What is an ionic bond?
An ionic bond is the strong electrostatic force of attraction between a positive ion (cation) and a negative ion (anion). It forms when a metal transfers valence electrons to a non-metal. The metal becomes a cation and the non-metal becomes an anion, and the opposite charges attract.
Formation of sodium chloride
Sodium (2.8.1) has one valence electron; chlorine (2.8.7) needs one. Sodium donates its electron to chlorine:
Na → Na+ + e− and Cl + e− → Cl−
The Na+ (2.8) and Cl− (2.8.8) now both have stable octets. Their attraction gives the ionic compound NaCl.
Worked example / 例题
Form the ionic compound between calcium (2.8.8.2) and chlorine (2.8.7). Ca must lose 2 electrons to form Ca2+, but each Cl gains only 1 electron. So one Ca2+ combines with two Cl−: CaCl2. The charges balance: (+2) + 2(−1) = 0.
Ionic charges depend on the group
| Group 1 | +1 (Na+, K+) |
| Group 2 | +2 (Mg2+, Ca2+) |
| Group 16 | −2 (O2−, S2−) |
| Group 17 | −1 (Cl−, Br−) |
Key idea / 要点
In the final formula the total positive charge must exactly cancel the total negative charge. Cross-multiply the charges: for Al3+ and O2− the formula is Al2O3.
Remember / 记住
- Ionic bonds form between a metal and a non-metal.
- Cations = metals losing electrons; anions = non-metals gaining electrons.
- The compound is electrically neutral overall.