S2.2 The covalent modelIB Chemistry HL: Revision notes

A covalent bond is the electrostatic attraction between a shared pair of electrons and the positively charged nuclei. The octet rule is the tendency of atoms to gain a valence shell of 8 electrons. Lewis formulas show all valence electrons, including lone pairs.

Single, double and triple bonds have one, two and three shared pairs; more pairs mean shorter, stronger bonds.

In a coordination bond, both electrons of the shared pair come from the same atom, e.g. NH₄⁺, H₃O⁺, CO.

Electron domains (lone pairs, single or multiple bonds) repel to be as far apart as possible; lone pairs repel more.

• 2 domains: linear 180°
• 3 domains: trigonal planar 120°; with 1 lone pair, bent
• 4 domains: tetrahedral 109.5°; 1 lone pair trigonal pyramidal ~107°; 2 lone pairs bent ~105°

Bond polarity comes from an electronegativity difference; molecular polarity depends on bond polarity and geometry. Symmetrical molecules (CO₂, CCl₄) have no net dipole.

Covalent networks: diamond (4 bonds per C, non-conductor), graphite (3 bonds per C, delocalised electrons, conducts, layers slide), graphene (one graphite layer), C₆₀ fullerene (molecules, weak London forces between them), silicon (diamond-like, weaker Si–Si bonds) and SiO₂ (each Si bonded to 4 O).

Intermolecular forces, for comparable molar mass: London < dipole–dipole < hydrogen bonding; dipole–induced dipole forces act between polar and non-polar molecules. Simple molecular substances are volatile and non-conducting, and dissolve in solvents with similar forces.

Chromatography separates by relative attraction to the stationary and mobile phases; Rf = distance moved by component ÷ distance moved by solvent front.

Atoms in period 3 and beyond (e.g. P, S, Cl, Xe) can have an expanded octet, with more than eight electrons around the central atom.

Five domains, electron domain geometry trigonal bipyramidal (90° and 120°):

• 5 bonds: trigonal bipyramidal (PCl₅)
• 4 bonds + 1 lone pair: seesaw (SF₄)
• 3 bonds + 2 lone pairs: T-shaped (ClF₃)
• 2 bonds + 3 lone pairs: linear (XeF₂)

Six domains, electron domain geometry octahedral (90°):

• 6 bonds: octahedral (SF₆)
• 5 bonds + 1 lone pair: square pyramidal (BrF₅)
• 4 bonds + 2 lone pairs: square planar (XeF₄)

In five-domain species, lone pairs occupy equatorial positions, where they have fewer 90° repulsions.

Formal charge (FC) = valence electrons − non-bonding electrons − ½ × bonding electrons. The formal charges in a species add up to its overall charge.

The preferred Lewis formula has formal charges as close to zero as possible; if charges are unavoidable, the negative charge should be on the more electronegative atom. For N₂O, N≡N–O (FC 0, +1, −1) is preferred to N=N=O (−1, +1, 0).

Resonance structures occur when there is more than one possible position for a double bond in a molecule or ion; atoms stay in place and only electrons move. The real structure is a resonance hybrid. Examples: O₃, NO₃⁻ (three equivalent N–O bonds), CO₃²⁻, carboxylate ions.

Benzene is a key example. Evidence against alternating single and double bonds (the Kekulé structure): all C–C bonds are 0.140 nm (between 0.154 and 0.134 nm); its enthalpy of hydrogenation (−208 kJ mol⁻¹) is much less exothermic than 3 × −120 kJ mol⁻¹, so it is about 152 kJ mol⁻¹ more stable; it undergoes substitution rather than addition; and only one 1,2-disubstituted isomer exists. The six p electrons are delocalised in a π ring above and below the plane.

A sigma (σ) bond forms by head-on overlap of orbitals; electron density is concentrated along the bond axis. A pi (π) bond forms by sideways (lateral) overlap of p orbitals; electron density is above and below the bond axis.

Single bond = 1σ; double bond = 1σ + 1π; triple bond = 1σ + 2π.

Hybridisation is the mixing of atomic orbitals to form new, equivalent hybrid orbitals for bonding:

• sp³: 4 domains, tetrahedral, 109.5° (C in CH₄, N in NH₃, O in H₂O)
• sp²: 3 domains, trigonal planar, 120°, one unhybridised p orbital for a π bond (C in C₂H₄, BF₃, benzene)
• sp: 2 domains, linear, 180°, two unhybridised p orbitals for two π bonds (C in C₂H₂, CO₂, HCN)

Work both ways: domains → hybridisation → geometry, or geometry → hybridisation.