What an electron configuration tells you
An electron configuration is the address book for an atom’s electrons: it lists which subshells (1s, 2s, 2p, 3s, …) hold electrons and how many each holds, written as a superscript. Read it left to right and the superscripts add up to the atomic number Z: the number of protons, and in a neutral atom, the number of electrons. Carbon (Z = 6), for example, is 1s² 2s² 2p²: two electrons in each of 1s and 2s, and two of the six possible 2p electrons.
Drag the slider (or use the −/+ buttons, or arrow keys when the slider is focused) to step through every element from hydrogen to krypton and watch the configuration rebuild itself.
The Aufbau order
Electrons fill the lowest-energy subshell available first: the Aufbau (“building-up”) principle. The catch is that energies overlap: the 4s subshell sits slightly below 3d, so it fills first. That gives the famous order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. Each subshell type has a fixed capacity: s holds 2, p holds 6, d holds 10. When written in standard notation, subshells are grouped by shell number, so iron appears as 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s².
Shells and the two exceptions
The Bohr diagram sums each principal shell n, whose capacities are 2, 8, 18, and 32. Iron’s electrons land as 2, 8, 14, 2 across its four shells.
Two elements in this range break the simple pattern, because a half-filled or completely filled 3d subshell is unusually stable. One 4s electron drops into 3d:
- Chromium (Z = 24): [Ar] 3d⁵ 4s¹, not 3d⁴ 4s²
- Copper (Z = 29): [Ar] 3d¹⁰ 4s¹, not 3d⁹ 4s²
Land on either element to see the exception flagged. Turn on noble-gas shorthand to compress the inner core, e.g. iron as [Ar] 3d⁶ 4s².
Using this with a class
Project it and play “name that element” from a configuration, or have students predict Cr and Cu before revealing the exceptions. The widget is free to embed on a class site or LMS page.