically? is very dense positive and intensely “something” Why? Because positive. must be the foil let most relatively small. of the positive So: The atoms alphas go straight through. must also They were possess a neither repelled relatively large nor attracted. region that is mostly empty space. Figure 1.13 Interpreting the gold foil experiment Chapter 1 Atoms, Elements, and Compounds MHR 17  nucleus electrons around nucleus Protons and a Nucleus Rutherford’s atomic model is shown in Figure 1.14. Rutherford’s reasoning led him to propose that the atom had the following features: A nucleus: a central region that is positively charged, extremely small, and yet contains almost all of the atom’s mass. Such a nucleus would be far too dense to be a cloud or shell of positive charge, as Thomson had suggested. Rather, Rutherford visualized the nucleus as containing tiny, relatively massive particles, each with a single positive charge — protons. A proton is now known to have about the same mass as 1836 electrons. Electrons: particles with a single negative charge, located in the outer region of the atom. Electrons Figure 1.14 Rutherford’s “solar-system” atomic model are much less massive than protons and neutrons. of 1911. This diagram distorts the size of the nucleus of the Rutherford’s model suggests that the electrons move atom. If atoms were really the size shown here, the nucleus around the nucleus rather like planets orbiting the Sun. would still be far too small to be visible. Electrical attraction from oppositely charged protons in the nucleus would keep the electrons in orbit, just as the Sun’s gravity keeps planets in orbit. Empty space: a volume of space surrounding the nucleus that is very large, compared to the nucleus. The electrons exist in this space, but it is otherwise empty. Since the gold foil was a solid, Rutherford assumed that atoms in it Canadian Harriet Brooks, shown were packed closely together. He reasoned that most alpha particles could below, was probably the only pass through the foil due to space within individual atoms, not between them. person to work with all three leading investigators of the atom at the beginning of the twentieth Evidence of the Neutron century: Rutherford, Marie Curie, and J.J. Thomson. Brooks was an Hydrogen is the element with the least-massive atoms. Rutherford hypothesized excellent researcher in her own that hydrogen would have the simplest possible atom: one proton and one right. She discovered that the electron. It seemed logical that helium, the next-lightest element, would have radioactive emissions from the two protons and two electrons. That would make a helium atom twice as element thorium were actually a simpler atom — the gas radon. massive as a hydrogen atom. According to experimental evidence, however, She thus became the first person helium atoms are four times more massive than hydrogen atoms. to recognize that one element could To explain these results, Rutherford hypothesized that there was a third change (“transmute”) into another. subatomic particle in the atom. He hypothesized that this particle had the same mass as the proton, but no electrical charge. For this reason, he called the particle a neutron. A helium atom, then, would contain two protons and two neutrons (as well as two electrons), making it four times as massive as a hydrogen atom. The neutron hypothesis could also explain the existence of isotopes. Isotopes are atoms of the same element that differ in mass but are chemically alike. All atoms of a given element were thought to contain the same number of protons, which accounted for their identical chemical properties. If atoms of the same element could have different numbers of neutrons, however, they would have different masses, as shown in Figure 1.15 on the next page. 18 MHR Unit 1 Energy and Matter in Chemical Change  one isotope of neon another isotope of neon (neon-10) (neon-12) The alpha particles used in proton Rutherford’s experiment, which had the same charge as two protons, were four times more " 0 " 0 massive than hydrogen atoms. " neutron " 0 0 Alpha particles are now known to 0 0 0 0 0 " be helium nuclei, each consisting 0 " " 0 0 " of two protons and two neutrons. " " " " " 0 0 0 0 " 0 " 0 " 0 0 0 " " 0 " " Figure 1.15 All neon atoms have 10 protons. The isotope on the left has 10 neutrons, however, while the other isotope has 12 neutrons. Because they have no electric charge, neutrons were extremely difficult to isolate and study. In 1932 Rutherford and his colleague English physicist James Chadwick (1891–1974) gave clear experimental evidence of the existence of neutrons. Evidence of Energy Levels Two of the main objections to Rutherford’s “solar-system” atom came from other physicists. According to physical theory, electrons moving around a nucleus should constantly emit energy in the form of light or radio waves. This process would cause the electrons to spiral into the nucleus, and the atom would collapse. Atoms, however, do not collapse. There is also no evidence that their electrons emit energy under normal conditions. In a gas discharge tube, however, gases do emit light, but only when electrical energy is supplied to them. As well, each element emits specific colours of light. These colours are different for each element. The colours of light correspond to specific wavelengths. This means that the colours have different energies. Rutherford’s atomic theory had no explanation for this behaviour. Figure 1.17 on the next page shows how instruments called spectroscopes separate light into different colours. Figure 1.18 shows the spectrum of light from hydrogen gas Figure 1.16 Niels Bohr used in a gas discharge tube. mathematics to describe the The Danish physicist Niels Bohr (1885–1962) came up with an explanation atom. In later life, Bohr was for these observations. He hypothesized that electrons in an atom have certain a leading advocate of international allowed energies that enable the atom to remain stable. These allowed energies co-operation in developing peaceful could be thought of as electron shells or energy levels. Electrons would be uses of atomic energy. associated with specific energy levels. In addition, electrons could move only from one allowed energy level to another. They could not exist between the energy levels. By absorbing a specific quantity of energy, an electron could move to a higher energy level. By emitting the same quantity of energy, the electron could move back to its original energy level. Figure 1.19 shows how Bohr pictured electron energy levels. The diagram shows the energy levels in two dimensions for simplicity. In three dimensions, they would have the shape of spherical shells. Chapter 1 Atoms, Elements, and Compounds MHR 19  slit prism gas discharge tube detector Figure 1.17 Instruments called spectroscopes separate light into different colours. Figure 1.18 Each coloured line in this hydrogen spectrum is produced by light with a certain energy. energy levels Bohr proposed this new atomic theory in 1913. The theory fit very well with observations of light emitted from discharge tubes. It seemed reasonable that electrons in atoms of different elements would have different allowed nucleus energy levels, and would therefore absorb and emit light of different energies (different colours). The energy levels of electrons in an atom are the key feature of Bohr’s theory. Bohr showed how to derive mathematical equations that described these energy levels. Figure 1.19 In Bohr’s atom, The equations could be solved for the hydrogen atom (the simplest atom) by electrons can exist only at assuming that electrons moved in circular paths. As the electron absorbed certain energy levels. energy, the equation showed that it could move farther from the positively charged nucleus. Bohr calculated the average distance of electrons in different energy levels from the nucleus of a hydrogen atom. Table 1.1 Maximum Number of Atoms with more than one electron were too complex for Bohr to analyze Electrons in First Two Energy Levels mathematically in the same way as hydrogen. He did establish, however, that Energy level Maximum atoms with two or more electrons could have only a certain number of electrons number of in each energy level. This meant that atoms of each element would have a electrons characteristic arrangement of electrons in different energy levels. Table 1.1 1 2 shows the maximum number of electrons that can occupy the first two 2 8 energy levels. Atomic Theory Evolves www.mcgrawhill.ca/links/sciencefocus10 Current models of the atom are far more complex The neutrino is a subatomic particle that is produced in than Bohr’s electron energy levels and Rutherford’s great numbers by the Sun. Canadian researchers study neutrinos nuclear model. For example, electron energy levels at the Sudbury Neutrino Observatory, a unique world-class centre are now thought to be divided into sublevels. In for specialized research into the properties of neutrinos. Find out more about neutrino research in Canada by going to many cases, electrons in the same energy level are the web site above. Click on Web Links to find grouped in pairs. In addition, scientists now believe out where to go next. that neutrons and protons are made of even smaller 20 MHR Unit 1 Energy and Matter in Chemical Change  Find Out Presenting the Atom It is often easier to understand a concept if you a poster summarizing the key points can see it represented visually. In this activity, your of the theory group will research an atomic theory. Your group Decide how to divide up the work among will then use three methods of communication to group members. explain its details to the rest of your class. 3. Use this textbook to do your research. As time Materials permits, use other texts or Internet resources modelling materials, such as polystyrene balls, to supplement your research. The information marbles, cardboard, modelling clay, and wire you present should include: posterboard and drawing materials such information about the key experiment or as markers experiments that provided evidence for computer three-dimensional modelling software the theory (optional) details of the theory itself explanation of why the theory needed to Procedure Communication and Teamwork be modified (i.e., what observations did 1. Your teacher will divide the class into the theory fail to explain?) four groups. Each group will present an What Did You Find Out? Analyzing and Interpreting atomic theory according to one of the following scientists: 1. Based on your group’s research and on the presentations of other groups, create a Dalton point-form summary of the development Rutherford of atomic theory. Thomson Bohr 2. Each of the models of the atom that you have studied has some flaws. In other words, not all 2. Each group will learn about atomic theory observations of matter can be explained using according to Dalton, Thomson, Rutherford, or these models. Without knowing the details, do Bohr. As a group, you will communicate what you think that the most modern atomic theory you have learned in the following three ways: is likely to represent the best description of a 10-minute presentation reality? Explain your answer. a 3-D model or computer simulation showing how the scientist you researched 3. Although Bohr’s atomic theory has flaws, it might have pictured the atom is still used today. Explain why a theory with known flaws is still taught and used. particles called quarks. Instead of just three subatomic particles (neutrons, protons, and electrons), scientists have identified dozens of fundamental particles. It seems likely that with new observations and discoveries, theories of the atom will continue to evolve. Chapter 1 Atoms, Elements, and Compounds MHR 21  approximately 10-10 m A Working Model of the Atom nucleus Physicists and chemists have developed a complex and detailed model of the atom. A more simplified version explains many observations about chemicals and chemical changes. What are the key features of this model? Protons and neutrons cluster together to form the central core, or nucleus, of an atom. For this reason, protons and neutrons are called nucleons. Electrons occupy the space that surrounds the nucleus of the atom. Table 1.2 and Figure 1.20 summarize the general features and properties of atoms and their three types of subatomic particles. A atom Table 1.2 Properties of Protons, Neutrons, and Electrons Subatomic Relative Symbol Mass (in g) Radius (in m) proton particle charge (positive charge) proton 1" p" 1.67 $ 10#24 10#15 neutron neutron 0 n0 1.67 $ 10#24 10#15 (no charge) electron 1# e# 9.02 $ 10#28 smaller than 10#18 approximately 10-14 m B nucleus An average atom is about 10#10 m in diameter. Such a tiny size is hard to visualize. If an average atom were the size of a grain of sand, a strand of Figure 1.20 This illustration your hair would be about 60 m in diameter! shows a simplified modern model of an atom. Notice that a fuzzy, cloud-like region surrounds the Nuclear Notation atomic nucleus. Electrons exist As you have learned, elements often have two or more isotopes. In other words, in this region at certain allowed atoms of a given element always have the same number of protons, but may energy levels. have differing numbers of neutrons. How do scientists keep track of the number of protons and neutrons in an atom? The composition of an atom is often represented using just two numbers. The atomic number is the number of protons in the nucleus, which identifies the element. The mass number is the total number of protons and neutrons. Along with the atomic number, the mass number identifies a particular isotope of the element. Figure 1.21 shows how to use these numbers. 1 proton 1 proton 1 proton 0 neutrons 1 neutron 2 neutrons mass number 1 2 3 atomic number 1 H 1 H 1 H hydrogen-1 hydrogen-2 hydrogen-3 (deuterium) (tritium) 6 protons 6 protons 6 protons 6 neutrons 7 neutrons 8 neutrons mass number 12 13 14 atomic number 6 C 6 C 6 C carbon-12 carbon-13 carbon-14 Figure 1.21 Scientists represent isotopes using element symbols and by adding the mass number to an element name. These examples show how to represent isotopes of carbon and hydrogen. Hydrogen-2 and hydrogen-3 are more commonly known as deuterium and tritium. 22 MHR Unit 1 Energy and Matter in Chemical Change You can determine the number of neutrons in a nucleus by subtracting the atomic number from the mass number. number of neutrons % mass number # atomic number For example, how many neutrons are in lithium-7? The number 7 in its name tells you that lithium-7 has a mass number of 7. Determine the atomic number of lithium by examining the periodic table in Appendix B. Since lithium is the third element on the periodic table, its atomic number is 3. Therefore, the number of neutrons in lithium-7 is: number of neutrons % 7 # 3 % 4 What about electrons? Atoms are electrically neutral, so they have no charge. Therefore, the number of negatively charged electrons must equal the number of positively charged protons. The atomic number of a neutral atom, then, is the same as the number of electrons in that atom. Answer the Practice Problems below to be sure you understand how to work with nuclear notation. Practice Problems 5. State the number of neutrons in each of the following isotopes. 22 (a) 10 Ne 4 (b) 2 He 40 (c) 20 Ca 27 (d) 13 Al 6. State the number of each of the following subatomic particles in oxygen-17. (a) protons (b) electrons (c) neutrons 7. A certain isotope has a mass number of 35. The isotope has 18 neutrons. (a) What is the atomic number of the isotope? (b) What is the atomic symbol of the isotope? (c) Use nuclear notation to represent the isotope. 8. How many nucleons are in sodium-23? Section 1.2 Summary In this section, you examined some of the evidence that led to an atomic theory that involves protons and neutrons in a nucleus surrounded by electrons in energy levels. In the next section, you will see how the arrangement of these electrons affects how compounds form. Chapter 1 Atoms, Elements, and Compounds MHR 23  Check Your Understanding 1. Make a table that summarizes the following features of the electron, proton, and neutron: location within the atom, relative mass, and electrical charge. 2. Explain why Thomson’s theory of the atom could not explain the results of Rutherford’s gold foil experiment. 3. Explain how the following observations provided evidence for the existence of neutrons. (a) A helium atom has four times the mass of a hydrogen atom. (b) Different atoms of the same element can have different masses. 4. In your notebook, copy and complete the following table. Neutral isotopes Number of neutrons Number of protons Number of electrons carbon-12 6 6 6 lithium-7 (a) 3 (b) sodium-23 12 (c) (d) chlorine-37 (e) (f) (g) 5. Describe the reasoning that links each observation-hypothesis pair below. Scientist Observation Hypothesis Thomson Cathode rays are attracted to positively charged Atoms contain tiny, negatively charged particles. plates. They curve with a measurable radius. Atoms also contain sufficient positive charge to counteract the negative charge. Rutherford Most alpha particles pass through a gold foil, Atoms have a tiny, dense nucleus, but are mostly but a few rebound. empty space. Bohr Hydrogen spectra consist of only a few, specific Electrons in atoms exist in specific energy levels. colours of light. ScientistObservation Hypothesis 6. Thinking Critically Give an example from the development of atomic theory that illustrates each of the following features of scientific thought. (a) A hypothesis allows you to make predictions that can be tested. (b) Theories must explain experimental observations. 7. Thinking Critically Rutherford studied for his undergraduate degree in New Zealand, did graduate studies to get a Ph.D. at Cambridge University in England, did further research at McGill University in Montréal, and then returned to England, to Manchester University. Do you think all this moving around was a disadvantage or an advantage to Rutherford as a scientist? Explain your answer. 8. Apply If an element contains two or more naturally occurring isotopes, is it a pure substance? Explain your answer. 24 MHR Unit 1 Energy and Matter in Chemical Change 1.3 Electrons and the Formation of Compounds Group 18 Under normal conditions, Group 1 Group 2 hydrogen and oxygen are metals colourless, odourless gases. metalloids If you ignite a mixture of Period 1 non-metals hydrogen and oxygen, it burns Period 2 explosively, forming water. Transition metals Period 3 (Groups 3 to 12) Water’s physical and chemical properties are different from those of the two original elements. What if you had never heard of hydrogen, oxygen, or even water? As long as you understood some basic facts about the organization Figure 1.22A Patterns and trends of the periodic table, you could predict that hydrogen and oxygen should in the periods and groups of the periodic table combine to form a new compound with the formula H2O. The periodic table could also help you predict some properties of this new compound. The periodic table, shown in Figure 1.22A, arranges elements into periods (horizontal rows) and groups (vertical column). Across periods, elements appear in order of their atomic number. Down groups, elements have similar properties. Groups of elements are also called “families.” Several of the groups have names. For example, elements in group 1 are called alkali metals, elements in group 2 are called alkaline earth metals, elements in group 17 are called halogens, and elements in group 18 are called noble gases or inert gases. Figure 1.22A also shows the three major sections of the periodic table. The dark “staircase” line separates metals from non-metals. Elements that border this line — metalloids — have some metallic and some non-metallic properties. Table 1.3 summarizes characteristic physical properties of these three groups of elements. Table 1.3 Properties of Metals, Metalloids, and Non-Metals State Appearance Conductivity Malleability and ductility Metals solids at room shiny lustre good conductors malleable temperature, of heat and ductile except for mercury electricity (a liquid) Non-metals some gases at not very shiny poor conductors brittle room temperature of heat and not ductile some solids electricity one liquid (bromine) Metalloids solids at room some are shiny, some conduct brittle temperature others are dull electricity somewhat not ductile poor conductors of heat Chapter 1 Atoms, Elements, and Compounds MHR 25  Patterns of Electron Arrangements in Periods The periodic table represents patterns related to the arrangement of electrons in atoms. These patterns help explain how substances behave during a chemical change. For example, the periodic table can help you answer questions such as: Group 18 elements, the noble gases, are very unlikely to take Why do elements in the same group have similar chemical properties? part in chemical reactions. Why How can you predict the kinds of compounds elements are likely to form? might that be? Record your ideas, and your reasons for them, in The answer to both of these questions comes from the electrons. your notebook. As you learned in section 1.2, Neils Bohr inferred that electrons orbit the nucleus of the atom in fixed energy levels. Each energy level can hold a certain number of electrons but no more. For example, the first energy level can hold a maximum of two electrons. The second energy level holds a maximum of eight electrons. Figure 1.22B shows electron arrangements for atoms of the first 20 elements. 1 Groups 18 1 2 1 H He 2 13 14 15 16 17 3 4 5 6 7 8 9 10 2 Li Be B C N O F Ne Periods 11 12 13 14 15 16 17 18 Figure 1.22B Occupied energy 3 Na Mg Al Si P S Cl Ar levels for the first 20 elements in the periodic table. The diagrams 19 20 do not represent the position or 4 K Ca path of electrons as they move in the atom. Notice that the two elements in period 1 have a single occupied energy level. Recall that the first energy level can hold up to two electrons. Helium, the second element in period 1, has a full complement of two electrons in its energy level. Hydrogen, the first element in period 1, has only one electron. Turn your attention to period 2, which has two occupied energy levels. The first energy level — the shell that is closest to the nucleus — is full. The second energy level contains different numbers of electrons. Lithium has one electron in its second energy level. As you move from one element to the next across period 2, one more electron is added to the second energy level of each atom. The second energy level can contain a maximum of eight electrons. The arrangement of eight electrons in the outermost occupied energy level is called a stable octet. Since neon’s second energy level has this maximum, the second period must end with neon. Elements in period 3 have a third occupied energy level. What do you notice about the number of electrons in the first two energy levels of period 3 elements? What about the number of electrons in the third energy level? As in period 2, the outer occupied energy level of period 3 elements can have a maximum of eight electrons. period number of an element % number of occupied energy levels of its atoms 26 MHR Unit 1 Energy and Matter in Chemical Change Patterns of Electron Arrangements in Groups The key to recognizing the group-related pattern of the periodic table is the number of electrons in the outer occupied energy level. For example, turn back to Figure 1.22B and examine the group 1 elements. Notice that atoms of each element in group 1 have only one electron in their outer occupied energy level. Now examine group 2. As with group 1, you will notice that each element has the same number of electrons in its outer occupied energy level. Group 2 elements have two electrons in their outer occupied energy level. This pattern holds for groups 1, 2, and 13 through 18. Notice that all group 18 elements have a filled outer energy level. Helium has two electrons, while neon and argon both have eight electrons in their outer energy level. The outermost occupied energy level of an atom is called its valence energy level. The electrons in the valence energy level are called valence electrons. Elements in the same group on the periodic table have atoms with the same number of valence electrons. Table 1.4 summarizes some of the properties of groups 1, 2, 17, and 18. Try the Practice Problems below to apply what you have learned. Table 1.4 Characteristics of Groups 1, 2, 17, and 18 Group Group name Number of Appearance Properties number valence electrons 1 alkali metals 1 shiny solids very soft, malleable, and ductile react easily with water and other substances 2 alkaline earth 2 shiny solids malleable and ductile metals react with oxygen to form substances called oxides 17 halogens 7 chlorine and fluorine are gases react with metallic elements to form substances bromine is a liquid called salts iodine is a solid 18 noble gases 8 (helium has 2) colourless gases very unlikely to take part in chemical reactions Practice Problems 9. Based on the patterns of the periodic table, identify the number of valence electrons for each of the following elements. (You will need to consult the full periodic table in Appendix B.) (a) chlorine, Cl (c) cesium, Cs (e) bromine, Br (b) magnesium, Mg (d) strontium, Sr (f ) silicon, Si 10. State what you would expect the appearance of sodium, Na, to be. 11. Identify the name and symbol of the element in each of the following locations on the periodic table. (a) group 1, period 2 (c) group 15, period 3 (b) group 14, period 2 (d) group 18, period 1 12. Based on the patterns of the periodic table, identify the number of occupied energy levels for each of the following elements. (a) calcium, Ca (c) sulfur, S (b) krypton, Kr (d) iodine, I Chapter 1