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D − Electrons fill shells and subshells of lowest energy first. The order of filling is: 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d • The ground state electron configuration of an atom refers to electrons in their lowest energy level. Any other configurations represent the atom in an excited state and in a higher energy level. 5. Multiple choice questions 6. 1. Which of the following is incorrect? A Ernest Rutherford was famous for his ‘gold-foil’ experiment. B The cathode-ray tube was the basis of much experimental evidence for the initial development of modern atomic theory. C Pierre and Marie Curie were the first scientists to discover radioactivity. D The idea of atoms was proposed as far back as 400 BC by Democritus. 2. Which of the following ideas of John Dalton’s atomic theory is no longer true? A All matter is composed of tiny indestructible particles called atoms. B Atoms of the same element are alike in every way. C Atoms of different elements are different. D Atoms can combine together in small numbers to form molecules. 3. J. J. Thomson’s basis for the ‘plum-pudding’ model for atomic structure was that: A he discovered electrons by investigating cathode rays in a specially designed cathoderay tube B he found that cathode rays travelled towards the positively charged anode, so the particles in the rays must be negatively charged C he measured the amount of deflection of cathode rays attracted by a positively charged plate D he postulated that an atom consists of a core of positive charges surrounded by negatively charged electrons. 4. Ernest Rutherford discovered a nucleus in atoms. What experimental evidence did he have? A Alpha particles were bombarded at the atoms of a thin gold foil. B Most of the alpha particles passed directly through the foil because gold atoms were mainly made of empty space. C A small number of alpha particles bounced back because they hit the centre of the atom which was made up of a core of negatively charged particles. 48 UNIT 1 The big ideas of chemistry 7. 8. 9. 10. He proposed the nuclear model of the atom, which suggested that the atom has a small central core of protons surrounded by electrons orbiting around it. Which of the following particle pairs has approximately the same mass? A a proton and an electron B a proton and a neutron C a neutron and an electron D an electron and a hydrogen atom Three atoms, I, II and III, each have an atomic number of 12. Atom I has 12 neutrons, atom II has 13 neutrons and atom III has 14 neutrons. Which of the following sentences is correct? A Atoms I, II and III are allotropes of each other. B Atoms I, II and III are isotopes of the same element. C Atom I is a neutral atom while atoms II and III are cations of atom I. D Atom I is a neutral atom while atoms II and III are anions of atom I. A neutral atom of the isotope 136 C would consist of: A 6 protons, 13 neutrons, 13 electrons B 0 protons, 13 neutrons, 13 electrons C 13 protons, 7 neutrons, 13 electrons D 6 protons, 7 neutrons, 6 electrons. Which of the following features is not present in a mass spectrometer? A The gas sample is bombarded by alpha particles. B The gas sample is ionised into positively charged particles. C The positive ions are accelerated by an electric field. D A magnetic field forces the particles to separate in curved paths according to their mass-tocharge ratio. Which of the following statements regarding subatomic particles is correct? A Protons are positively charged particles and neutrons are negatively charged. B The relative masses of an electron, a proton and a neutron are all about 1 unit. C In a neutral atom, the number of neutrons is equal to the number of protons. D Isotopes of an element have the same number of protons but a different number of neutrons. Bohr’s theory of the atom proposed that: A electrons orbit the nucleus like planets move around the sun B no more than two electrons are allowed in any energy level C electrons move around the nucleus in fixed orbits, each of which has a different energy level D energy shells have subshells which contain regions of space called orbitals. 11. The maximum number of electrons that can be placed in the shell n = 3 is: A 18 C 2 B 8 D 32. 12. Which of these electron configurations represents an atom in an excited energy state? A 1s22s22p43s2 B 1s22s22p63s23p4 C 1s22s22p6 D 1s22s22p63s23p63d34s2 35 13. The electron configuration of 17 C1– is: 2 2 6 2 5 1 A 1s 2s 2p 3s 3p 3d B 1s22s22p63s23p44s2 C 1s22s22p63s23p6 D 1s22s22p63s23p43d14s1. 14. The electron configuration 1s22s22p63s23p6 represents which of the following ions? A O2– C Al3+ 2– B S D Na+ 15. The electron configuration of an atom X is 1s22s22p63s23p1. Which of the following formulae is most likely to be a compound formed with X? A XF2 B CaX C XCl3 D MgX2 16. The ground state electron configuration of a neutral atom with an atomic number of 19 is: A 1s22s22p63s23p63d1 B 1s22s22p63s23p7 C 1s22s22p63s23p64s1 D 1s22s22p63s23p53d14s1. Review questions 1. (a) What did the alchemists contribute to the growth of chemistry? (b) Would you consider the alchemists to be scientists? Justify your answer. 2. Examine the points made by Dalton in his atomic theory. Consider each statement, and record: (a) whether or not each suggestion still holds (b) which other scientists were able to contribute to refining or changing each proposal, and in what way (c) our current understanding of each statement. 3. From the results of Rutherford’s ‘gold-foil’ experiment, he suggested that the protons of the atoms in the metal must be concentrated in the centre, or nucleus, of the atom, while the electrons are outside the nucleus. If this experiment were to be repeated, what results would have been obtained if: (a) Thomson’s ‘plum-pudding’ model was correct (b) the electrons were concentrated in the nucleus and the protons orbited the nucleus? 4. Complete the following table of the structural properties of atoms. Atomic number Element (Z ) Atomic Number of: mass Protons Neutrons Electrons (A) 1 nickel 59 gold 28 197 osmium 76 190 silver 47 107 silicon mercury 0 79 6 8 19 20 16 80 14 120 5. Oxygen consists of three isotopes: 16O, 17O and 18 O. Show how many different masses an oxygen molecule, O2, can have. 6. Naturally-occurring chromium consists of the following four isotopes: 4.31% 50Cr (relative isotopic mass 49.946) 83.76% 52Cr (relative isotopic mass 51.941) 9.55% 53Cr (relative isotopic mass 52.941) 2.38% 54Cr (relative isotopic mass 53.939). (a) Calculate the relative atomic mass of chromium. (b) Explain the difference between ‘relative atomic mass’ and ‘mass number’, selecting appropriate data from the above list. 7. A sample of carbon is found to contain two isotopes with relative isotopic masses of 12.00 and 13.00. If the relative atomic mass of carbon is 12.01, calculate the relative abundances of each of the isotopes. 8. Suppose that you are a research chemist and you have just discovered a new element. How would you identify its different isotopes and determine its relative atomic mass? 9. Explain the difference between: (a) a ‘shell’ and a ‘subshell’ (b) an atomic ‘orbit’ and an ‘orbital’. 10. What is the maximum number of electrons that may be found in: (a) a 3p subshell (b) a 2s subshell (c) a 4d subshell (d) the third shell? 11. Write the electron configuration of each of the following in their ground states: (a) sodium atom (b) nitrogen atom (c) argon atom (d) iron atom (e) copper atom (f) calcium ion (g) chloride ion. CHAPTER 2 The atomic theory of matter 49