Ionic bonding
The idea
When a metal meets a nonmetal, electrons change hands rather than being shared. The metal, which holds its few outer electrons loosely, gives them up to become a positive cation; the nonmetal, hungry to complete its octet, accepts them and becomes a negative anion. The oppositely charged ions then attract each other in every direction, locking into a rigid, repeating crystal lattice. You already know why each side wants this from periodic trends: both ions end up with a stable noble-gas electron arrangement.
Writing the formula is a charge-balancing exercise: the compound must be electrically neutral overall, so you need the smallest whole-number ratio of cations to anions whose charges cancel. The quick route is to find the least common multiple of the two charges. Strong attractions throughout the lattice are also why ionic compounds tend to be brittle, high-melting solids that conduct only when melted or dissolved.
The big misconception is reading a formula like NaCl as a molecule — one sodium handcuffed to one chlorine. There are no molecules in an ionic solid. The formula only reports the ratio of ions in an endless three-dimensional lattice where every ion touches several oppositely charged neighbors.
Worked example
Aluminum reacts with oxygen to form an ionic compound. Predict the charges on each ion, work out the formula, and name the compound.
- Read the charges off the table: aluminum is in group 13 with 3 valence electrons, so it loses all 3 to form Al³⁺; oxygen is in group 16 with 6 valence electrons, so it gains 2 to form O²⁻.
- Balance the charges with the smallest whole-number ratio. The least common multiple of 3 and 2 is 6, so you need two Al³⁺ (total 6+) for every three O²⁻ (total 6−).
- Write the ratio as subscripts, cation first: Al₂O₃.
- Verify neutrality: 2 × (+3) + 3 × (−2) = +6 − 6 = 0, so the formula is electrically neutral as required. The name is aluminum oxide — the metal keeps its name and the nonmetal takes an -ide ending.
Answer. The compound is Al₂O₃, aluminum oxide, built from Al³⁺ and O²⁻ ions in a 2:3 ratio.
Check your understanding
- Why does the metal lose electrons and the nonmetal gain them, rather than the other way around?
- How does the lattice structure explain why ionic compounds are brittle and have high melting points?
- What does a subscript in an ionic formula physically describe, given that no individual molecules exist in the crystal?
- How would the formula change if you swapped aluminum for a group 2 metal like calcium, and why?
Build the foundations first
Ionic bonding builds on these concepts. If any feel shaky, start there.