3.2 Ionic bonding

Ionic bonding is the strong electrostatic attraction between oppositely charged ions: positive cations and negative anions. The attraction acts in all directions between neighbouring ions in the ionic lattice.

A metal transfers one or more outer-shell electrons to a non-metal. The metal becomes a positively charged cation and the non-metal becomes a negatively charged anion; each ion may thereby reach a stable outer-shell arrangement supported by the source.

The oppositely charged ions arrange in a regular repeating lattice. Each ion is attracted to oppositely charged ions around it, so the solid is overall electrically neutral and the strong attraction requires substantial energy to overcome.

Keep the mechanism distinct: electron transfer creates the ions, whereas the ionic bond is the electrostatic attraction between them. Do not describe an ionic compound as a single covalent molecule or add detailed lattice-energy calculations; specific charge-balance examples belong to the neighbouring card.

Use a fixed charge-balance method: identify the metal and non-metal, infer the ions formed from their outer electrons or group positions, write each ion charge, and choose the smallest whole-number combination whose total charge is zero.

For sodium chloride, Na⁺ and Cl⁻ combine 1:1 to give NaCl. For magnesium oxide, Mg²⁺ and O²⁻ combine 1:1 to give MgO. For calcium fluoride, Ca²⁺ requires two F⁻ ions, giving CaF₂. In every case, electron transfer creates the ions and electrostatic attraction holds the lattice together.

Check the formula by adding the ionic charges after writing the subscripts: the total must be zero. The subscript counts ions in the formula unit; it does not change the charge of an individual ion. The same method supports the SME examples Li₃N and Al₂O₃.

Do not write subscripts before balancing charge, confuse charge magnitude with the number of atoms, or treat the formula as a discrete molecule when it represents an ionic lattice. Detailed lattice-energy calculations and broader property explanations are outside this card’s objective.