7. Electron Configuration

Shells and how they fill

Electrons sit in shells (energy levels) around the nucleus. Shell 1 is closest and lowest in energy.

For the first 20 elements, use this rule:

Shell	Holds up to
1st	2 electrons
2nd	8 electrons
3rd	8 electrons (for the first 20 elements)
4th	the rest

The electron configuration lists how many electrons are in each shell, separated by commas.

The first 20 elements

Element	Z	Configuration	Element	Z	Configuration
H	1	1	Na	11	2, 8, 1
He	2	2	Mg	12	2, 8, 2
Li	3	2, 1	Al	13	2, 8, 3
Be	4	2, 2	Si	14	2, 8, 4
B	5	2, 3	P	15	2, 8, 5
C	6	2, 4	S	16	2, 8, 6
N	7	2, 5	Cl	17	2, 8, 7
O	8	2, 6	Ar	18	2, 8, 8
F	9	2, 7	K	19	2, 8, 8, 1
Ne	10	2, 8	Ca	20	2, 8, 8, 2

Bohr diagrams

A Bohr diagram shows the nucleus in the centre and the electrons as dots or crosses on circles.

Oxygen has 6 outer electrons. Sodium has 1 outer electron (shown in red).

Valence electrons

The electrons in the outer shell are called valence electrons. They are the ones that take part in reactions.

For groups 1 and 2, the number of valence electrons equals the group number.
For groups 13 to 18, it is the group number minus 10 (so group 17 has 7).
The number of shells used equals the period (row) number.

Sodium is 2, 8, 1: 3 shells, so period 3. 1 valence electron, so group 1.

Why atoms react: the full outer shell

Atoms with a full outer shell are very stable. The noble gases (helium 2, neon 2, 8, argon 2, 8, 8) already have one, which is why they hardly react at all.

Other atoms gain, lose or share electrons to get a full outer shell, like a noble gas.

Sodium 2, 8, 1 loses 1 electron: $N a^{+}$ is 2, 8, like neon.
Chlorine 2, 8, 7 gains 1 electron: $C l^{-}$ is 2, 8, 8, like argon.

Electron configuration

Shells fill inside out

1st: 2
2nd: 8
3rd: 8 (first 20)

Writing it

Sodium 2, 8, 1
Calcium 2, 8, 8, 2

Valence electrons

Outer shell
Gives the group
Shells give the period

Stability

Full outer shell
Noble gases

Ions

Metals lose e⁻
Non-metals gain e⁻

Year 11Subshells: s, p, d and f

Each shell is made of subshells. Each subshell holds a set number of electrons.

Subshell	Holds
s	2
p	6
d	10
f	14

Shell 1 has 1s. Shell 2 has 2s and 2p (2 + 6 = 8). Shell 3 has 3s, 3p and 3d (2 + 6 + 10 = 18).

Subshells fill in order of energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p... Notice that 4s fills before 3d. That is why potassium is 2, 8, 8, 1 and not 2, 8, 9.

Sodium: $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{1}$
Chlorine: $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{5}$
Iron (26): $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 4 s^{2} 3 d^{6}$

Quick summary

Electrons fill shells from the inside out: 2, then 8, then 8, for the first 20 elements.
Write configurations like 2, 8, 1 (sodium).
Valence electrons are in the outer shell. They decide how an atom reacts and its group.
Number of shells = period number.
Atoms gain, lose or share electrons to get a full outer shell like a noble gas.

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