Chemical Bonding (Advanced)

Why atoms bond: the octet rule and Lewis symbols, counting shared and lone pairs, the molecules that break the rule, and the electron sea model of metals.

Key ideas

Atoms tend to gain, lose or share electrons until they have eight in the valence shell; hydrogen stops at two.
The octet rule is a guiding principle, not a law: stable molecules break it.
A Lewis symbol puts one dot round the symbol for each valence electron.
Every shared pair and every lone pair round an atom counts two electrons in its valence shell.
Incomplete octets: BeCl₂ (4), BF₃ (6); expanded octets: PCl₅ (10), SF₆ (12); odd-electron molecules: NO (11), NO₂ (17).
A metal is positive ions in a sea of free electrons; the non-directional bond explains conduction, malleability and ductility.

Remember

Electrons round an atom	2 × (shared pairs + lone pairs)
Valency	valence electrons if 1–4, otherwise 8 − valence electrons
Ionic formula	cross the valencies and simplify: Al (3) + O (2) → Al₂O₃
Odd-electron molecule	total valence electrons odd → at least one unpaired electron

Watch out

Counting a shared pair as one electron instead of two.
Forgetting lone pairs when counting electrons round the central atom.
Calling the octet rule a law with no exceptions.

Worked example

How many chlorine atoms must bond with one nitrogen atom (atomic number 7) so that nitrogen completes its octet by sharing?

Working: nitrogen has 5 valence electrons and needs 3 more; each chlorine atom shares one

Answer: 3