1.3.4—Electronic configurations to include

Read an electron configuration from left to right: the leading number identifies the principal shell, the letter identifies the sub-shell, and the superscript gives the number of electrons in that sub-shell. For example, 2p⁴ means four electrons in the p sub-shell of shell n = 2.

Determine the species’ electron count before checking the notation: a neutral atom has electrons equal to its atomic number, a positive ion has lost electrons, and a negative ion has gained electrons. The superscripts in a complete configuration must add to that count.

A full configuration lists occupied sub-shells from the beginning; shorthand replaces the inner-electron part with the nearest noble-gas symbol in brackets. The outer occupied sub-shells identify the valence-electron region relevant to the species’ outer electronic structure.

Use the notation to report electron arrangement, not as a substitute for the separate filling-order rules. Check charge and total superscripts before accepting an ion configuration, and do not confuse a sub-shell superscript with an orbital count or with the atomic number.