Atomic Structure IB Past Paper PDF

02 Atomic structure M02_CHSL_SB_IBD_9069_U02.indd 56 15/10/2014 16:30  Essential ideas Picture of individual atoms....

02 Atomic structure M02_CHSL_SB_IBD_9069_U02.indd 56 15/10/2014 16:30  Essential ideas Picture of individual atoms. This is a scanning tunnelling 2.1 The mass of an atom is concentrated in its minute, positively micrograph of gold atoms on charged nucleus. a graphite surface. The gold atoms are shown in yellow, 2.2 The electron configuration of an atom can be deduced from its red, and brown and the graphite (carbon) atoms are atomic number. shown in green. ‘All things are made from atoms.’ This is one of the most important ideas that the Richard Feynman: ‘…if all human race has learned about the universe. Atoms are everywhere and they make up of scientific knowledge everything. You are surrounded by atoms – they make up the foods you eat, the liquids were to be destroyed, and you drink and the fragrances you smell. Atoms make up you! To understand the world only one sentence passed on to the next generation and how it changes you need to understand atoms. of creatures, what The idea of atoms has its origins in Greek and Indian philosophy nearly 2500 years statement would contain the most information ago, but it was not until the 19th century that there was experimental evidence to in the fewest words? I support their existence. Although atoms are too small ever to be seen directly by a believe it is the atomic human eye, they are fundamental to chemistry. All the atoms in a piece of gold foil, hypothesis…that all things for example, have the same chemical properties. The atoms of gold, however, have are made of atoms.’ Are the models and theories different properties from the atoms of aluminium. This chapter will explain how they which scientists create differ. We will explore their structure and discover that different atoms are made from accurate descriptions different combinations of the same sub-atomic particles. of the natural world, or are they primarily This exploration will take us into some difficult areas because our everyday notion of useful interpretations for particles following fixed trajectories does not apply to the microscopic world of the prediction, explanation atom. To understand the block structure of the Periodic Table we need to use quantum and control of the natural world? theory and adopt a wave description of matter. These ideas are revolutionary. As Niels Bohr, one of the principal scientists involved in the development of quantum theory said, ‘Anyone who is not shocked by quantum theory has not understood it.’ The hydrogen atom shown as a nucleus (a proton, pink), and an electron orbiting in a wavy path (light blue). It is necessary to consider the wave properties of the electron to understand atomic structure in detail. A billion of your atoms once made up Shakespeare, another billion made up Beethoven, another billion St. Peter and another billion the Buddha. Atoms can rearrange in chemical reactions but they cannot be destroyed. 57 M02_CHSL_SB_IBD_9069_U02.indd 57 15/10/2014 16:30  02 Atomic structure 2.1 The nuclear atom Understandings: Atoms contain a positively charged dense nucleus composed of protons and neutrons (nucleons). Guidance Relative masses and charges of the sub-atomic particles should be known, actual values are given in section 4 of the IB data booklet. The mass of the electron can be considered negligible. Negatively charged electrons occupy the space outside the nucleus. The mass spectrometer is used to determine the relative atomic mass of an element from its isotopic composition. Guidance The operation of the mass spectrometer is not required. Applications and skills: Use of the nuclear symbol notation AZ X to deduce the number of protons, neutrons, and electrons in atoms and ions. Calculations involving non-integer relative atomic masses and abundance of isotopes from given data, including mass spectra. Guidance Specific examples of isotopes need not be learned. Dalton’s model of the atom One of the first great achievements of chemistry was to show that all matter is built An element is a substance that cannot from about 100 elements. As mentioned in Chapter 1, the elements are substances be broken down into which cannot be broken down into simpler components by chemical reactions. They simpler substances by a are the simplest substances and their names are listed in your IB data booklet (section chemical reaction. 5). Different elements have different chemical properties but gold foil, for example, reacts in essentially the same way as a single piece of gold dust. Indeed if the gold dust The word ‘atom’ comes is cut into smaller and smaller pieces, the chemical properties would remain essentially from the Greek words for the same until we reached an atom. This is the smallest unit of an element. There are ‘not able to be cut’. only 92 elements which occur naturally on Earth and they are made up from only 92 different types of atom. (This statement will be qualified when isotopes are discussed later in the chapter.) Scanning tunnelling microscope (STM) image of the surface of pure gold. STM provides a magnification of 250 000 times, which records the surface structure at the level of the individual atoms. Gold exists in many forms – gold foil, nuggets, blocks, etc., which all contain the same type of atoms. The ‘rolling hills’ structure seen here is the result of changes in the surface energy as the gold cooled from its molten state. STM is based on quantum mechanical effects. 58 M02_CHSL_SB_IBD_9069_U02.indd 58 15/10/2014 16:30  NATURE OF SCIENCE CHALLENGE The idea that matter is made up from elements and atoms dates back to the Indian philosophy of the sixth century BCE and the Greek philosophy of Democritus (460 BCE to 370 BCE). These YOURSELF ideas were speculative as there was little evidence to support them. A significant development 1 Can you think of any for chemistry came with the publication of Robert Boyle’s Skeptical Chemist of 1661 which evidence based on simple emphasized the need for scientific knowledge to be justified by evidence from practical observations that supports investigations. Boyle was the first to propose the modern concept of an element as a substance the idea that water is made which cannot be changed into anything simpler. up from discrete particles? The modern idea of the atom dates from the beginning of the 19th century. John Dalton noticed that the elements hydrogen and oxygen always combined together in fixed proportions. To explain this observation he proposed that: all matter is composed of tiny indivisible particles called atoms; atoms cannot be created or destroyed; atoms of the same element are alike in every way; atoms of different elements are different; atoms can combine together in small numbers to form molecules. Using this model we can understand how elements react together to make new substances called compounds. The compound water, for example, is formed when two hydrogen atoms combine with one oxygen atom to produce one water molecule. If we repeat the reaction on a larger scale with 2 × 6.02 × 1023 atoms of hydrogen and 6.02 × 1023 atoms of oxygen, 6.02 × 1023 molecules of water will be formed. This leads to the conclusion (see Chapter 1) that 2.02 g of hydrogen will react with 16.00 g of oxygen to form 18.02 g of water. This is one of the observations Dalton was trying to explain. Although John Dalton (1766– 1844) was a school teacher NATURE OF SCIENCE from Manchester in England, his name has passed into other Dalton was a man of regular habits. For fifty-seven years… he measured the rainfall, the languages. The internationally temperature… Of all that mass of data, nothing whatever came. But of the one searching, almost recognized term for colour- childlike question about the weights that enter the construction of simple molecules – out of blindness, Daltonisme in that came modern atomic theory. That is the essence of science: ask an impertinent question: French, for example, derives and you are on the way to the pertinent answer. ( J. Bronowski) from the fact the he suffered from the condition. Dalton was the first person to assign chemical symbols to the different elements. ‘What we observe is not nature itself but nature exposed to our mode of questioning.’ (Werner Heisenberg). How does CHALLENGE YOURSELF the knowledge we gain about the natural 2 It is now known that some of these substances are world depend on the not elements but compounds. Lime, for example, is questions we ask and the a compound of calcium and oxygen. Can you find experiments we perform? any other examples in this list and explain why the elements had not been extracted at this time? A compound is a substance made by chemically combining two or more elements. It has different properties John Dalton’s symbols for the from its constituent elements. elements. 59 M02_CHSL_SB_IBD_9069_U02.indd 59 15/10/2014 16:30  02 Atomic structure Following his example, the formation of water (described above) can be written using modern notation: 2H +O → H2O But what are atoms really like? It can be useful to think of them as hard spheres (Figure 2.1) but this tells us little about how the atoms of different elements differ. To understand this, it is necessary to probe deeper. NATURE OF SCIENCE Figure 2.1 A model of a Dalton’s atomic theory was not accepted when it was first proposed. Many scientists, such water molecule made from as Kelvin for example, considered it as nothing more than a useful fiction which should not two hydrogen atoms and one be taken too seriously. Over time, as the supporting evidence grew, there was a general shift oxygen atom. Dalton’s picture in thinking which led to its widespread acceptance. These revolutions in understanding or of the atom as a hard ball is ‘paradigm shifts’ are characteristic of the evolutions of scientific thinking. the basis behind the molecular models we use today. Atoms contain electrons − + The first indication that atoms were destructible came at the end of the 19th century − when the British scientist J. J. Thomson discovered that different metals produce − − + + a stream of negatively charged particles when a high voltage is applied across two − − + − electrodes. As these particles, which we now know as electrons, were the same + + + regardless of the metal, he suggested that they are part of the make-up of all atoms. − + − + NATURE OF SCIENCE The properties of electrons, or cathode rays as they were first called, could only be investigated once powerful vacuum pumps had been invented – and once advances had been made in Figure 2.2 Thomson’s ‘plum the use and understanding of electricity and magnetism. Improved instrumentation and new pudding’ model of the atom. technology have often been the drivers for new discoveries. The electrons (yellow) are scattered in a positively As it was known that the atom had no net charge, Thomson pictured the atom as a charged sponge-like substance (pink). ‘plum pudding’, with the negatively charged electrons scattered in a positively charged sponge-like substance (Figure 2.2). When Geiger and Rutherford’s model of the atom Marsden reported to Ernest Rutherford (1871–1937) and his research team working at Manchester Rutherford that they had University in England, tested Thomson’s model by firing alpha particles at a piece seen nothing unusual with most of the alpha of gold foil. We now know that alpha particles are helium nuclei, composed of two particles passing straight protons and two neutrons, with a positive charge. They are emitted by nuclei with too through the gold and many protons to be stable. If Thomson’s model was correct, the alpha particles should a small number being either pass straight through or get stuck in the positive ‘sponge’. Most of the alpha deflected by small angles, he asked them to look particles did indeed pass straight through, but a very small number were repelled and and see if any of the alpha bounced back. Ernest Rutherford recalled that ‘It was quite the most incredible thing particles had bounced that has happened to me. It was as if you had fired a (artillery) shell at a piece of tissue back. This was a very paper and it came back and hit you.’ unusual suggestion to make at the time, with little The large number of undeflected paths led to the conclusion that the atom is mainly logical justification. What empty space. Large deflections occur when the positively charged alpha particles is the role of intuition in collide with and are repelled by a dense, positively charged centre called the nucleus the pursuit of scientific knowledge? (Figure 2.3). The fact that only a small number of alpha particles bounce back suggests that the nucleus is very small. 60 M02_CHSL_SB_IBD_9069_U02.indd 60 15/10/2014 16:30  incident alpha Figure 2.3 Rutherford’s model particles of the atom accounts for the experimental observations. Most of the alpha particles pass straight through but a small number collide with and are repelled by a positively charged nucleus. NATURE OF SCIENCE Our knowledge of the nuclear atom came from Rutherford’s experiments with the relatively newly discovered alpha particles. Scientific knowledge grows as new evidence is gathered as a result of new technologies and instrumentation. Sub-atomic particles A hundred years or so after Dalton first proposed his model, these experiments and The European many others showed that atoms are themselves made up from smaller or sub-atomic Organization for Nuclear particles. The nucleus of an atom is made up of protons and neutrons, collectively Research (CERN) is run by called nucleons. Both the protons and neutrons have almost the same mass as a twenty European Member States, with involvements hydrogen nucleus and account for the most of the mass of the atom. Electrons, which from scientists from have a charge equal and opposite to that of the proton, occupy the space in the atom many other countries. It outside of the nucleus. operates the world’s largest particle physics research These particles are described by their relative masses and charges which have no units. centre, including particle The absolute masses and charges of these fundamental particles are given in section 4 accelerators and detectors of the IB data booklet. used to study the fundamental constituents Particle Relative mass Relative charge of matter. proton 1 +1 PET (positron-emission electron 0.0005 −1 tomography) scanners give three-dimensional images neutron 1 0 of tracer concentration in the body, and can be used to detect cancers. NATURE OF SCIENCE The patient is injected with a tracer compound The description of sub-atomic particles offered here is sufficient to understand chemistry but labelled with a positron- incomplete. Although the electron is indeed a fundamental particle, we now know that the emitting isotope. The protons and neutrons are both themselves made up from more fundamental particles called positrons collide with quarks. We also know that all particles have anti-particles. The positron is the anti-particle of an electrons after travelling electron; it has the same mass but has an equal and opposite positive charge. When particles a short distance (≈1 mm) and anti-particles collide they destroy each other and energy in the form of high-energy within the body. Both photons called gamma rays. Our treatment of sub-atomic particles is in line with the principle particles are destroyed of Occam’s razor, which states that theories should be as simple as possible while maximizing with the production of two explanatory power. photons, which can be collected by the detectors surrounding the patient, and used to generate an image. 61 M02_CHSL_SB_IBD_9069_U02.indd 61 15/10/2014 16:30  02 Atomic structure View of a patient undergoing a positron-emission tomography (PET) brain scan. A radioactive tracer is injected into the patient’s bloodstream, which is then absorbed by active tissues of the brain. The PET scanner detects photons emitted by the tracer and produces ‘slice’ images. As you are made from atoms, you are also mainly empty space. The particles which make up your mass would occupy the same volume as a flea if they were all squashed together, but a flea with your mass. This gives you an idea of the density of the nucleus. Bohr model of the hydrogen atom The Danish physicist Niels Bohr pictured the hydrogen atom as a small ‘solar system’, None of these sub- with an electron moving in an orbit or energy level around the positively charged atomic particles can be nucleus of one proton (Figure 2.4). The electrostatic force of attraction between the (or ever will be) directly observed. Which ways oppositely charged sub-atomic particles prevents the electron from leaving the atom. of knowing do we use to The nuclear radius is 10–15 m and the atomic radius 10–10 m, so most of the volume of interpret indirect evidence the atom is empty space. gained through the use of technology? The existence of neutrally charged neutrons is crucial for the stability of nuclei of later elements, which have more than one proton. Without the neutrons, the positively Figure 2.4 The simplest charged protons would mutually repel each other and the nucleus would fall apart. atom. Only one proton and one electron make up the hydrogen atom. The nuclear radius is 10–15 m and the Atomic number and atomic radius 10–10 m. Most proton in electron orbit mass number of the volume of the atom is the nucleus + We are now in a position to empty – the only occupant is the negatively charged understand how the atoms electron. It is useful to think − electron of different elements differ. of the electron orbiting the − electron They are all made from the nucleus in a similar way to the planets orbiting the sun. + proton same basic ingredients, the The absence of a neutron sub-atomic particles. The only is significant – it would be difference is the recipe – how many of each of essentially redundant as there these sub-atomic particles are present in the atoms is only one proton. of different elements. If you look at the Periodic n1 Table, you will see that the elements are each given 2 2 Figure 2.5 A helium atom. 1n a number which describes their relative position The two neutrons allow the two protons, which repel each in the table. This is their atomic number. We now 2 electron other, to stay in the nucleus. know that the atomic number, represented by Z, 1 proton is the defining property of an element as it tells n neutron us something about the structure of the atoms of The atomic number is defined as the number of the element. The atomic number is defined as the protons in the nucleus. number of protons in the atom. 62 M02_CHSL_SB_IBD_9069_U02.indd 62 15/10/2014 16:31  As an atom has no overall charge, the positive charge of the protons must be balanced Make sure you have a by the negative charge of the electrons. The atomic number is also equal to the number precise understanding of of electrons. the terms identified in the subject guide. The atomic The electron has such a very small mass that it is essentially ignored in mass number, for example, is calculations. The mass of an atom depends on the number of protons and neutrons defined in terms of the only. The mass number, given the symbol A, is defined as the number of protons plus number of protons, not electrons. the number of neutrons in an atom. An atom is identified in the following way: mass number The mass number (A) is A the number of protons ZX symbol of element plus the number of atomic number neutrons (total number of nucleons) in an atom. As it gives the total We can use these numbers to find the composition of any atom. number of nucleons number of protons (p) = number of electrons = Z in the nucleus it is sometimes called the number of neutrons (n) = A − number of protons = A − Z nucleon number. Consider an atom of aluminium: n 5 27 2 13 5 14 27 13Al p 5 13 e 5 13 An aluminium atom is made from 13 protons and 13 electrons. An atom of gold on the other hand has 79 protons and 79 electrons. Can you find gold in the Periodic Table? Isotopes Find chlorine in the Periodic Table. There are two numbers associated with the element, as shown below. 2 8 9 10 Atomic number = 17 O F Ne Oxygen Fluorine Neon 16.00 19.00 24.16 16 17 18 S Cl Ar Phosphorous Sulfur Chlorine Argon 32.06 35.45 39.95 34 35 36 Se Br Kr Selenium Bromine Krypton 78.95 79.90 83.80 Relative atomic 128 127 131 mass = 35.45 Te I Xe How can an element have a fractional relative atomic mass if both the proton and neutron have a relative mass of 1? One reason is that atoms of the same element with different mass numbers exist, so it is necessary to work within an average value – as discussed in Chapter 1. To have different mass numbers, the atoms must have different numbers of neutrons – Isotopes are atoms of all the atoms have the same number of protons as they are all chlorine atoms. Atoms of the same element with the same element with different numbers of neutrons are called isotopes. different mass numbers. The isotopes show the same chemical properties, as a difference in the number of neutrons makes no difference to how they react and so they occupy the same place in the Periodic Table. 63 M02_CHSL_SB_IBD_9069_U02.indd 63 15/10/2014 16:31  02 Atomic structure The word ‘isotope’ derives Chlorine exists as two isotopes, 35Cl and 37Cl. The average relative mass of the isotopes from the Greek for is, however, not 36, but 35.45. This value is closer to 35 as there are more 35Cl atoms in ‘same place’. As isotopes nature – it is the more abundant isotope. In a sample of 100 chlorine atoms, there are are atoms of the same 77.5 atoms of 35Cl and 22.5 atoms of the heavier isotope, 37Cl. element, they occupy the same place in the Periodic To work out the average mass of one atom we first have to calculate the total mass of Table. the hundred atoms: total mass = (77.5 × 35) + (22.5 × 37) = 3545 A common error is to misunderstand the total mass 3545 meaning of ‘physical relative average mass = = = 35.45 number of atoms 100 property’. A difference in the number of neutrons The two isotopes are both atoms of chlorine with 17 protons and 17 electrons. is not a different physical 35 Cl; number of neutrons = 35 − 17 = 18 property. A physical 37 property of a substance Cl; number of neutrons = 37 − 17 = 20 can be measured without Although both isotopes essentially have the same chemical properties, the difference changing the chemical composition of the in mass does lead to different physical properties such as boiling and melting points. substance, e.g. melting Heavier isotopes move more slowly at a given temperature and these differences can be point, density. used to separate isotopes. Exercises Radioisotopes are used 1 State two physical properties other than boiling and melting point that would differ for the two in nuclear medicine for isotopes of chlorine. diagnostics, treatment, 2 Explain why the relative atomic mass of tellurium is greater than the relative atomic mass of iodine, and research, as tracers even though iodine has a greater atomic number. in biochemical and pharmaceutical research, and as ‘chemical clocks’ Uranium exists in nature as two isotopes, uranium-235 and uranium-238. One key stage in in geological and the Manhattan Project (the development of the atomic bomb during World War II) was the archaeological dating. enrichment of uranium with the lighter and less abundant isotope, as this is the atom which splits more easily. It is only 0.711% abundant in nature. First the uranium was converted to a gaseous compound (the hexafluoride UF6). Gaseous molecules with the lighter uranium isotope Radioactive isotopes are move faster than those containing the heavier isotope at the same temperature and so the extremely hazardous and isotopes could be separated. Isotope enrichment is employed in many countries as part of their use is of international nuclear energy and weaponry programmes. This is discussed in more detail in Chapter 14. concern. The International Atomic Energy Agency (IAEA) promotes the NATURE OF SCIENCE peaceful use of nuclear Science is a collaborative endeavour and it is common for scientists to work in teams between energy. The organization disciplines, laboratories, organizations, and countries. The Manhattan Project, which produced was awarded the Nobel the first nuclear bomb, employed more than 130 000 people working in secret at different Peace Prize in 2005. production and research sites. Today such collaboration is facilitated by virtual communication which allows scientists around the globe to work together. Ions The atomic number is defined in terms of number of protons because it is a fixed characteristic of the element. The number of protons identifies the element in the same way your fingerprints identify you. The number of protons and neutrons never When an atom loses changes during a chemical reaction. It is the electrons which are responsible for electrons, a positive ion is formed and when chemical change. Chapter 4 will examine how atoms can lose or gain electrons to form it gains electrons, a ions. When the number of protons in a particle is no longer balanced by the number negative ion is formed. of electrons, the particles have a non-zero charge. When an atom loses electrons Positive ions are called it forms a positive ion or cation, as the number of protons is now greater than the cations and negative number of electrons. Negative ions or anions are formed when atoms gain electrons. ions are called anions. The magnitude of the charge depends on the number of electrons lost or gained. The 64 M02_CHSL_SB_IBD_9069_U02.indd 64 15/10/2014 16:31  loss or gain of electrons makes a very big difference to the chemical properties. You swallow sodium ions, Na+, every time you eat table salt, whereas (as you will discover in Chapter 3) sodium atoms, Na, are dangerously reactive. An aluminium ion is formed when the atom loses three electrons. There is no change in the atomic or mass numbers of an ion because the number of protons and neutrons remains the same. The atom has lost 3 electrons. 27 31 e 5 13 2 3 5 10 13Al The element radium was first discovered by the Polish– p 5 13 French scientist Marie Curie. n 5 27 2 13 5 14 She is the only person to win Nobel Prizes in both Physics Oxygen forms the oxide ion when the atom gains two electrons. and Chemistry. The Curies were a remarkable family for The atom has gained 2 electrons. scientific honours – Marie e 5 8 1 2 5 10 shared her first prize with 16 22 8O husband Pierre, and her daughter Irène shared hers p58 with her husband Frédéric. n 5 16 2 8 5 8 All the Curies’ prizes were for work on radioactivity. Worked example Identify the sub-atomic particles present in an atom of 226Ra. Solution The number identifying the atom is the atomic number. We can find the atomic number from the IB data booklet (section 5). We have Z = 88 and A = 226 In other words: number of protons (p) = 88 number of electrons (e) = 88 number of neutrons (n) = 226 − 88 = 138 Worked example Most nutrient elements in food are present in the form of ions. The calcium ion 40 Ca2+, for example, is essential for healthy teeth and bones. Identify the sub-atomic particles present in the ion. Solution We can find the atomic number from the IB data booklet (section 5). We have Z = 20 and A = 40: number of protons (p) = 20 number of neutrons (n) = 40 − 20 = 20 As the ion has a positive charge of 2+ there are two more protons than electrons: number of electrons = 20 − 2 = 18 65 M02_CHSL_SB_IBD_9069_U02.indd 65 15/10/2014 16:31  02 Atomic structure Worked example Identify the species with 19 protons, 20 neutrons and 18 electrons. Solution the number of protons tells us the atomic number; Z = 19 and the element is potassium: K the mass number = p + n = 19 + 20 = 39: 39 19K the charge will be = p – e = 19 – 18 = +1 as there is one less electron: 39 19K + Exercises 3 Use the Periodic Table to identify the sub-atomic particles present in the following species. Species No. of protons No. of neutrons No. of electrons 7 (a) Li 1 (b) H 14 (c) C 19 – (d) F 56 (e) Fe3+ 4 Isoelectronic species have the same number of electrons. Identify the following isoelectronic species by giving the correct symbol and charge. You will need a Periodic Table. The first one has been done as an example. Species No. of protons No. of neutrons No. of electrons 40 Ca2+ 20 20 18 (a) 18 22 18 (b) 19 20 18 (c) 17 18 18 5 Which of the following species contain more electrons than neutrons? 2 11 16 2− 19 − A 1H B 5B C 8O D 9F 71 + 6 Which of the following gives the correct composition of the Ga ion present in the mass spectrometer when gallium is analysed. Protons Neutrons Electrons A 31 71 30 B 31 40 30 C 31 40 32 D 32 40 31 Relative atomic masses of some elements An instrument known as a mass spectrometer can be used to measure the mass of individual atoms. The mass of a hydrogen atom is 1.67 × 10−24 g and that of a carbon atom is 1.99 × 10−23 g. As the masses of all elements are in the range 10−24 to 10−22 g and these numbers are beyond our direct experience, it makes more sense to use relative 66 M02_CHSL_SB_IBD_9069_U02.indd 66 15/10/2014 16:31  values. The mass needs to be recorded relative to some agreed standard. As carbon is a very common element which is easy to transport and store because it is a solid, its isotope, 12C, was chosen as the standard in 1961. As discussed in Chapter 1 this is given a relative mass of exactly 12, as shown below. Element Symbol Relative atomic mass carbon C 12.011 The relative atomic mass chlorine Cl 35.453 of an element (Ar) is the average mass of an hydrogen H 1.008 atom of the element, taking into account all iron Fe 55.845 its isotopes and their Standard isotope Symbol Relative atomic mass relative abundance, compared to one atom 12 carbon-12 C 12.000 of carbon-12. Carbon-12 is the most abundant isotope of carbon but carbon-13 and carbon-14 also 80 exist. This explains why the average value for the element is greater than 12. 70 60 % abundance 50 Mass spectra 40 The results of the analysis by the mass spectrometer are presented in the form of a 30 mass spectrum. The horizontal axis shows the mass/charge ratio of the different ions 20 on the carbon-12 scale, which in most cases can be considered equivalent to their 10 mass. The percentage abundance of the ions is shown on the vertical axis. 0 69 70 71 The mass spectrum of gallium in Figure 2.6 shows that in a sample of 100 atoms, 60 mass/charge have a mass of 69 and 40 have a mass of 71. We can use this information to calculate the relative atomic mass of the element. Figure 2.6 Mass spectrum for gallium. The number of total mass of 100 atoms = (60 × 69) + (40 × 71) = 6980 lines indicates the number of isotopes (two in this case), the total mass 6980 value on the x-axis indicates relative average mass = = = 69.80 number of atoms 100 their mass number (69 and 71) and the y-axis shows the percentage abundance. Worked example Deduce the relative atomic mass of the 80 77 element rubidium from the data given in 70 Figure 2.7. 60 % abundance 50 Solution 40 Consider a sample of 100 atoms. 30 23 20 total mass of 100 atoms = (85 × 77) + 10 (87 × 23) = 8546 0 relative atomic mass = average mass of 85 86 87 88 total mass 8546 mass/charge atom = = = 85.46 number of atoms 100 Figure 2.7 Mass spectrum for rubidium. 67 M02_CHSL_SB_IBD_9069_U02.indd 67 15/10/2014 16:31  02 Atomic structure Worked example Boron exists in two isotopic forms, 10B and 11B. 10B is used as a control for nuclear reactors. Use your Periodic Table to find the abundances of the two isotopes. Solution Consider a sample of 100 atoms. Let x atoms be 10B atoms. The remaining atoms are 11B. number of 11B atoms = 100 − x total mass = x × 10 + (100 − x) × 11 = 10x + 1100 − 11x = 1100 − x total mass 1100 − x average mass = = number of atoms 100 From the Periodic Table, the relative atomic mass of boron = 10.81. 1100 − x 10.81 = 100 1081 = 1100 − x x = 1100 − 1081 = 19 The abundances are 10B = 19% and 11B = 81% Exercises 7 What is the same for an atom of phosphorus-26 and an atom of phosphorus-27? A atomic number and mass number B number of protons and electrons C number of neutrons and electrons D number of protons and neutrons 8 Use the Periodic Table to find the percentage abundance of neon-20, assuming that neon has only one other isotope, neon-22. 9 The relative abundances of the two isotopes of chlorine are shown in this table: Isotope Relative abundance 35 Cl 75% In 1911, a 40 kg meteorite 37 fell in Egypt. Isotopic Cl 25% and chemical analysis of Use this information to deduce the mass spectrum of chlorine gas, Cl2. oxygen extracted from this 10 Magnesium has three stable isotopes – 24Mg, 25Mg and 26Mg. The lightest isotope has an abundance meteorite show a different of 78.90%. Calculate the percentage abundance of the other isotopes. relative atomic mass to that of oxygen normally 11 The Geiger–Marsden experiment, supervised by Ernest Rutherford, gave important evidence for the found on Earth. This value structure of the atom. Positively charged alpha particles were fired at a piece of gold foil. Most of the matched measurements particles passed through with only minor deflections but a small number rebounded from the foil. made of the Martian How did this experiment change our knowledge of the atom? atmosphere by the Viking A It provided evidence for the existence of discrete atomic energy levels. landing in 1976, showing B It provided evidence for a positively charged dense nucleus. that the meteorite had C It provided evidence that electrons move in unpredictable paths around the nucleus. originated from Mars. D It provided evidence for the existence of an uncharged particle in the nucleus. 68 M02_CHSL_SB_IBD_9069_U02.indd 68 15/10/2014 16:31  2.2 Electron configuration Understandings: Emission spectra are produced when photons are emitted from atoms as excited electrons return to a lower energy level. (a) The line emission spectrum of hydrogen provides evidence for the existence of electrons in discrete energy levels, which converge at higher energies. Guidance The names of the different series in the hydrogen line spectrum are not required. The main energy level or shell is given an integer number, n, and can hold a maximum number of electrons, 2n2. A more detailed model of the atom describes the division of the main energy level into s, p, d and f sub-levels of successively higher energies. Sub-levels contain a fixed number of orbitals, regions of space where there is a high probability of finding an electron. Each orbital has a defined energy state for a given electronic configuration and chemical environment and can hold two electrons of opposite spin. (b) Applications and skills: Description of the relationship between colour, wavelength, frequency, and energy across the electromagnetic spectrum. Guidance Details of the electromagnetic spectrum are given in the IB data booklet in section 3. Distinction between a continuous spectrum and a line spectrum. Description of the emission spectrum of the hydrogen atom, including the relationships between the lines and energy transitions to the first, second, and third energy levels. Recognition of the shape of an s orbital and the p , p , and p atomic orbitals. x y z Application of the Aufbau principle, Hund’s rule, and the Pauli exclusion principle to write electron configurations for atoms and ions up to Z = 36. Guidance Full electron configurations (e.g. 1s22s22p63s23p4) and condensed electron configurations (e.g. [Ne] (c) 3s23p4) should be covered. Orbital diagrams should be used to represent the character and relative energy of orbitals. Orbital diagrams refer to arrow-in-box diagrams, such as the one given below. N: 1s 2s 2p The electron configurations of Cr and Cu as exceptions should be covered. Atoms of different elements give out light of a distinctive colour when an electric discharge is passed through a vapour of the element. Similarly, metals can be identified by the colour of the flame produced when their compounds are heated in a Bunsen burner. Analysis of the light emitted by different atoms has given us insights into the electron configurations within the atom. Flame tests on the compounds To interpret these results we must consider the nature of electromagnetic radiation. of (a) sodium, (b) potassium, and (c) copper. The electromagnetic spectrum Electromagnetic radiation comes in different forms of differing energy. The visible Flame colours can be light we need to see the world is only a small part of the full spectrum, which ranges used to identify unknown compounds. from low-energy radio waves to high-energy gamma rays. All electromagnetic waves 69 M02_CHSL_SB_IBD_9069_U02.indd 69 15/10/2014 16:31  02 Atomic structure All electromagnetic waves travel at the same speed (c) but can be distinguished by their different wavelengths (λ) travel at the same speed, (Figure 2.8). Different colours of visible light have different wavelengths; red light, for c = 3.00 × 108 m s−1. This example, has a longer wavelength than blue light. The full electromagnetic spectrum is is the cosmic speed limit given in section 3 of the IB data booklet. as, according to Einstein’s Theory of Relativity, wavelength (l) nothing in the universe can travel faster than this. crest crest Figure 2.8 Snapshot of a wave at a given instant. The trough distance between successive crests or peaks is called the wavelength (λ). The number of waves which pass a particular point in 1 s is called the frequency (ν); the shorter the wavelength, the higher the frequency. Blue light has a higher frequency The distance between than red light. two successive crests (or troughs) is called The wavelength and frequency are related by the equation: the wavelength (λ). The frequency (ν) c = νλ of the wave is the number of waves where c is the speed of light. which pass a point in 1s. The wavelength and White light is a mixture of light waves of differing wavelengths or colours. We see this frequency are related when sunlight passes through a prism to produce a continuous spectrum or as a by the equation c = νλ rainbow when light is scattered through water droplets in the air. where c is the speed of light. A continuous spectrum is produced when white light is passed through a prism. The different colours merge smoothly into one another. The two spectra below the illustration of the prism show, (top) a continuous spectrum with a series of discrete absorption lines and (bottom) a line emission spectrum. As well as visible light, atoms emit infrared radiation, which has a longer wavelength than red light, and ultraviolet radiation, with a shorter wavelength than violet light. The complete electromagnetic spectrum is shown in Figure 2.9. 70 M02_CHSL_SB_IBD_9069_U02.indd 70 15/10/2014 16:31  Figure 2.9 The changing wavelength (in m) of 10216 10214 10212 10210 1028 1026 1024 1022 100 102 104 106 108 wavelength/m electromagnetic radiation through the spectrum is shown by the trace across the g rays X rays UV IR microwaves radio waves top. At the short wavelength end (on the left) of the spectrum are γ rays, X rays and ultraviolet light. In the centre of the spectrum are wavelengths that the human eye can see, known as visible 4 3 1027 5 3 1027 6 3 1027 7 3 1027 wavelength/m light. Visible light comprises light of different wavelengths, energies, and colours. At the energy increasing longer wavelength end of the spectrum (on the right) are infrared radiation, microwaves, and radio waves. The visible Atomic absorption and emission line spectra spectrum gives us only a small window to see the world. When electromagnetic radiation is passed through a collection of atoms some of the radiation is absorbed and used to excite the atoms from a lower energy level to a higher energy level. The spectrometer analyses the transmitted radiation relative to the incident radiation and an absorption spectrum is produced. sample in low Figure 2.10 The origin of energy level absorption and emission absorption spectrum spectra. An absorption spectrum shows the radiation absorbed as atoms move from a lower to a higher energy sample in high level. An emission spectrum energy level emission spectrum is produced when an atom moves from a higher to a lower level. When white light is passed through hydrogen gas, an absorption line spectrum is produced with some colours of the continuous spectrum missing. If a high voltage is applied to the gas, a corresponding emission line spectrum is produced. Investigating flame tests Full details of how to carry out this experiment with a worksheet are available on your eBook. Visible emission spectrum of hydrogen. These lines form the Balmer series and you should note that they converge at higher energies. Similar series are found in the ultraviolet region – the Lyman Series – and in the infrared region – the Paschen series. The colours present in the emission spectrum are the same as those that are missing Emission spectra could be from the absorption spectra. As different elements have different line spectra they observed using discharge can be used like bar codes to identify unknown elements. They give us valuable tubes of different gases information about the electron configurations of different atoms. and a spectroscope. 71 M02_CHSL_SB_IBD_9069_U02.indd 71 15/10/2014 16:31  02 Atomic structure Electromagnetic waves The element helium was discovered in the Sun before it was found on Earth. Some allow energy to be unexpected spectral lines were observed when the absorption spectra of sunlight was transferred across the analysed. These lines did not correspond to any known element. The new element was universe. They also carry named after the Greek word helios, which means ‘Sun’. Emission and absorption spectra information. Low-energy can be used like barcodes to identify the different elements. radio waves are used in radar and television, for example, and the higher Evidence for the Bohr model energy gamma rays are used as medical tracers. How can a hydrogen atom absorb and emit energy? A simple picture of the atom The precision with which was considered earlier with the electron orbiting the nucleus in a circular energy we view the world is level. Niels Bohr proposed that an electron moves into an orbit or higher energy level limited by the wavelengths further from the nucleus when an atom absorbs energy. The excited state produced of the colours we can see. This is why we will never is, however, unstable and the electron soon falls back to the lowest level or ground be able to see an atom state. The energy the electron gives out when it falls into lower levels is in the form directly; it is too small to of electromagnetic radiation. One packet of energy (quantum) or photon, is released interact with the relatively for each electron transition (Figure 2.11). Photons of ultraviolet light have more long waves of visible light. energy than photons of infrared light. The energy of the photon is proportional to the What are the implications of this for human frequency of the radiation. knowledge? excited state The elemental composition hν hν of stars can be determined by analysing their absorption spectra. This is discussed in more detail in Chapter 14. ground state Figure 2.11 Emission and When an atom falls from an When an atom moves from excited state to the ground the ground state to an excited absorption spectra are the state, light of a specific state, light of a specific result of an energy transition frequency n is emitted. frequency n is absorbed. between the ground and excited states. The energy of the photon of light emitted is equal to the energy change in the atom: ∆Eelectron = Ephoton A continuous It is also related to the frequency of the radiation by the Planck equation: spectrum shows an Ephoton = hν unbroken sequence of frequencies, such This equation and the value of h (the Planck constant) are given in sections 1 and 2 of as the spectrum of the IB data booklet.) visible light. A line emission This leads to: spectrum has only certain frequencies of ∆Eelectron = hν light as it is produced by excited atoms and This is a very significant equation as it shows that line spectra allow us to glimpse the ions as they fall back inside of the atom. The atoms emit photons of certain energies which give lines of to a lower energy certain frequencies, because the electron can only occupy certain orbits. The energy level. levels can be thought of as a staircase. The electron cannot change its energy in a A line absorption continuous way, in the same way that you cannot stand between steps; it can only spectrum is a change its energy by discrete amounts. This energy of the atom is quantized. The line continuous spectrum except for certain spectrum is crucial evidence for quantization: if the energy were not quantized, the colours which are emission spectrum would be continuous. absorbed as the atoms are excited to higher energy levels. 72 M02_CHSL_SB_IBD_9069_U02.indd 72 15/10/2014 16:31  NATURE OF SCIENCE When asked to distinguish between The idea that electromagnetic waves can be thought of as a stream of photons or quanta is one a line spectrum and a aspect of quantum theory. The theory has implications for human knowledge and technology. continuous spectrum, The key idea is that energy can only be transferred in discrete amounts or quanta. Quantum references should be theory shows us that our everyday experience cannot be transferred to the microscopic world made to discrete or of the atom and has led to great technological breakthroughs such as the modern computer. continuous energy It has been estimated that 30% of the gross national product of the USA depends on the levels and all or specific applications of quantum theory. In the modern world our scientific understanding has led to colours, wavelengths, or many technological developments. These new technologies in their turn drive developments in frequencies. science. The implications of the quantum theory for the electron are discussed in more detail later (page 74). Note that ‘discrete’ has a different meaning to ‘discreet’. The amount of light absorbed at particular frequency depends on the identity and concentration of atoms present. Atomic absorption spectroscopy is used to measure the concentration of metallic elements. Figure 2.12 When an electron The hydrogen spectrum n=6 is excited from a lower to a n=5 higher energy level, energy The hydrogen atom gives out energy n=4 is absorbed and a line in when an electron falls from a higher n=3 the absorption spectrum is to a lower energy level. Hydrogen n=2 produced. When an electron falls from a higher to a lower produces visible light when the n=1 energy level, radiation is given electron falls to the second energy atom out by the atom and a line level (n = 2). The transitions to the first absorbs in the emission spectrum is energy level (n = 1) correspond to a energy produced. higher energy change and are in the The energy of a photon ultraviolet region of the spectrum. of electromagnetic Infrared radiation is produced when radiation is directly an electron falls to the third or higher proportional to its energy levels (Figure 2.12). atom emits energy frequency and inversely proportional to its The pattern of the lines in Figure 2.13 wavelength. It can be gives us a picture of the energy levels in the atom. The lines converge at higher calculated from the Planck equation (E = hν), energies because the energy levels inside the atoms are closer together at high energy. which is given in section When an electron is at the highest energy n = ∞, it is no longer in the atom and the 1 of the IB data booklet. atom has been ionized. The energy needed to remove an electron from the ground state of each atom in a mole of The first ionization gaseous atoms, ions, or 8 ∞ energy of an element 7 6 is the minimum energy molecules is called the 5 4 excited needed to remove one ionization energy. 3 Paschen states mole of electrons from Ionization energies can also series one mole of gaseous be used to support this model 2 atoms in their ground Balmer series state. of the atom. Figure 2.13 Energy levels of the hydrogen atom showing the transitions which produce UV Lyman the Lyman, Balmer, and series Paschen series. The transition 1 → ∞ corresponds to ionization: H(g) → H+(g) +e− 1 ground This is discussed in more detail state later. 73 M02_CHSL_SB_IBD_9069_U02.indd 73 15/10/2014 16:31