How to know when an outer valence shell is filled or not:
🧠 The basic rule: 8 electrons = full outer shell
For most elements you're learning in intro biology/chemistry:
An atom wants 8 electrons in its outer (valence) shell.
There are exceptions, especially hydrogen and helium, which are full with 2 electrons.
🔢 How do you know how many valence electrons an atom has?
Look at its group (column) on the periodic table:
Group | Valence electrons | What it wants to do |
|---|---|---|
1 | 1 | Lose 1 → +1 |
2 | 2 | Lose 2 → +2 |
13 | 3 | Usually lose 3 → +3 |
14 | 4 | Share 4 |
15 | 5 | Gain 3 → −3 |
16 | 6 | Gain 2 → −2 |
17 | 7 | Gain 1 → −1 |
18 | 8 | Already full! |
⭐ Super important pattern:
1st shell → holds up to 2
2nd shell → holds up to 8
3rd shell → usually think up to 8 for basic chemistry
So:
Na = 2, 8, 1 → wants to lose 1
Cl = 2, 8, 7 → wants to gain 1
Ne = 2, 8 → already FULL ✅
🧪 Let's use Na and Cl
Sodium (Na) is in Group 1.
Na has 1 valence electron:
Na: 2, 8, 1
This means:
1st shell: 2 electrons ✅
2nd shell: 8 electrons ✅
3rd shell: 1 electron ← outer/valence shell
So sodium has 11 total electrons: 2 + 8 + 1 = 11
And that last 1 is important because it's the electron sodium can easily give away.
The outer shell has 1, so it's NOT full.
It can lose that 1 electron:
Na → Na⁺
Now it looks like:
2, 8 ✅
Its outer shell is full!
Chlorine (Cl) is in Group 17.
Cl has 7 valence electrons:
Cl: 2, 8, 7
1st shell → 2
2nd shell → 8
3rd (outer) shell → 7
2 + 8 + 7 = 17 electrons
The outer shell has 7 electrons, so it's NOT full
It wants 1 more to reach 8:
Cl + 1 electron → Cl⁻
Now:
2, 8, 8 ✅
Its outer shell is full!
⭐ The easiest way to remember it
Think of the outer shell like 8 seats on a bus 🚌:
1–7 electrons → seats aren't full
8 electrons → FULL ✅
Atoms will lose, gain, or share electrons to try to get to 8.
That's why Na gives 1 electron to Cl:
Na (1) → gives 1 → Na⁺
Cl (7) → takes 1 → Cl⁻
Both end up with 8 electrons in their outer shell.