S2.2 The covalent modelIB Chemistry SL: Revision notes

A covalent bond is the electrostatic attraction between a shared pair of electrons and the positively charged nuclei of the two bonded atoms. The octet rule is the tendency of atoms to gain a valence shell of 8 electrons (hydrogen: 2).

Lewis formulas show all valence electrons as dots, crosses or lines, including lone pairs. Steps: count valence electrons (add one per negative charge, remove one per positive charge), join atoms with single bonds, complete octets on outer atoms, then use multiple bonds if the central atom lacks an octet. Some atoms, e.g. B in BF₃, have fewer than 8.

Single, double and triple bonds have one, two and three shared pairs. More shared pairs mean more electron density between the nuclei, so bonds are shorter and stronger: C≡C < C=C < C–C in length.

A coordination bond is a covalent bond in which both electrons of the shared pair come from the same atom. Once formed, it is identical to any other covalent bond.

Examples: NH₄⁺ (N donates its lone pair to H⁺); H₃O⁺ (O donates to H⁺); CO (O donates a lone pair to C); NH₃BF₃ (N donates to the electron-deficient B).

To identify one: look for an atom with a lone pair next to an atom or ion that is short of an octet.

The VSEPR model: electron domains around a central atom repel each other and arrange themselves as far apart as possible. A domain is a lone pair, a single bond or a multiple bond (a double or triple bond counts as one domain). Lone pairs repel more than bonding pairs, closing bond angles by about 2.5° each.

• 2 domains: linear, 180° (CO₂, HCN)
• 3 domains: trigonal planar, 120° (BF₃); with 1 lone pair, bent ~117° (SO₂)
• 4 domains: tetrahedral, 109.5° (CH₄); 1 lone pair trigonal pyramidal ~107° (NH₃); 2 lone pairs bent ~105° (H₂O)

The electron domain geometry includes lone pairs; the molecular geometry describes only the atoms.

Bond polarity results from a difference in electronegativity (values in the data booklet): the more electronegative atom gets a partial negative charge (δ−). A larger difference gives a more polar bond.

Molecular polarity depends on bond polarity and geometry. If bond dipoles cancel by symmetry, the molecule has no net dipole moment (CO₂, BF₃, CH₄, CCl₄). If they do not cancel, the molecule is polar (H₂O, NH₃, CH₃Cl, CHCl₃).

Diamond: each C bonded to 4 others tetrahedrally; very hard, very high melting point, no mobile electrons so non-conductor. Graphite: each C bonded to 3 others in hexagonal layers; one delocalised electron per C so it conducts along layers; weak London forces between layers let layers slide (soft, lubricant); very high melting point. Graphene: a single layer of graphite; extremely strong, very thin, excellent conductor. Fullerene C₆₀: a molecule of 60 C atoms in a hollow sphere, each bonded to 3; weak London forces between molecules, so lower sublimation point; poor conductor. Silicon: tetrahedral network like diamond, but longer, weaker Si–Si bonds; semiconductor. Silicon dioxide (SiO₂): each Si bonded to 4 O, each O to 2 Si; hard, high melting point, insoluble, non-conductor.

The strength of intermolecular forces depends on the size and polarity of molecules.

• London (dispersion): in all molecules; from instantaneous dipoles; stronger with more electrons / larger molecules.
• Dipole–induced dipole: between a polar and a non-polar molecule.
• Dipole–dipole: between polar molecules.
• Hydrogen bonding: when H is bonded to N, O or F and attracted to a lone pair on N, O or F in another molecule.

For comparable molar mass: London < dipole–dipole < hydrogen bonding.

Properties: simple molecular substances are volatile (only weak intermolecular forces to overcome), do not conduct (no ions or delocalised electrons), and dissolve in solvents with similar intermolecular forces ('like dissolves like').

Chromatography separates the components of a mixture according to their relative attractions (intermolecular forces) to a stationary phase and a mobile phase. Components more strongly attracted to the mobile phase travel further.

Retardation factor: Rf = distance moved by component ÷ distance moved by solvent front, both from the baseline. Rf is always between 0 and 1 and is characteristic of a substance under fixed conditions (same solvent, stationary phase and temperature), so it can identify components by comparison with known values.