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Atomic Structure

Electron Configuration

OxfordAQA International AS and A-level Chemistry

Shells, sub-shells and orbitals

Electrons occupy shells. A shell is divided into sub-shells, and a sub-shell is made of orbitals. One orbital holds 2 electrons, in any sub-shell.

Sub-shellOrbitals in itElectrons it holdsLowest shell it appears in
s121
p362
d5103

A superscript counts the electrons in the whole sub-shell, not in one orbital: 3d10 is 5 full orbitals, and 3d9 is four full orbitals and one holding a single electron.

The order the sub-shells fill

SUB-SHELL ENERGY energy MAX 4p 6 3d 10 4s 2 3p 6 3s 2 2p 6 2s 2 1s 2 4s is below 3d, so 4s fills first fill from the bottom up: 1s first, 4p last
4s lies below 3d on the energy axis, so 4s fills first when the atom is built.
Method: full configuration of an atom
  1. The number of electrons equals the proton number.
  2. Fill in order of increasing energy, 1s 2s 2p 3s 3p 4s 3d 4p, taking each sub-shell to its maximum before starting the next.
  3. Stop when the superscripts add up to the number of electrons.
AtomFull electron configuration
Mg1s2 2s2 2p6 3s2
Zn1s2 2s2 2p6 3s2 3p6 4s2 3d10
Ga1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p1

Where the opening of a configuration is already given as 1s2, [Ne] or [Ar], carry it on from where it stops: sulfur is [Ne] 3s2 3p4.

Writing the configuration of an ion

Method: full configuration of an ion
  1. Write the full configuration of the neutral atom first.
  2. For a positive ion, take electrons out of the outermost shell: in a transition metal that is the 4s before the 3d.
  3. For a negative ion, put the electrons into the next spaces in the filling order.
  4. Check the superscripts add up to the proton number minus the charge.
IonFull electron configurationElectrons removed or added
Al3+1s2 2s2 2p6the 3p, then both 3s
N3−1s2 2s2 2p6three into 2p
Ti+1s2 2s2 2p6 3s2 3p6 4s1 3d2one 4s, the other 4s stays
Mn2+1s2 2s2 2p6 3s2 3p6 3d5both 4s, 3d untouched
Fe3+1s2 2s2 2p6 3s2 3p6 3d5both 4s, then one 3d
Ge+1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p1one 4p, the outermost sub-shell

The charge on an ion also fixes the formula of its compounds, because the charges balance: lithium nitride is Li3N and beryllium nitride is Be3N2.

Copper and chromium

The filling order alone would give copper a full 4s. A copper atom in fact has a single 4s electron and a full 3d sub-shell.

SpeciesFull electron configuration
Cu1s2 2s2 2p6 3s2 3p6 4s1 3d10
Cu+1s2 2s2 2p6 3s2 3p6 3d10
Cu2+1s2 2s2 2p6 3s2 3p6 3d9

Chromium breaks the filling order the same way: 1s2 2s2 2p6 3s2 3p6 4s1 3d5.

Working back from a configuration

Method: reading a configuration backwards
  1. Add the superscripts for the number of electrons in the species.
  2. For an atom, that total is the proton number, which names the element.
  3. For a positive ion, add the charge to that total to get the proton number.

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