Valence electrons — the outer shell
What "valence" means
An atom's electrons sit in shells. Inner shells fill first. The outermost shell is called the valence shell, and the electrons in it are valence electrons.
Why we care: valence electrons are the ones that interact with other atoms. Bonding, reactivity, whether something is a metal — all driven by valence.
The pattern for rows 1-3
| Group | Row 1 | Row 2 | Row 3 | Valence |
|---|---|---|---|---|
| 1 | H | Li | Na | 1 |
| 2 | He* | Be | Mg | 2 |
| 13 | B | Al | 3 | |
| 14 | C | Si | 4 | |
| 15 | N | P | 5 | |
| 16 | O | S | 6 | |
| 17 | F | Cl | 7 | |
| 18 | Ne | Ar | 8 |
* He is a noble gas — its K-shell holds 2 electrons total. We call its valence 2 (full shell).
Same pattern every row: 1, 2, [skip transition metals — not relevant yet], 3, 4, 5, 6, 7, 8. Group number → valence count.
Why this matters — the octet rule
Atoms are "happy" with 8 valence electrons (2 for the first shell — H and He). They will give away, take, or share electrons to get there.
- Sodium (1): gives away its 1 electron → leaves it with a full shell underneath. Becomes Na⁺.
- Chlorine (7): takes 1 electron → makes 8. Becomes Cl⁻.
- Na⁺ + Cl⁻ → NaCl (table salt). Ionic bond.
- Oxygen (6): wants 2 more → grabs them, often by sharing.
- Water: H–O–H. Each H shares its 1 with O. O's outer shell now effectively has 8 (2 from H's, 6 of its own). Done.
- Carbon (4): equally happy giving up or taking 4 → forms 4 bonds. That's why carbon is the universal builder for life.
Cheat sheet for bonds
The number of bonds an atom usually makes:
- Group 1 (H, Li, Na): 1 bond
- Group 2 (Be, Mg): 2 bonds
- C (group 14): 4 bonds
- N (group 15): 3 bonds (= 8 − 5)
- O (group 16): 2 bonds (= 8 − 6)
- F, Cl (group 17): 1 bond (= 8 − 7)
- Ne, Ar (group 18): 0 bonds (noble — already full)
Rule of thumb: groups 1-4, make as many bonds as your valence;
groups 5-7, make 8 − valence bonds.
Practice
Pick any pair of elements from rows 1-3. Count their valences. Ask yourself: do they want to give, take, or share? That tells you the bond type and the formula.