Chemical Bonding and Intermolecular Forces
This domain tests why substances have the properties they do: bond type (ionic, covalent, metallic), molecular shape and polarity, and the intermolecular forces that set boiling point, vapor pressure, and solubility.
Bond types
- Ionic bonding: a metal transfers electrons to a nonmetal. The result is a crystal lattice of ions with high melting point, brittleness, and conductivity only when molten or dissolved.
- Covalent bonding: nonmetals share electrons. A small electronegativity difference gives a nonpolar bond, a moderate difference a polar bond.
- Metallic bonding: cations in a sea of delocalized electrons, which explains malleability, ductility, and conductivity as a solid.
Shape and polarity
Electron domains repel, so two domains give linear (180°), three give trigonal planar (120°), and four give tetrahedral (109.5°). Lone pairs convert four domains into trigonal pyramidal (NH₃) or bent (H₂O). A molecule with polar bonds can still be nonpolar if symmetry cancels the dipoles, as in CO₂ and CCl₄.
Intermolecular forces
From weakest to strongest: London dispersion, dipole–dipole, hydrogen bonding. Every molecule has dispersion forces, which grow with more electrons. Hydrogen bonding requires H bonded directly to N, O, or F. Stronger forces mean higher boiling point, higher heat of vaporization, and lower vapor pressure.
Worked example
Water boils at 100 °C but hydrogen sulfide boils at about −60 °C. Both are bent with polar bonds, yet oxygen's high electronegativity lets water form hydrogen bonds, while H₂S relies on weaker dipole–dipole and dispersion forces. More energy is needed to separate water molecules.
Traps
- Claiming solid ionic compounds conduct electricity; only molten or dissolved ions move freely.
- Assuming any molecule with polar bonds is polar overall.
- Describing hydrogen bonding as a bond inside a molecule; it is between molecules.
- Saying covalent bonds break when a molecular solid melts; only intermolecular forces break.