Chapter 2: Atoms, Molecules, and Ions

Chapter 2 Practice

2.1 Early Ideas in Atomic Theory [Go to section 2.1]

  1. Which of the following pairs of compounds can be used to illustrate the law of multiple proportions?
    1. [latex]\ce{NH4}[/latex] and [latex]\ce{NH4Cl}[/latex]
    2. [latex]\ce{ZnO2}[/latex] and [latex]\ce{ZnCl2}[/latex]
    3. [latex]\ce{H2O}[/latex] and [latex]\ce{HCl}[/latex]
    4. [latex]\ce{NO}[/latex] and [latex]\ce{NO2}[/latex]
    5. [latex]\ce{CH4}[/latex] and [latex]\ce{CO2}[/latex]
  2. A sample of chemical X is found to contain 5.0 grams of oxygen, 10.0 grams of carbon, and 20.0 grams of nitrogen. The law of definite proportion would predict that a 70 gram sample of chemical X should contain how many grams of carbon?
  3. How many of the following postulates of Dalton’s atomic theory are still scientifically accepted?
    1. All atoms of the same element are identical.
    2. Compounds are combinations of different atoms.
    3. A chemical reaction changes the way atoms are grouped together.
    4. Atoms are indestructible.
Show Selected Solutions
  1. D
  2. I, II, and III


2.2 Evolution of Atomic Theory [Go to section 2.2]

  1. Predict and test the behavior of [latex]\alpha[/latex] particles fired at a “plum pudding” model atom.
    1. Predict the paths taken by [latex]\alpha[/latex] particles that are fired at atoms with a Thomson’s plum pudding model structure. Explain why you expect the [latex]\alpha[/latex] particles to take these paths.
    2. If [latex]\alpha[/latex] particles of higher energy than those in (a) are fired at plum pudding atoms, predict how their paths will differ from the lower-energy [latex]\alpha[/latex] particle paths. Explain your reasoning.
    3. Now test your predictions from (a) and (b). Open the Rutherford Scattering simulation and select the “Plum Pudding Atom” tab. Set “Alpha Particles Energy” to “min,” and select “show traces.” Click on the gun to start firing [latex]\alpha[/latex] particles. Does this match your prediction from (a)? If not, explain why the actual path would be that shown in the simulation. Hit the pause button, or “Reset All.” Set “Alpha Particles Energy” to “max,” and start firing [latex]\alpha[/latex] particles. Does this match your prediction from (b)? If not, explain the effect of increased energy on the actual paths as shown in the simulation.
  2. Predict and test the behavior of [latex]\alpha[/latex] particles fired at a Rutherford atom model.
    1. Predict the paths taken by [latex]\alpha[/latex] particles that are fired at atoms with a Rutherford atom model structure. Explain why you expect the [latex]\alpha[/latex] particles to take these paths.
    2. If [latex]\alpha[/latex] particles of higher energy than those in (a) are fired at Rutherford atoms, predict how their paths will differ from the lower-energy [latex]\alpha[/latex] particle paths. Explain your reasoning.
    3. Predict how the paths taken by the [latex]\alpha[/latex] particles will differ if they are fired at Rutherford atoms of elements other than gold. What factor do you expect to cause this difference in paths, and why?
    4. Now test your predictions from (a), (b), and (c). Open the Rutherford Scattering simulation and select the “Rutherford Atom” tab. Due to the scale of the simulation, it is best to start with a small nucleus, so select “20” for both protons and neutrons, “min” for energy, show traces, and then start firing [latex]\alpha[/latex] particles. Does this match your prediction from (a)? If not, explain why the actual path would be that shown in the simulation. Pause or reset, set energy to “max,” and start firing [latex]\alpha[/latex] particles. Does this match your prediction from (b)? If not, explain the effect of increased energy on the actual path as shown in the simulation. Pause or reset, select “40” for both protons and neutrons, “min” for energy, show traces, and fire away. Does this match your prediction from (c)? If not, explain why the actual path would be that shown in the simulation. Repeat this with larger numbers of protons and neutrons. What generalization can you make regarding the type of atom and effect on the path of [latex]\alpha[/latex] particles? Be clear and specific.
  3. Who was the first scientist to show that atoms emit any negative particles?
  4. Several old experiments detailed the structure and nature of the atom indirectly. Which of the following experiments listed below did not give the results described?
    1. The Rutherford experiment proved the Thomson “plum-pudding” model of the atom to be essentially correct.
    2. The Rutherford experiment was useful in determining the nuclear charge on the atom.
    3. Millikan’s oil-drop experiment showed that the charge on any particle was a simple multiple of the charge on the electron.
    4. The electric discharge tube proved that electrons have a negative charge.
    5. All of the above experiments gave the results described.
Show Selected Solutions
  1. The Rutherford atom has a small, positively charged nucleus, so most a particles will pass through empty space far from the nucleus and be undeflected. Those a particles that pass near the nucleus will be deflected from their paths due to positive-positive repulsion. The more directly toward the nucleus the a particles are headed, the larger the deflection angle will be. (b) Higher-energy a particles that pass near the nucleus will still undergo deflection, but the faster they travel, the less the expected angle of deflection. (c) If the nucleus is smaller, the positive charge is smaller and the expected deflections are smaller—both in terms of how closely the a particles pass by the nucleus undeflected and the angle of deflection. If the nucleus is larger, the positive charge is larger and the expected deflections are larger—more a particles will be deflected, and the deflection angles will be larger. (d) The paths followed by the a particles match the predictions from (a), (b), and (c).  
  2. A, B, C


2.3 Atomic Structure and Symbolism [Go to section 2.3]

  1. Determine the number of protons, neutrons, and electrons in the following isotopes that are used in medical diagnoses:
    1. atomic number 9, mass number 18, charge of 1–
    2. atomic number 43, mass number 99, charge of 7+
    3. atomic number 53, atomic mass number 131, charge of 1–
    4. atomic number 81, atomic mass number 201, charge of 1+
    5. Name the elements in parts (a), (b), (c), and (d).
  2. K-40 is an ion essential to our diet. Determine the number of protons, neutrons and electrons in this isotope and name them.
  3. An element has the following natural abundances and isotopic masses: 90.92% abundance with 19.99 amu, 0.26% abundance with 20.99 amu, and 8.82% abundance with 21.99 amu. Calculate the average atomic mass of this element.
  4. List the number of protons, neutrons and electrons for each atom below.
    1. [latex]\ce{Te}[/latex]-125
    2. [latex]\ce{Na}[/latex]-23
    3. [latex]\ce{N}[/latex]-15
    4. [latex]\ce{Ag}[/latex]-109
    5. [latex]\ce{S}[/latex]-32
  5. Variations in average atomic mass may be observed for elements obtained from different sources. Lithium provides an example of this. The isotopic composition of lithium from naturally occurring minerals is 7.5% [latex]\ce{6Li}[/latex] and 92.5% [latex]\ce{7Li}[/latex], which have masses of 6.01512 amu and 7.01600 amu, respectively. A commercial source of lithium, recycled from a military source, was 3.75% [latex]\ce{6Li}[/latex] (and the rest [latex]\ce{7Li}[/latex]). Calculate the average atomic mass values for each of these two sources.
  6. Knowing that silver has two naturally occurring isotopes [latex]\ce{Ag}[/latex]-107 and [latex]\ce{Ag}[/latex]-109. The masses and their respective abundances are 106.905 amu at 51.82% and 108.905 amu at 48.18%. What is the average atomic mass for silver you would see on the periodic table?
  7. The 18O:16O abundance ratio in some meteorites is greater than that used to calculate the average atomic mass of oxygen on earth. Is the average mass of an oxygen atom in these meteorites greater than, less than, or equal to that of a terrestrial oxygen atom? 
  8. Forensic Geology is the practice of using elemental differences from one part of the world or one part of a country compared to another. Isotope ratios for some elements will be different in one region vs another and this allows Forensic Scientists to determine the birth country of a victim or the region of manufacture for a bullet, etc. The average atomic masses of some elements may vary, depending upon the sources of their ores. Vanadium has two common isotopes ([latex]\ce{V}[/latex]-51, 50.94396 amu and [latex]\ce{V}[/latex]-50, 49.94716 amu). The reported atomic mass of vanandium can vary from 50.942 to 50.939, depending on whether the mineral source is from Russia or Canada. Calculate the percent abundances leading to the two values of the average atomic masses of vanadium from these two countries.
  9. Naturally occurring element X exists in three isotopic forms: X-28 (27.977 amu, 92.23% abundance), X-29 (28.976 amu, 4.67% abundance), and X-30 (29.974 amu, 3.10% abundance). Calculate the atomic weight of X.
  10. Knowing that 37.1% of naturally occurring rhenium is [latex]\ce{Re}[/latex]-185 and the atomic mass of rhenium is 186.2 amu, then what is the percent occurrence of the other isotope and what is the other stable isotope?
  11. Which one of the following statements about atomic structure is false?
    1. An atom is mostly empty space.
    2. Almost all of the mass of the atom is concentrated in the nucleus.
    3. The protons and neutrons in the nucleus are very tightly packed.
    4. The number of protons and neutrons is always the same in the neutral atom.
    5. All of the above statements (A–D) are true.
  12. The average atomic mass of nitrogen atom is 14.011 amu. If you had the ability to pick up just a single nitrogen atom, what are the odds that you would select one with an exact mass of 14.011 amu?
  13. The element rhenium [latex]\ce{(Re)}[/latex] exists as two stable isotopes and 18 unstable isotopes. What does Rhenium-185 have in its nucleus?
  14. Which of the following atomic symbols is incorrect and why?
    1. [latex]\ce{^{14}_7C}[/latex]
    2. [latex]\ce{^{14}_7N}[/latex]
    3. [latex]\ce{^7_3Li}[/latex]
    4. [latex]\ce{^{72}_{31}Ge}[/latex]
  15. How many protons, neutrons, and electrons does [latex]\ce{_{20}^{40}Ca^2+}[/latex] have?
  16. Of the following 5 atoms which would represent a set of isotopes? Atomic nuclei containing:
    1. 20 protons and 20 neutrons
    2. 21 protons and 19 neutrons
    3. 18 protons and 22 neutrons
    4. 20 protons and 22 neutrons
    5. 22 protons and 20 neutrons
  17. How many protons, neutrons, and electrons are in [latex]\ce{_{19}^{39}K+}[/latex]
  18. Which of the following is a species with 12 protons and 10 electrons?
    1. [latex]\ce{Ne^{2+}}[/latex]
    2. [latex]\ce{Ti{2+}}[/latex]
    3.  [latex]\ce{Mg^2+}[/latex]
    4. [latex]\ce{Mg}[/latex]
    5. [latex]\ce{Ne^2-}[/latex]
  19. What does the formula of water, [latex]\ce{H2O}[/latex], suggest?
    1. There is twice as much mass of hydrogen as oxygen in each molecule.
    2. There are two hydrogen atoms and one oxygen atom per water molecule.
    3. There is twice as much mass of oxygen as hydrogen in each molecule.
    4. There are two oxygen atoms and one hydrogen atom per water molecule.
    5. None of these.
Show Selected Solutions
  1. potassium 19 protons, 18 electrons, and 21 neutrons
  2. The answers are as follows:
    1. 52 protons, 73 neutrons, 52 electrons
    2. 11 protons, 12 neutrons, 11 electrons
    3. 7 protons, 8 neutrons, 7 electrons
    4. 47 protons, 62 neutrons, 47 electrons
    5. 16 protons, 16 neutrons, 16 electrons
  3. 107.8686 amu
  4. Two items, the percentage of each isotope, are unknown. As both unknowns are related through an equation that says that the sum of the two fractions is equal to 1, we can write: Russian source:  99.803 % [latex]\ce{V}[/latex]-51 and 0.197 % [latex]\ce{V}[/latex]-50 Canadian source:  99.502 % [latex]\ce{V}[/latex]-51 and 0.498 % [latex]\ce{V}[/latex]-50
  5. 62.9% 186.9 amu, [latex]\ce{Re}[/latex]-187
  6. The odds are 0, or there is a 0% chance you could select a nitrogen atom with a mass of exactly 14.011 amu since that mass is a weighted average of all the nitrogen isotopes.
  7. Choices a and d are incorrect. Carbon can not have 7 protons, only 6 and germanium can not have 31 protons, all germanium atoms must have 32 protons.
  8. Answers a and d are isotopes because they contain the same number of protons but differing numbers of neutrons.
  9. c. Magnesium with a +2 charge. All magnesium atoms/ions have 12 protons, if the ion has a +2 charge that means it must have lost 2 electrons. So it would have 10 electron compared to the 12 protons it has.


2.4 The Periodic Table [Go to section 2.4]

  1. The Periodic Table, Try-it Metal or non-metal #2) Using the periodic table, classify each of the following elements as a metal or a nonmetal, and then further classify each as a main-group (representative) element, transition metal, or inner transition metal:
    1. cobalt
    2. europium
    3. iodine
    4. indium
    5. lithium
    6. oxygen
    7. cadmium
    8. terbium
    9. Rhenium
  2. Using the periodic table, identify the heaviest member of each of the following groups:
    1. alkali metals
    2. chalcogens
    3. noble gases
    4. alkaline earth metals
  3. Use the periodic table to give the name and symbol for each of the following elements:
    1. the noble gas in the same period as germanium
    2. the alkaline earth metal in the same period as selenium
    3. the halogen in the same period as lithium
    4. the chalcogen in the same period as cadmium
  4. Use the periodic table to give the name and symbol for each of the following elements:
    1. the halogen in the same period as the alkali metal with 11 protons
    2. the alkaline earth metal in the same period with the neutral noble gas with 18 electrons
    3. the noble gas in the same row as an isotope with 30 neutrons and 25 protons
    4. the noble gas in the same period as gold
  5. Write a symbol for each of the following neutral isotopes. Include the atomic number and mass number for each.
    1. the chalcogen with a mass number of 125
    2. the halogen furthest down on the periodic table
    3. the noble gas, used in lighting, with 10 electrons and 10 neutrons
    4. the lightest alkali metal with three neutrons
  6. Which of the following are incorrectly paired?
    1. [latex]\ce{K}[/latex], alkali metal
    2. [latex]\ce{Ba}[/latex], alkaline earth metal
    3. [latex]\ce{O}[/latex], halogen
    4. [latex]\ce{Ne}[/latex], noble gas
    5. [latex]\ce{Ni}[/latex], transition metal
  7. Which of the following are incorrectly paired?
    1. Copper, [latex]\ce{Cu}[/latex]
    2. Carbon, [latex]\ce{C}[/latex]
    3. Cobalt, [latex]\ce{Co}[/latex]
    4. Calcium, [latex]\ce{Ca}[/latex]
    5. Cesium, [latex]\ce{Ce}[/latex]
  8. All of the following are characteristics of metals except:
    1. good conductors of heat
    2. malleable
    3. ductile
    4. often lustrous
    5. tend to gain electrons in chemical reactions
Show Selected Solutions
  1. Main Group Nonmetals C, F Main Group Metals E Transition Metals A, D, G, I Inner Transition Metals B, H
  2. The answers are as follows:
    1. krypton, [latex]\ce{Kr}[/latex]
    2. calcium, [latex]\ce{Ca}[/latex]
    3. fluorine, [latex]\ce{F}[/latex]
    4. tellurium, [latex]\ce{Te}[/latex]
  3. The answers are as follows:
    1. [latex]\ce{^{125}_{52}Te}[/latex]
    2. [latex]\ce{^{293}_{117}Ts}[/latex]
    3. [latex]\ce{^{20}_{10}Ne}[/latex]
    4. [latex]\ce{^6_3Li}[/latex]
  4. e. cesium, [latex]\ce{Cs}[/latex]


2.5 The Mole Concept [Go to section 2.5]

  1. Iron is biologically important in the transport of oxygen by red blood cells from the lungs to the various organs of the body. In the blood of an adult human, there are approximately 2.64 × 1013 red blood cells with a total of 2.90 g of iron. On average, how many iron atoms are present in each red blood cell? (molar mass [latex]\ce{Fe}[/latex] = 55.85 g/mol)
Show Selected Solutions
  1. 1.18 × 109 iron atoms per red blood cell


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