S2.1 The ionic modelIB Chemistry HL: Revision notes

When metal atoms lose electrons they form positive ions called cations; when non-metal atoms gain electrons they form negative ions called anions. Atoms tend to lose or gain electrons to reach a noble-gas configuration.

Predicting charge from electron configuration: count the outer (valence) electrons. Mg, [Ne]3s², loses 2 → Mg²⁺. Al, [Ne]3s²3p¹, loses 3 → Al³⁺. N, 1s²2s²2p³, gains 3 → N³⁻. S, [Ne]3s²3p⁴, gains 2 → S²⁻. 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 Roman numeral gives the charge, e.g. iron(III) = Fe³⁺.

An ionic bond is the electrostatic attraction between oppositely charged ions. It is not a bond between one pair of ions: each ion is attracted to all the oppositely charged ions around it.

Ionic compounds exist as three-dimensional lattice structures of alternating cations and anions (in NaCl, each Na⁺ is surrounded by six Cl⁻ and vice versa). Because there are no discrete molecules, ionic compounds are represented by empirical formulas giving the ratio of ions, e.g. NaCl, MgCl₂.

The attraction is stronger when the ions have higher charges and smaller radii.

To write a formula, balance the total positive and negative charges: Mg²⁺ + N³⁻ → Mg₃N₂.

Binary ionic compounds (two elements) are named cation first, then the anion with the suffix 'ide': sodium chloride, calcium sulfide, magnesium nitride, aluminium oxide.

Polyatomic ions to know: ammonium NH₄⁺, hydroxide OH⁻, nitrate NO₃⁻, hydrogencarbonate HCO₃⁻, carbonate CO₃²⁻, sulfate SO₄²⁻, phosphate PO₄³⁻. Use brackets when more than one polyatomic ion is needed: Ca₃(PO₄)₂, (NH₄)₂SO₄, Fe₂(SO₄)₃.

Volatility: ionic compounds have low volatility and high melting points, because many strong electrostatic attractions throughout the lattice must be overcome. Higher ion charges and smaller ions give higher melting points (MgO 2852 °C, NaCl 801 °C).

Electrical conductivity: solids do not conduct, because ions are fixed in the lattice. When molten or aqueous, the ions are free to move and carry charge, so they conduct.

Solubility: many are soluble in polar solvents such as water, because the partial charges on water molecules attract the ions and pull them out of the lattice. They are insoluble in non-polar solvents such as hexane. Some (e.g. Al₂O₃, CaCO₃) are insoluble in water, because their lattice attractions are too strong.