Ionic bonding: definition

· Ionic bonding = electrostatic attraction between oppositely charged ions.
· Positive ions are cations; negative ions are anions.
· Ionic bonding usually forms between a metal and a non-metal.
· Metals form positive ions by losing electrons; non-metals form negative ions by gaining electrons.
· The bond is not a single line between two atoms: it is a strong attraction throughout an ionic lattice.

Sodium chloride, NaCl

· Sodium forms Na⁺ by losing one electron.
· Chlorine forms Cl⁻ by gaining one electron.
· The ions have charges +1 and −1, so the formula is NaCl.
· In solid sodium chloride, many Na⁺ and Cl⁻ ions are arranged in a giant ionic lattice.
· Each ion is strongly attracted to many oppositely charged ions around it.

The diagram shows the giant ionic lattice of sodium chloride. Purple spheres represent sodium ions, Na⁺, and green spheres represent chloride ions, Cl⁻. Source

Magnesium oxide, MgO

· Magnesium forms Mg²⁺ by losing two electrons.
· Oxygen forms O²⁻ by gaining two electrons.
· The ions have charges +2 and −2, so the formula is MgO.
· The ionic bond is the electrostatic attraction between Mg²⁺ and O²⁻ ions.
· MgO contains a giant ionic lattice, not separate MgO molecules.

This model represents magnesium oxide as a lattice of Mg²⁺ and O²⁻ ions. It helps show that ionic compounds are extended structures rather than individual molecules. Source

Calcium fluoride, CaF₂

· Calcium forms Ca²⁺ by losing two electrons.
· Each fluorine atom forms F⁻ by gaining one electron.
· One Ca²⁺ ion requires two F⁻ ions to balance charge, so the formula is CaF₂.
· The ionic bond is the electrostatic attraction between Ca²⁺ and F⁻ ions.
· Always balance total positive and negative charge when writing formulas.

Calcium fluoride has a giant ionic lattice containing Ca²⁺ and F⁻ ions. The 1:2 ratio is needed because one Ca²⁺ ion balances two F⁻ ions. Source

Writing formulas from ionic charges

· The formula of an ionic compound must have overall charge = 0.
· Na⁺ + Cl⁻ → NaCl because +1 and −1 cancel.
· Mg²⁺ + O²⁻ → MgO because +2 and −2 cancel.
· Ca²⁺ + 2F⁻ → CaF₂ because +2 is balanced by two −1 ions.
· Do not write charges in the final formula: write CaF₂, not Ca²⁺F⁻₂.

Common exam mistakes

· Do not define ionic bonding as “electron transfer”; electron transfer forms ions, but the bond is electrostatic attraction.
· Do not describe ionic compounds as molecules; they are usually giant ionic lattices.
· Do not forget charges when deriving formulas: MgCl₂, not MgCl.
· Do not say cations and anions “share electrons”; sharing electrons is covalent bonding.
· Always use the terms cation, anion, oppositely charged ions and electrostatic attraction in definitions.