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💡 Why atoms bond
Atoms are happiest with a full outer shell (an octet). To get there they lose, gain or share electrons — and that number is the valency.
The outermost shell is the valence shell ; the electrons in it are valence electrons . A full outermost shell of 8 electrons (or 2 for helium) is an octet — such atoms are stable and unreactive.
What is valency?
- The number of electrons an atom gains, loses or shares to complete its octet.
- Fewer than 4 valence electrons → tends to lose (e.g. Na: 2,8,1 → valency 1).
- More than 4 → tends to gain (e.g. O: 2,6 → valency 2).
- Exactly 4 → tends to share (e.g. C: 2,4 → valency 4).
Important Definitions
- Valence shell — the outermost shell of an atom that contains electrons.
- Valence electrons — the electrons present in the valence (outermost) shell.
- Octet — a valence shell containing 8 electrons (2 for helium); makes the atom stable and unreactive.
- Valency — the number of electrons gained, lost or shared by an atom to complete its octet.
