Periodic table with electron configurations

How to read an electron configuration

Each part of a configuration names a subshell and the number of electrons in it. In 1s² 2s² 2p⁶, the first number is the shell (energy level), the letter is the type of subshell and the superscript is the number of electrons. An s subshell holds up to 2 electrons, p holds 6, d holds 10 and f holds 14, and the superscripts always add up to the atomic number.

Writing out every subshell gets long, so this table uses noble-gas shorthand: the symbol of the previous noble gas in square brackets stands for all of its electrons. Sulfur, 1s² 2s² 2p⁶ 3s² 3p⁴, becomes [Ne] 3s² 3p⁴. The cells are colored by block and each one shows the element's configuration.

The filling order

Electrons fill the lowest-energy subshells first, a rule called the aufbau principle. The order is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, so 4s fills before 3d and 4f only after 6s. You do not need to memorize it: reading each period from left to right takes you through the blocks in filling order, from the s block through the f block (the rows at the bottom, in periods 6 and 7) and the d block to the p block.

This table writes subshells in order of shell number rather than filling order, so iron appears as [Ar] 3d⁶ 4s². When a transition metal forms an ion, it loses its outer s electrons first, which makes Fe²⁺ [Ar] 3d⁶.

Exceptions to the rules

About twenty elements do not follow the simple filling order. The best known are chromium, [Ar] 3d⁵ 4s¹, and copper, [Ar] 3d¹⁰ 4s¹, where one electron moves from 4s into 3d, leaving the d subshell exactly half full or completely full. Silver and gold follow copper's pattern, and palladium, [Kr] 4d¹⁰, has no electrons in its outermost s subshell at all. Configurations for the superheavy elements are calculated predictions rather than measurements.

Worked example: chlorine

The position of an element tells you its configuration. Apart from palladium, the period number is the outermost shell that holds electrons, and the block tells you which subshell is filling. Chlorine is in period 3 and is the fifth element of the p block, so its configuration ends in 3p⁵: 1s² 2s² 2p⁶ 3s² 3p⁵, or [Ne] 3s² 3p⁵. The superscripts add up to 17, chlorine's atomic number, and its outer shell holds 2 + 5 = 7 valence electrons.