Comparing Elements Based on Their Properties PDF

EXPLORATION 1 Comparing Elements Based on Their Properties The gases in Earth’s atmosphere are an example of matter in the Earth system. FIGURE 1: Magnesium (left), zinc (center), and copper Matter can be classified as either a pure substance or...

EXPLORATION 1 Comparing Elements Based on Their Properties The gases in Earth’s atmosphere are an example of matter in the Earth system. FIGURE 1: Magnesium (left), zinc (center), and copper Matter can be classified as either a pure substance or a mixture. The metals (right) in hydrochloric acid. shown in Figure 1 are all pure substances. Collaborate Discuss the following with a partner: In Figure 1, three different metals are combined with an acid. What differences do you notice in the way these metals react? What do you think causes these differences in reactivity? The metals shown in Figure 1 are all elements, or pure substances that cannot be broken down into simpler substances. The particles that make up these elements are called atoms. Each element contains only one type of atom. Classifying Elements Certain groups of elements have similar properties and can be classified together. The most general way to classify elements is as metals, nonmetals, or metalloids. Metals are shiny, and they are generally good conductors of electricity and heat. They can be bent or hammered into sheets easily, and almost all metals are solid at room temperature. In contrast, nonmetals are generally very poor conductors of electricity and heat. Many nonmetals, such as nitrogen, oxygen, fluorine, and chlorine, are gases at room temperature. The nonmetals that are solids at room temperature, such as carbon, © Houghton Mifflin Harcourt Publishing Company Image Credits: (tl) ©Turtle Rock Scientific/Science phosphorus, sulfur, selenium, and iodine, tend to be brittle and dull rather than shiny. Source; (bl) ©lucentius/E+/Getty Images; (bc) ©Chip Clark/Fundamental Photographs; (br) ©Chip FIGURE 2: Elements are categorized into broad categories based on similar properties. a Metals b Nonmetals c Metalloids Clark/Fundamental Photographs Metalloids have characteristics of metals and nonmetals. Metalloids are solids at room temperature. They are not as brittle as nonmetals. Metalloids do conduct electric current but not as well as metals. The “semiconducting” properties of metalloids, such as boron and silicon, make them useful in computer chips. APPLY Describe some examples of metals and nonmetals from your daily life. How can you tell which type of elements are metals and which are not? 58 Unit 2 Atoms and Elements  Understanding Atoms People’s understanding of the atom has changed over time. The idea that matter is made up of smaller, individual units was proposed many centuries ago. Empirical evidence to support the existence of atoms did not come until much later. By the mid 1800s, most scientists agreed that each element was made up of a unique type of atom. However, they saw atoms as tiny, indivisible balls, differing only in mass. MODEL Draw a diagram to show how a scientist in the 1800s might have modeled atoms. Include different types of atoms in your diagram and indicate how they differ in this model. The theory that atoms were indivisible units differing only in mass did not fully explain the patterns that scientists observed in the properties of elements. For example, the metals you tested in the lab react with hydrochloric acid, but not with water. But some metals, such as sodium and potassium, are so reactive that they ignite when they are combined with water. Then there are elements, such as the gases shown in Figure 3, that almost never react with other substances. These gases are part of a group of elements known as the noble gases. Because the noble gases appeared not to have any chemical properties that could be used to compare them, early chemists struggled to organize them into a classification scheme. Even passing an electric current through these gases did not cause a reaction. It did, however, cause the gases to produce light of different colors. © Houghton Mifflin Harcourt Publishing Company Image Credits: ©Richard Megna/Fundamental FIGURE 3: Each sign spells out the element symbol of the gas with which it is filled: helium, neon, and argon. Collaborate Discuss these questions with a partner: Why do you think the elements in Figure 3 give off different colors of light when an electric current passes through them? How might the colors given off be related to the type of atom that makes up each element? Photographs Over time, scientists concluded that something other than mass alone must be causing the patterns they observed in the properties of elements. Further experimentation would allow scientists to determine what made one type of atom different from others. Evidence Notebook How might the properties of different elements, such as the noble gases and the metals you tested, be related to the phenomenon of the aurora polaris? Lesson 1 Modeling Atomic Structure 61 EXPLORATION 2 Investigating Atomic Structure Explore Online You may have experienced a shock when touching a door knob after walking FIGURE 4: A negatively across a carpeted floor. When you walk across carpet, a charge can build up charged balloon attracts a stream of water. on the surface of your body. When you touch a metal object, the charge is transferred with a shock. In a similar way, a balloon rubbed with a cloth builds up a negative charge. When the balloon is placed near a thin stream of water, as shown in Figure 4, the water is attracted to the balloon. PREDICT In Figure 4, a negatively charged balloon attracts a thin stream of water. Why do you think this happens? Identifying Electrons Like charges repel, and opposite charges attract. Thus, the attraction between a negatively charged balloon and a stream of water is evidence that opposite charges are present. If atoms were simply tiny indivisible spheres, as scientists once theorized, how could these opposite charges form? In the late 1800s, experiments with cathode rays led to the © Houghton Mifflin Harcourt Publishing Company Image Credits: (tl) ©Houghton Mifflin Harcourt; discovery of a charged particle called the electron. A cathode-ray tube, as shown in Figure 5, is a glass tube containing a gas at very low pressure. At one end, it has a cathode, a metal disk connected to the negative terminal of the energy source. At the other end, it has an anode, a metal disk connected to a positive terminal. When an electric current is passed through the tube, a glowing stream of particles called a cathode ray can be observed. In 1897, a scientist named J.J. Thomson noted that a magnetic or electric field could cause the cathode ray to bend. The ray bent toward a positive charge and away from a negative charge. Explore Online FIGURE 5: A cathode ray bends away from a magnet. (b) ©Charles D. Winters/Science Source EXPLAIN Select the correct terms to complete the statement. Opposite | like charges attract one another. So, if an electric current causes a cathode ray to bend toward a positive charge, the ray must be positively | negatively charged. 62 Unit 2 Atoms and Elements  Further experimentation with cathode rays allowed J.J. Thomson to calculate the charge- to-mass-ratio of the particles that made them up. He noticed that this ratio remained constant even when different gases or metals were used in the cathode ray tube. The very small, negatively charged particles Thomson identified later became known as electrons. –31 It was determined that electrons had a mass of 9.109 x 10 kg, or 1/1836 the mass of a hydrogen atom. Because electrons are so much smaller than an atom, and atoms have no overall charge, it was clear that atoms could not be made up solely of electrons. INFER What could be inferred about atomic structure after the discovery of the electron? Select all correct answers. a. Atoms are made up of smaller “subatomic” particles. b. Atoms of different elements contain the same number of electrons. c. The electrons in an atom are attracted to each other due to their negative charge. d. Atoms must also contain a positively charged component. FIGURE 6: A plum pudding The discovery of the electron led scientists to develop a new atomic model. In this model, called the “plum pudding model,” negatively charged electrons are evenly distributed within a mass of positively charged material. Thus, the electrons are like raisins in a cake, as shown in Figure 6. The cake itself represents the area of positive charge that surrounds the electrons. Identifying the Nucleus In the early 1900s, a physicist and former student of Thomson’s named Ernest Rutherford devised an experiment to learn more about atomic structure. In this experiment, called the gold foil experiment, positively charged particles called alpha (α) particles were focused into a narrow beam and shot at a very thin piece of gold. Rutherford hypothesized that if the plum pudding model was correct, the area of positive charge in the gold atoms would be too spread out to repel the positively charged alpha © Houghton Mifflin Harcourt Publishing Company Image Credits: ©magnetix/Shutterstock particles. So, most of the particles should pass straight through the foil undeflected. FIGURE 7: Gold foil experiment Explore Online a few α particles are scattered most particles pass through foil source of α particles thin metal foil beam of α particles screen to detect scattered α particles Collaborate With a partner, analyze the diagram of the gold foil experiment shown in Figure 7 and answer these questions: What happened when the alpha particles encountered the gold foil? What do you think these results indicate about atomic structure? Lesson 1 Modeling Atomic Structure 63  In the gold foil experiment, most of the alpha particles passed straight through the gold foil as expected. However, some alpha particles were deflected at large angles, as shown in Figure 8. A few alpha particles were even deflected backwards from the foil. Rutherford was very surprised by the results, later saying that it was almost as incredible as if you fired a cannonball at a piece of tissue paper and it came back to hit you. FIGURE 8: The results of the gold foil experiment led to a new model of the atom. small deflection beam of positive particles nucleus electrons surrounding large deflection nucleus The model that Rutherford developed to explain his results depicted atoms as being made up of mostly empty space. In the center of the atom is a small, dense, positively-charged core, or nucleus, that makes up most of an atom’s mass. The much lighter electrons surround the nucleus in a relatively large electron cloud. Evidence Notebook Explain how evidence from the gold foil experiment supports each of these claims. How can you apply these ideas to the atomic model in your unit project? atoms have a very small, dense core the core of an atom has a positive charge atoms are made up of mostly empty space Describing Atomic Structure Properties of Subatomic Particles The gold foil experiment showed that the nucleus of an atom contains a small, dense nucleus with a positive charge. Further Particles Electric charge Actual mass (kg) studies of the nucleus showed that it is composed of protons, © Houghton Mifflin Harcourt Publishing Company −31 Electron −1 9.109 × 10 which are positively charged particles, and neutrons, which have no −27 charge. Protons and neutrons are much larger than electrons, with a Proton +1 1.673 × 10 proton having about 1836 times the mass of an electron. The much −27 Neutron 0 1.675 × 10 smaller, negatively-charged electrons surround the nucleus. INFER If atoms are neutral (have no overall charge), what can you infer about the number of protons and electrons in an atom? How should these numbers compare? Explain your thinking. 64 Unit 2 Atoms and Elements  Scale, Proportion, and Quantity Scale and Atomic Models Atoms are too small to observe directly. So, to help scientists understand the structure of atoms, they develop models. Atomic models are useful because they allow scientists to visualize the structure of atoms. However, it can be difficult to develop a model of the atom that is to scale. The models of atoms shown in textbooks are not to FIGURE 9: An atom with a nucleus the size of a grain of scale because it would be impossible to represent the sand would be the size of a baseball stadium. proportions of the atom in a two-dimensional diagram. In a scale model, the electron cloud would be 100 000 times the diameter of the nucleus. To visualize this, imagine the nucleus were the size of a grain of sand. An atom with a nucleus this size would be as large around as a baseball stadium, such as the one shown in Figure 9. In addition, atoms are three-dimensional objects, so any drawing does not show its true shape. Scientists acknowledge that their atomic models are not drawn to scale and that the distances between the nucleus and electrons are not accurate. PREDICT What types of information do you think scientists can gain from a computational, physical, or two-dimensional atomic model that is not to scale? © Houghton Mifflin Harcourt Publishing Company Image Credits: ©Ron Niebrugge/Alamy Some models can be useful even when they are not to scale. For example, models of very large objects, such as the solar system, are not to scale. This is because the interplanetary distances involved are so great that it would be hard to make an accurate scale model. Consider this: If a model of the solar system were on a football field, the sun would be about the size of a dime. The planet Neptune would be 60 yards (55 m) away and about the diameter of the lead on a mechanical pencil. Some models, however, must be made to scale. For example, blueprints for a building are a type of model. It is important that they be made to scale to make sure the rooms, plumbing, and electrical wiring are all properly constructed and fit in the required space. Collaborate Work with a partner to develop criteria for situations that require a scale model or do not require a scale model. Evidence Notebook Summarize what you have learned about the structure of the atom. How do you think atomic structure might be related to the aurora polaris phenomenon? Lesson 1 Modeling Atomic Structure 65