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Class 9 Chemical Bonding (Advanced): cheat sheet
CBSE Science. 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
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