2.4 · Chemical Bonding and Molecular StructurePractice · 化学键与分子结构

In an ionic solid, cations and anions are held by strong electrostatic attraction (ionic bonds), so melting takes a lot of energy and the melting point is high. In the solid the ions can’t move, so it doesn’t conduct; once melted or dissolved, the ions move freely and it conducts. When struck, layers shift so like charges line up and repel, and the crystal shatters: hard but brittle. Ionic compounds usually don’t dissolve in nonpolar solvents such as gasoline.

Ca has 2, 8, 8, 2 electrons per shell, with 2 valence electrons that are easy to lose; O has 2, 6, so it gains 2 readily. Ca gives 2 electrons to O: $\ce{Ca^2+}$ ends up with 2, 8, 8 (like Ar) and $\ce{O^2-}$ with 2, 8 (like Ne), and the oppositely charged ions attract in an ionic bond. Their electronegativities differ by 2.44, so the electrons are transferred, not shared.

A single bond is 1 σ; a double bond is 1 σ + 1 π; a triple bond is 1 σ + 2 π. Acrylonitrile has 3 C–H single bonds, 1 C–C single bond, 1 C=C double bond, and 1 C≡N triple bond: 3 + 1 + 1 + 1 = 6 σ bonds, and 1 + 2 = 3 π bonds. Counting every line as a σ bond would give 9.

The more electrons two atoms share, the more strongly they pull the nuclei together, so the bond gets shorter and stronger: C≡C 120 pm, 839 kJ/mol; C=C 134 pm, 614 kJ/mol; C–C 154 pm, 348 kJ/mol. Note that a double bond (614) is weaker than two single bonds (696), because a π bond is weaker than a σ bond.

Metallic bonding is the attraction between metal cations and the sea of delocalized electrons. Each alkali metal atom gives just 1 electron, so every cation is +1, but going down the group the ions get larger ($\ce{Li+}$ 76 pm, $\ce{Na+}$ 102 pm, $\ce{K+}$ 138 pm). The electron sea is farther from each ion’s center, the attraction is weaker, and less energy is needed to melt the metal. Heavier atoms melt lower here, so mass isn’t the cause, and metals have no covalent bonds.

S has 6 valence electrons, each of the 4 O atoms has 6, and the 2− charge adds 2 more: 6 + 4 × 6 + 2 = 32. Add the charge for an anion; subtract it for a cation.

Valence electron totals: $\ce{CO2}$ 16, $\ce{ClO2}$ 19, $\ce{SO2}$ 18, $\ce{NH3}$ 8. Only $\ce{ClO2}$ has an odd number (19), so one electron is always left unpaired and at least one atom falls short of an octet. Molecules with an unpaired electron are called free radicals. Chlorine dioxide is used to disinfect drinking water.

Sulfite has 26 valence electrons. The S has 3 bonding domains and 1 lone pair, so 4 electron domains in a tetrahedral arrangement; with one of them a lone pair, the ion is trigonal pyramidal. Compare $\ce{SO3}$, which has no lone pair on S and is trigonal planar: the 2 extra electrons in the ion form that lone pair. (The Chinese textbook formula, adding the charge to a, gives the same count: ½(8 − 3 × 2) = 1.)

Whether a molecule is polar depends on whether its shape lets the bond dipoles cancel. Lone pairs on the central atom: $\ce{CS2}$ 0, linear; $\ce{NF3}$ 1, trigonal pyramidal; $\ce{SiCl4}$ 0, tetrahedral; $\ce{OF2}$ 2, bent; $\ce{BCl3}$ 0, trigonal planar. With identical outer atoms, a central atom without lone pairs gives a symmetric, nonpolar molecule; $\ce{NF3}$ and $\ce{OF2}$ have lone pairs on the central atom, so they are asymmetric and polar.

The three are isomers with the same molar mass, all nonpolar, with only London dispersion forces between molecules and no hydrogen bonding (every H is on a C). What differs is shape: the long chain of pentane lets molecules lie close along their length, while nearly spherical neopentane touches its neighbors over a small area, so its dispersion forces are weakest and it boils lowest. Boiling doesn’t break covalent bonds inside the molecules.