How to write electron configurations, step by step

What the notation means

Each part of a configuration has three pieces. In 2p⁴, the 2 is the shell (the principal energy level), the letter p is the type of subshell, and the superscript 4 is the number of electrons in that subshell. The superscripts always add up to the total number of electrons, which for a neutral atom equals the atomic number.

There are four types of subshell, each with a fixed capacity: s holds 2 electrons (1 orbital), p holds 6 (3 orbitals), d holds 10 (5 orbitals) and f holds 14 (7 orbitals). Every orbital holds at most 2 electrons.

The aufbau order

Electrons fill the lowest-energy subshells first. This is the aufbau principle, from the German word for “building up.” The filling order is 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p.

Notice that 4s fills before 3d, and 6s fills before 4f. The n + l rule explains this: add the shell number n to l (0 for s, 1 for p, 2 for d, 3 for f) and fill from the lowest total up; on a tie, the smaller n fills first. Many students instead draw a diagonal chart of subshells and follow the arrows.

The periodic table shows the same order: across a period you pass through the s-block (groups 1–2), the d-block (groups 3–12, one shell lower) and the p-block (groups 13–18), with the f-block (two shells lower) fitting into periods 6 and 7.

Worked example: oxygen

Oxygen has atomic number 8, so it has 8 electrons to place. Put 2 in 1s (6 left), then 2 in 2s (4 left), then the last 4 in 2p, which could hold up to 6. The result is 1s² 2s² 2p⁴. Check: 2 + 2 + 4 = 8.

The highest shell is 2, with 2 + 4 = 6 electrons, so oxygen has 6 valence electrons. Inside the 2p subshell, Hund’s rule says electrons occupy the three p orbitals singly before pairing up, so an oxygen atom has two unpaired electrons.

Worked example: iron

Iron has atomic number 26, so it has 26 electrons. Fill in the aufbau order: 1s² (2 placed), 2s² (4), 2p⁶ (10), 3s² (12), 3p⁶ (18), 4s² (20), and the remaining 6 go into 3d. The full configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶.

Many textbooks, and this site, list the subshells in order of shell number instead: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s². Both describe the same atom. When iron forms ions, it loses its 4s electrons first, so Fe²⁺ is [Ar] 3d⁶ and Fe³⁺ is [Ar] 3d⁵.

Noble-gas shorthand

Writing every subshell gets long for heavy atoms, so chemists replace the inner electrons with the symbol of the previous noble gas in square brackets. [He] stands for 1s², [Ne] for 1s² 2s² 2p⁶ and [Ar] for 1s² 2s² 2p⁶ 3s² 3p⁶. The heavier cores are [Kr] (36 electrons), [Xe] (54) and [Rn] (86).

To use it, write the previous noble gas in brackets and add only the electrons that come after it. Oxygen becomes [He] 2s² 2p⁴, sodium becomes [Ne] 3s¹ and iron becomes [Ar] 3d⁶ 4s².

Exceptions: chromium and copper

A few elements do not follow the aufbau order exactly. Chromium (24 electrons) is predicted to be [Ar] 3d⁴ 4s², but its actual configuration is [Ar] 3d⁵ 4s¹. Copper (29 electrons) is predicted to be [Ar] 3d⁹ 4s², but it is actually [Ar] 3d¹⁰ 4s¹.

In both cases one 4s electron moves into 3d. The 4s and 3d subshells are very close in energy, and a half-filled (d⁵) or completely filled (d¹⁰) d subshell is especially stable. Some heavier elements behave the same way: molybdenum is [Kr] 4d⁵ 5s¹, silver is [Kr] 4d¹⁰ 5s¹ and gold is [Xe] 4f¹⁴ 5d¹⁰ 6s¹. Around 20 elements in the d- and f-blocks have irregular configurations, so check a reliable table for heavier elements.