What is an electron configuration?

An electron configuration describes how an atom’s electrons are arranged in shells and subshells. It is written with the shell number (n), the subshell letter (s, p, d, f) and the number of electrons as a superscript. Oxygen, with 8 electrons, is 1s² 2s² 2p⁴.

The configuration tells you the valence electrons, the element’s position in the periodic table and the ions it is likely to form, which is why it underpins chemical bonding and periodic trends.

Shells and subshells

Electrons occupy the K, L, M, N... shells (n = 1, 2, 3, 4...). A shell holds at most 2n² electrons. Each shell is split into subshells, and each orbital holds at most 2 electrons.

Shell	n	Subshells	Maximum (2n²)
K	1	1s	2
L	2	2s 2p	8
M	3	3s 3p 3d	18
N	4	4s 4p 4d 4f	32

Subshell	Orbitals	Maximum electrons
s	1	2
p	3	6
d	5	10
f	7	14

The three filling rules

1. The Aufbau principle

Electrons fill subshells from the lowest energy upward, following the n + l rule; when n + l is equal, the subshell with the smaller n fills first:

1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d → 7p

The diagonal diagram is an easy way to remember it: write the subshells of each shell in rows and draw diagonal arrows from top right to bottom left. That is why 4s fills before 3d.

2. The Pauli exclusion principle

No two electrons in an atom can share all four quantum numbers, so one orbital holds at most two electrons with opposite spins (↑↓).

3. Hund’s rule

Within a set of equal-energy orbitals, electrons occupy empty orbitals singly with parallel spins before pairing up. Nitrogen (2p³) has three unpaired electrons: ↑ ↑ ↑, while oxygen (2p⁴) has one paired orbital: ↑↓ ↑ ↑.

Configuration by shell (Bohr model)

School textbooks often write electrons per shell, for example sodium 2 8 1 and chlorine 2 8 7. For main-group elements, each shell holds at most 2n² electrons and the outer shell at most 8, so potassium is 2 8 8 1 rather than 2 8 9.

For transition metals, derive the shell counts from the subshell configuration. Take iron, [Ar] 4s² 3d⁶: the M shell holds 3s² 3p⁶ 3d⁶ = 14 electrons and the N shell holds 4s², giving 2 8 14 2. The generator above shows the shell counts for every element.

Noble-gas notation

Long configurations are shortened with the previous noble gas in square brackets: sodium is [Ne] 3s¹ and iron is [Ar] 4s² 3d⁶. The cores are He (2), Ne (10), Ar (18), Kr (36), Xe (54) and Rn (86).

Exceptions to the Aufbau principle

Some elements break the filling order. The best known are chromium and copper: a half-filled (d⁵) or filled (d¹⁰) d subshell is more stable, so one 4s electron moves into 3d. Among the lanthanides and actinides, f and d subshells are so close in energy that other exceptions appear. The full list, based on PubChem data:

Electron configuration of ions

Cations form when an atom loses electrons. Electrons are removed from the outermost shell (largest n) first, not from the subshell filled last. So Fe²⁺ is [Ar] 3d⁶ and Fe³⁺ is [Ar] 3d⁵: both 4s electrons go first.

Anions form when an atom gains electrons, which fill the next subshell. Cl (2 8 7) becomes Cl⁻ [Ne] 3s² 3p⁶ = [Ar], like argon. Type Fe3+ or O2- in the generator to try it.

Finding valence electrons, group and period

• Period = the highest occupied shell number.
• Main groups: valence electrons = ns + np. Sodium [Ne] 3s¹ has 1, so it is in group 1, period 3.
• Transition metals: valence electrons = ns + (n−1)d. Iron 4s² 3d⁶ has 8, so it is in group 8, period 4.
• Block = the last subshell being filled: s, p, d or f.

Worked examples

Electron configurations of elements 1–36

For elements 37–118 use the generator above, the interactive periodic table or each element’s page.

Frequently asked questions

What is an electron configuration?

The arrangement of an atom’s electrons in shells and subshells, for example O = 1s² 2s² 2p⁴. It determines valence electrons and chemical behaviour.

What is the order of filling?

By the Aufbau principle: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d → 7p.

Why does 4s fill before 3d?

Because 4s has a lower n + l value (4 + 0 = 4) than 3d (3 + 2 = 5). When ions form, however, 4s electrons are removed first.

Which elements are exceptions?

In period 4: Cr and Cu. In period 5: Nb, Mo, Ru, Rh, Pd and Ag. In period 6: Pt and Au, plus several lanthanides and actinides. See the table above.

How do I find the group from the configuration?

Count the valence electrons: ns + np for main groups, ns + (n−1)d for transition metals. The period is the highest shell number.