Atoms and elements. Checkup. Checkups and follow-ups. 1 WHAT IS AN ATOM? (pp. 6 17) EST Questions 1 38 and A C

Transcription

1 EST Questions 1 38 and A C Checkup 1 WHAT IS AN ATOM? (pp. 6 17) 1. Why is it useful to represent an atom with a model? Because an atom is too small to be examined directly. 2. Look at the two photos below. A B a) Which photo represents a model in which matter can be infinitely divided? Explain your answer. The photo of the glass of water, because the water gives an impression of uniformity. b) Which photo represents a model in which matter cannot be infinitely divided? Explain your answer. The photo of the dish of sand, because the grains of sand represent atoms. Matter can be divided into small indivisible particles. 3. Here is a representation of a molecule of water (H 2 O). Does it respect the principles behind Dalton s atomic model? Explain your answer. No, this figure does not represent a water molecule correctly according to Dalton s atomic model. Dalton believed that the atoms of different elements have different masses and dimensions. The hydrogen atoms should therefore look different from the oxygen atom. 1

2 4. What conclusion did Thomson draw from each of the following observations? a) The cathode rays cause a small propeller inside the tube to turn. The rays are made up of particles of matter. b) The rays are identical regardless of the metal used to make the cathode. The rays are common to all elements. c) The cathode rays are attracted to the positive pole of an electrical field. The cathode rays are negatively charged. 5. What type of radiation did Rutherford use for his research on atomic structure? Alpha radiation 6. Look at the illustration below. Each letter corresponds to an observation made by Rutherford. Describe each observation and match it with one of Rutherford s conclusions. A C B A: These particles are not deflected. Rutherford concluded that most of an atom is empty space. B: These particles are strongly deflected. Rutherford concluded that the atom contains a very small, dense nucleus. C: These particles bounce back. Rutherford concluded that the nucleus of the atom is positive. 7. Which positively charged particle is found in the nucleus? The proton 8. The bright colours of fireworks amaze and delight us. Each colour is the result of the combustion of a specific substance. For example, potassium chloride produces a purple flame, and sodium chloride, an orange-yellow flame. Which theory, proposed by Bohr, explains this phenomenon? When an atom receives energy, its electrons become excited. They can then jump to electron shells farther from the nucleus (higher levels of energy) for a short period of time. When they return to their original shells, they release the surplus energy they had accumulated, emitting light of a specific colour. 2

3 9. Match a characteristic of the Rutherford-Bohr atomic model with each of the following statements. a) An atom contains as many protons as electrons. An atom is electrically neutral. b) Alpha particles pass easily through gold foil. An atom is mostly empty space. c) Alpha particles are strongly repelled by the nucleus. The nucleus of an atom is small, dense and positively charged. d) A heated gas emits light waves of a specific length. Electrons jump to a higher level of energy when they receive energy and then release the energy in the form of light. 10. What is the neutron s function in the atom? It holds the protons together in the nucleus. 11. Which particles of the atom account for most of its mass? The proton and the neutron 12. Which is the lightest particle of the atom? The electron 2 THE PERIODIC CLASSIFICATION OF THE ELEMENTS (pp ) 13. Where are the nonmetals in the periodic table? They are all located to the right of the staircase, except hydrogen. 14. What do elements of the same group have in common? They all have the same number of valence electrons, so they have similar chemical properties. 15. Hydrogen is a nonmetal, but it is in the same column as the alkali metals. Explain your answer. Because hydrogen has only one valence electron, like the alkali metals. 16. What are the elements in the second column of the periodic table called? What do they have in common? Alkaline earth metals. They all have two valence electrons. 3

4 17. In the atomic model of potassium below, which letter represents a valence electron? Explain your answer. The letter D, because this electron is in the outermost electron shell. 18. What do elements of the same period have in common? They have the same number of electron shells in their atoms. 19. The atomic radius is a periodic property of the elements. Explain how the atomic radius varies within a period. The attraction between the protons and the electrons increases within a period, causing the atom to shrink, so the atomic radius decreases. 20. How does the melting point vary within a period? Draw a graph showing the melting point as a function of the atomic number to explain your answer. Refer to the data in Appendix 1 and draw your graph on the sheet of graph paper at the end of this handout. The melting point increases gradually at the beginning of the period. It then falls abruptly for the nonmetals. 21. The table in Appendix 1 shows the first ionization energy of the elements of the periodic table. a) How does this property vary within a period? The first ionization energy increases within a period. b) How does this property vary within a group? The first ionization energy decreases within a group. 4

5 22. What does the atomic number of an element represent? It represents the number of protons in the nucleus of an atom. 23. What is the relative atomic mass of an element? It is the mass of an atom measured by comparison with a reference element, carbon An atom of argon contains 18 protons, and its mass number is 40. How many neutrons does it contain? 22 neutrons (40 18 = 22) 25. An atom has 25 protons, 25 electrons and 27 neutrons. What is its mass in atomic mass units? Round your answer to the nearest whole number and explain your calculation. Its atomic mass is approximately 52 u. Only protons and neutrons are included in the calculation of the mass. Electrons, which are much lighter than protons and neutrons, can be left out of the calculation. 26. Among the elements below, which are isotopes of the same element? Explain your answer B, C 23 and G are isotopes of the same element because they have the same atomic number but different mass numbers. 27. Which of these representations is impossible? Explain your answer A is impossible because the mass number (12) should be greater than the atomic number (24). 5

6 3 REPRESENTING ATOMS (pp ) 28. How is Lewis notation useful? Lewis notation illustrates only the valence electrons of an atom. 29. Are the atomic representations below correct Lewis structures? If not, explain explain your answer. a) Magnesium It is illustrated correctly. b) Argon It is not illustrated correctly because argon has eight valence electrons, not nine. c) It is illustrated correctly. Chlorine 30. Represent the following atoms in Lewis notation. a) b) c) Aluminum Iodine Barium 31. Are the atomic representations below correct according to the Rutherford-Bohr model? If not, explain your answer. a) b) Calcium Yes, it is correct. No, it does not follow the model. The protons should be in the nucleus, and the electrons should be in the shells around the nucleus. Neon 6

7 c) No, it does not follow the model. An atom must have the same number of protons and electrons. Chlorine d) No, it does not follow the model. An atom must have the same number of protons and electrons. Lithium 32. Are the following representations correct according to the simplified atomic model? If not, explain your answer. a) No, it does not follow the model. In the simplified atomic model, the number of neutrons in the nucleus should be indicated. Sodium b) Yes, it is correct. Sulphur c) d) Aluminum Helium No, it does not follow the model. The neutrons should be in the nucleus, and the electrons, in the arcs around the nucleus. No, it does not follow the model. The atom should contain as many electrons as protons. 7

8 33. Draw an atom of fluorine according to the simplified atomic model. 4 THE CONCEPT OF MOLE (pp ) 34. How many moles are there in each of the following samples of matter? a) 12 g of carbon 1 mol c) 80 g of argon 2 mol b) 20 g of calcium 0.5 mol d) 10 g of sodium 0.43 mol 35. What is the molar mass of each of the following molecules? a) LiBr 6.94 g/mol g/mol = g/mol b) CO g/mol + (16.00 g/mol 2) = g/mol c) H 2 SO 4 (1.01 g/mol 2) g/mol + (16.00 g/mol 4) = g/mol d) Mg(NO 3 ) g/mol + (14.01 g/mol g/mol 3) 2 = g/mol 36. A 0.25-mol sample of a certain substance has a mass of 10 g. a) What is the molar mass of this substance? m 10 g M = = N 0.25 mol = 40 g/mol b) Which element has the same molar mass? Argon 37. What is the mass, in grams, of atoms of gold? atoms of gold represent one mole and therefore a mass of g. 38. How many particles are there in a sample of matter that contains three moles of particles? = particles 8

9 REVIEW QUESTIONS A. The chemical reactivity of elements depends on their ability to give up their valence electrons. The most reactive metal in the periodic table is francium. Explain why, using the Rutherford- Bohr atomic model. Like all alkali metals, francium has only one valence electron. It therefore has a natural tendency to give up this electron to resemble the closest noble gas in the periodic table, namely, radon. In addition, francium has seven electron shells. As a result, its single valence electron is far from the nucleus and more loosely bound to it than the valence electrons of other alkali metals are bound to their nuclei. Consequently, francium reacts easily with other elements. B. Different models of the atom have been proposed over time. The changes from one model to the next reflect recent experimental findings. In certain contexts, however, an older model may still apply. From this point of view, which model Dalton s model, Thomson s model, the Rutherford-Bohr model or the simplified atomic model is the most relevant to an explanation of each of the following phenomena? a) Cathode ray screens depend on the emission of rays from the cathode of a gas discharge tube. Thomson s atomic model b) When magnesium burns, it emits a bright light. For this reason, it was previously used in camera flashes. The Rutherford-Bohr atomic model c) In a nuclear reaction, neutrons are frequently emitted. The simplified atomic model d) If you rub a balloon on your head, it gives off electrons (onto your hair), creating static electricity. Thomson s atomic model e) Iron combines with oxygen in a ratio of 2:3 to form rust (Fe 2 O 3 ). Dalton s atomic model f) Atoms are mostly empty space. The Rutherford-Bohr atomic model 9

10 g) The atomic mass of helium is 4 u. The simplified atomic model C. Prepare your own summary of Chapter 1 by building a concept map. See the Concept maps section in Guide B. Follow-up 1. The Amazon is not the only place affected by mercury contamination. Relatively high concentrations can also be found in Québec landfills. Name three commonplace objects that you use that might contain mercury. Explain your answers. Answers will vary. Examples: compact fluorescent light bulbs, because the symbol Hg appears on either the packaging or the bulb batteries, for the reasons discussed in the textbook, on page 20 old mercury thermometers. 2. Mercury is a heavy metal. Using atomic models and the periodic table of the elements, explain why it belongs to this category of elements. Answers will vary. Example: The atomic mass of mercury is u. Its atomic mass is therefore relatively high compared to that of gold, for example. Moreover, its atomic number is 80, and it belongs to the sixth period, which means it has a nucleus containing 80 protons, surrounded by six electron shells. 10

11 11 Checkup and follow-ups