2.2.7-1. (a).

2.2.7-2. Matter is made up of atoms. Think about (i) the size of atomic nuclei relative to that of atoms (Figure 2-9) and (ii) the fact that it is the nucleus where over 99% of the mass of an atom is located (Table 1). Consider reading this article.

2.2.7-3. (c).

2.2.7-4. (a), (b), and (d).

2.2.7-5. The number of protons is this element's atomic number. Element number 15 is phosphorus. As the isotope number is the sum of the number of protons and neutrons in the nucleus (15 + 16 = 31), the isotope is ³¹P (phosphorus-31). To answer the last question, you should either know the correct answer by heart (which is not required), or look up isotopes of phosphorus on the internet or in a handbook. Phosphorus-31 is the only stable naturally occurring isotope of phosphorus, so P is a monoisotopic element.

2.2.7-6. (a).

2.2.7-7. True, although in some cases replacing an atom in the reacting molecule by a different isotope of the same element may have a small influence on reaction rates of the substance.

2.2.7-8. The periodic table shows that fluorine's atomic number is 9 and atomic weight is 19. The atomic number indicates that fluorine has 9 protons and 9 electrons. Since F is monoisotopic, the atomic mass of the isotope is 19. This is the total number of protons and neutrons a fluorine atom contains. Therefore, the number of neutrons is 19 - 9 = 10.

2.2.7-10. Answer: As follows from the atomic number of Mg (12), the number of protons an atom of any isotope of magnesium has is 12. The number of neutrons in ²⁴Mg, ²⁵Mg, and ²⁶Mg is the atomic mass minus the atomic number = 12, 13, and 14, respectively. The atomic weight of Mg is the weighted average of atomic masses of its three isotopes: (24 x 0.79) + (25 x 0.10) + (26 x 0.11) = 24.3. Compare this calculated value with the atomic weight of magnesium in the periodic table.

2.3.5-1. (c).

2.3.5-2. Both. Being farther away from the nucleus, an electron in the 2s orbital is indeed less attracted by the nucleus than a 1s electron. Given that, however, a 2s electron has more, not less energy than a 1s electron.

2.3.5-3. False. Orbitals' boundaries are blurred. All orbital shapes drawn in the pictures depict the areas where an electron can be found with the highest probability.

2.3.5-4. (a).

2.3.5-5. (b).

2.3.5-6. True.

2.3.5-7. s-orbitals are spherical and p-orbitals are dumbbell shaped.

2.3.5-8. Just kidding. You are not expected to memorize the shapes of d- and f-orbitals for this introductory course.

2.3.5-9. False. The 1^st electron shell has only one orbital, which is an s-orbital.

2.3.5-10. (a), (b), and (e).

2.3.5-13. Smaller. See Figure 2-22 and recall that an atom of He is smaller than an atom of H because He has more protons and electrons within the same shell (1^st). The greater opposite charges enhance the attraction between the shell and the nucleus and, as a consequence, result in stronger contraction of the atom for He relative to H (Figure 2-22). The trend is the same along the 2^nd period for the same reason.

2.3.5-15. (b) and (d).

2.4.12-1. (b).

2.4.12-11. (c).

2.4.12-13. The O-H bonds are covalent polar bonds with the shared electron pairs shifted toward the more electronegative O atom. Consequently, partial positive charges are induced on each of the two H atoms of the molecule.

2.5.8-1. (b).

2.5.8-3. False. Nearly all salts are strong electrolytes, but an acid can be a strong, medium strength, or weak electrolyte.

2.5.8-4. False. Like most salts, BaSO₄ is a strong electrolyte. Solubility and degree of dissociation are different things and must not be mixed up. Some compounds are poorly soluble in water while being strong electrolytes, as the very little that dissolves dissociates to a large degree. Barium sulfate is one of those. On the contrary, some other compounds are easily soluble in water while being weak electrolytes; they dissolve readily yet dissociate poorly. Examples of such compounds include sugar, acetic acid, alcohol, and rubbing alcohol.

2.5.8-8. (b).

2.5.8-9. (d).

2.5.8-11. (e) and (h).

2.5.8-14. Air consists of mainly nitrogen and oxygen but also contains small quantities of inert gases and carbon dioxide, CO₂. Atmospheric CO₂ slowly dissolves in water to give carbonic acid, CO₂ + H₂O = H₂CO₃. Although H₂CO₃ is a weak electrolyte (Table 4), it does dissociate to a small extent to produce H⁺, which lowers the pH from 7 to 6.

2.5.8-15. Homolytic: (b), (c), (e), and (f). Heterolytic: (a) and (d). Hint: use the electronegativity table in Figure 2-52.

2.6.4-3. The reaction of BaCl₂. Atomic masses of Cl and Br are 35.5 and 80, respectively. As bromine is considerably heavier than chlorine, the atomic mass percentage of Ba is higher in BaCl₂ than in BaBr₂.

2.6.4-4. Answer: Regular (molecular) equation: NaCl + H₂SO₄ = NaHSO₄ + HCl↑. Complete ionic equation: Na⁺ + Cl^- + 2 H⁺ + SO₄^2- = Na⁺ + H⁺ + SO₄^2- + HCl↑. Net ionic equation: Cl^- + H⁺ = HCl↑. Hydrogen chloride is a strong electrolyte and is therefore dissociated in solution. However, gaseous HCl released in the reaction is not dissociated as the H-Cl bond is polar covalent, not ionic. Therefore, we present the formula in our ionic equations as HCl, not H⁺ + Cl^-. Gaseous HCl is produced in this reaction only if concentrated H₂SO₄ is used. In a dilute aqueous solution, this reaction would produce only very small quantities of HCl gas (if any) because HCl is easily soluble in water where it remains in the dissociated state.

2.6.4-5. (a), (c), (d), (e), (g), and (h). You are expected to remember that BaSO₄, AgCl, Cu(OH)₂, and CaCO₃ are insoluble in water and that CO₂ and H₂S are gases of limited solubility in water. If you are not sure if a product is soluble or not, look up its solubility on the Internet or elsewhere. For example, aluminum phosphate, AlPO₄, has not been previously mentioned in our course and you might not know that it is insoluble. Just go to Wikipedia to find out.