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Ionic, covalent & metallic models, VSEPR, IMFs, networks, polymers, bonding vs properties.

ionic · covalent · VSEPR & hybridization · polarity & IMFs · networks · metallic · continuum & materials · bonding vs properties

Ionic model

Formation. Metal atoms lose e⁻ → cations; non-metals gain e⁻ → anions. Ionic bond = electrostatic attraction in a giant lattice; formula empirical, charge-balanced.

Properties. High mp/bp, brittle; conduct when molten/aqueous, not solid; often soluble in polar solvents.

Lattice enthalpy. Magnitude increases with \(|q_+q_-|\) and decreases with ionic radius.

Covalent character. Polarizing power \(\propto\) charge density; small/high-charge cations + large/polarizable anions give covalent character.

Ionic radius. Cation < atom; anion > atom; in an isoelectronic series the radius decreases as \(Z\) increases.

Covalent model

Covalent bond. Shared e⁻ pair attracted to the nuclei; single/double/triple = 1/2/3 shared pairs; bond order ↑: length ↓, strength ↑. Coordinate bond: both e⁻ from the donor.

Lewis structures. Count valence e⁻ → connect skeleton → fill outer atoms → complete central → use multiple bonds → minimize formal charge.

Formal charge. \(FC=V-(N_\text{lone}+\tfrac12 N_\text{bond})\); preferred: small FC, negative on the more electronegative atom.

Resonance. Same connectivity, different e⁻ placement; the actual structure is delocalized, with equalized bond lengths and lower energy.

Expanded octet. Possible for period-3+ central atoms.

VSEPR & hybridization

Repulsion. LP–LP > LP–BP > BP–BP; multiple bonds count as one domain but repel more than single bonds.

Sigma & pi bonds. \(\sigma\): head-on overlap along the internuclear axis; \(\pi\): side-on p overlap above/below the axis. Double = \(1\sigma+1\pi\); triple = \(1\sigma+2\pi\). Rotation restricted about a \(\pi\) bond.

Polarity & IMFs

Polarity. Bond polarity from \(\Delta EN\); molecular polarity = vector sum of bond dipoles + geometry.

IMFs. London/dispersion in all particles, strength ↑ with electron number/surface area; dipole–dipole in polar molecules; H-bond when H is bonded to F/O/N and a lone pair sits on F/O/N.

Strength order. London < dipole–dipole < H-bond < ion–dipole < covalent/ionic/metallic.

Consequences. bp/mp, viscosity, volatility, solubility. Like dissolves like: polar/ionic in polar solvents; non-polar in non-polar.

Covalent networks

Diamond. Tetrahedral \(sp^3\), very hard, high mp, non-conductor.

Graphite/graphene. Trigonal planar \(sp^2\), delocalized \(\pi\) e⁻, conducts; layers held by London forces, lubricant.

Silicon dioxide. Tetrahedral network, high mp, hard, non-conductor.

Fullerenes/nanotubes. Molecular/extended carbon, delocalized, high tensile strength.

Metallic model

Metallic bond. Cation lattice + delocalized e⁻ sea. Strength increases with cation charge and smaller radius.

Properties. Electrical/thermal conductor, malleable/ductile, lustrous, variable mp.

Alloys. Distort the layers: harder/less malleable; interstitial/substitutional alloys.

Bonding continuum & materials

Bonding triangle. Ionic–covalent–metallic; location set by average electronegativity and electronegativity difference. Properties arise from dominant bonding + structure, not formula alone.

Polymers. Monomer repeat units, degree of polymerization \(n\). Addition polymer: alkene \(\ce{n CH2=CHR -> [-CH2-CHR-]_{n}}\). Condensation polymer: bifunctional monomers, small molecule lost (\(\ce{H2O}\)/\(\ce{HCl}\)) — polyester from diol + dicarboxylic acid; polyamide from diamine + dicarboxylic acid.

Plastics. Thermoplastic = softens on heating; thermoset = cross-linked; elastomer = flexible chains. Recycling: addition polymers chemically inert; condensation polymers hydrolyzable.

Bonding vs properties