Formation of Ions
Atoms are electrically neutral because they contain equal numbers of protons and electrons. But atoms don't always "stay neutral" — many elements gain or lose electrons to become ions, which are charged particles. Understanding why and how this happens is one of the keys to predicting chemical behaviour across the periodic table.
Why do atoms form ions?
Atoms "want" to achieve a stable electron arrangement, usually resembling that of the nearest noble gas (a full outer shell). This drive explains most ion formation:
- Metals (found on the left side of the periodic table, s-block and some p-block) tend to have few valence electrons. It's easier for them to lose these electrons than to gain many more. Losing electrons removes a negative charge, so the atom becomes a positively charged ion, called a cation.
- Non-metals (on the right side, p-block) tend to have almost full outer shells. It's easier for them to gain a few electrons to complete the shell. Gaining electrons adds negative charge, so the atom becomes a negatively charged ion, called an anion.
The number of electrons lost or gained is directly linked to the group number, since elements in the same group share the same number of valence electrons (Talbot, p. 233).
Predicting ion charge from group number
| Group | Valence electrons | Tends to... | Ion formed |
|---|---|---|---|
| 1 | 1 | lose 1 e⁻ | +1 cation |
| 2 | 2 | lose 2 e⁻ | +2 cation |
| 13 | 3 | lose 3 e⁻ | +3 cation |
| 15 | 5 | gain 3 e⁻ | −3 anion |
| 16 | 6 | gain 2 e⁻ | −2 anion |
| 17 | 7 | gain 1 e⁻ | −1 anion |
(Based on the period 3 pattern shown in Talbot, p. 132.)
Worked Example 1: Sodium
Sod