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IB Chemistry: Structure 2 · Models of bonding and structure

Ionic, covalent and metallic bonding, intermolecular forces. 4 subtopics, 16 key terms.

In this topic: S2.1 · S2.2 · S2.3 · S2.4

S2.1The ionic model

Key terms

Ion

A charged particle: cations are positive, anions are negative.

Ionic bond

Electrostatic attraction between oppositely charged ions.

Ionic lattice

A 3D repeating arrangement of alternating cations and anions.

Lattice enthalpy

Energy needed to separate one mole of an ionic solid into gaseous ions; a measure of ionic bond strength.

Metals lose electrons to form cations; non-metals gain electrons to form anions. The charge often follows the group: group 1 → +1, group 2 → +2, group 13 → +3, group 15 → −3, group 16 → −2, group 17 → −1. Transition metals can form more than one ion.

The full notes, worked example and check questions for this subtopic are in the app.

Exam tip: Explain properties with both structure and particles: say what moves or what is overcome, not just 'it is ionic'.

S2.2The covalent model

Key terms

Covalent bond

Electrostatic attraction between a shared pair of electrons and the positively charged nuclei.

Coordination bond

A covalent bond in which both shared electrons come from the same atom.

Electronegativity

Ability of an atom to attract a shared pair of electrons in a covalent bond.

VSEPR theory

Electron domains around a central atom repel and arrange themselves as far apart as possible.

Hydrogen bond

Strong dipole–dipole attraction between H bonded to N, O or F and a lone pair on N, O or F of another molecule.

Lewis formulas show all valence electrons as dots, crosses or lines. Most atoms reach an octet; exceptions include Be and B (fewer than 8).

The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.

Exam tip: Count electron domains first (including lone pairs), name the domain geometry, then name the molecular shape from the bonding pairs only.

S2.3The metallic model

Key terms

Metallic bond

Electrostatic attraction between a lattice of cations and a 'sea' of delocalized electrons.

Malleable

Can be hammered into shape.

Ductile

Can be drawn into wires.

Metals conduct electricity and heat because delocalized electrons can move through the lattice.

The full notes, worked example and check questions for this subtopic are in the app.

Exam tip: Compare metals using two factors: charge of the cation (number of delocalized electrons) and the cation's radius.

S2.4From models to materials

Key terms

Bonding continuum

Bonding is not purely ionic, covalent or metallic; most substances lie between these extremes.

Bonding triangle

A van Arkel–Ketelaar diagram that places a substance using the average and the difference of its elements' electronegativities.

Alloy

A mixture of a metal with other metals or non-metals, usually harder than the pure metal.

Polymer

A large molecule made of many repeating units (monomers).

Percentage ionic character increases with electronegativity difference. A large difference and low average electronegativity → ionic; small difference and high average → covalent; small difference and low average → metallic.

The full notes, worked example and check questions for this subtopic are in the app.

Exam tip: In a polymer repeat unit, the main chain only ever contains the two carbons that were in the C=C bond.

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