Electron configuration (Aufbau diagram)

Electron configuration (Aufbau diagram): what it is for

Type the symbol, name or atomic number and, if you want, the ion charge. The tool fills the electrons in the energy order of the Aufbau diagram, shows the noble gas shorthand, the number of electrons in each shell (K, L, M, N, O, P, Q), the valence shell and the highest-energy subshell. For cations, electrons leave the outermost shell, as the rule says, and the tool warns you when the element is a known exception, such as chromium and copper.

The calculations run in your browser, with IUPAC standard atomic masses.

How to use

1. Type the element.
2. Enter the charge if it is an ion.
3. See the configuration by subshells and shells.

When to use it

• Solving electron configuration exercises.
• Finding the valence shell.
• Studying transition metal ions.

Frequently asked questions

How does the Aufbau diagram work?

It shows the increasing energy order of the subshells: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p. Electrons fill the subshells in that order, with up to 2 electrons in s, 6 in p, 10 in d and 14 in f.

What about ions?

For an anion, add the electrons gained and fill as usual. For a cation, write the configuration of the neutral atom and remove electrons from the outermost shell (highest principal quantum number), not from the last subshell filled. That is why Fe²⁺ loses its 4s electrons before 3d.

Does the configuration always match reality?

For most elements, yes. Some, such as chromium (Cr), copper (Cu), silver (Ag) and gold (Au), have different real configurations because of d-subshell stability. The tool follows the Aufbau diagram, which is what exams ask for, and warns you when the element is one of these exceptions.

Is it really free?

Yes, 100% free. No sign-up, no subscription and no usage limits. All Tools123 tools are free and run right in your browser.

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