Electron Configuration Checker

Chromium and copper are the two that catch everyone — check your answer against the measured configuration.

Noble gas shorthand[Ar] 3d⁵ 4s¹

Electron configuration1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹Chromium, atomic number 24 — written by shell, which is the usual convention

Filling order1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹the order the subshells actually filled, which for the transition metals is not the order above

Valence electrons6counted outside the noble gas core

Electrons per shell2, 8, 13, 1

Subshells occupied7

Total electrons24

Chromium is one of the documented exceptions to the aufbau order: the configuration above is the measured one, not what the n plus l rule predicts. A half-filled or filled d subshell is worth more here than the promotion costs.

The formula

Madelung order: fill by increasing n + l, then increasing n

The n plus l rule

Orbitals fill in order of increasing n + l, and where two are tied, the one with the smaller n goes first. That single rule produces the whole diagonal filling pattern, including the awkward business of 4s filling before 3d: 4s has n + l = 4 while 3d has 5.

The exceptions are real

Chromium is [Ar]3d⁵4s¹ and copper [Ar]3d¹⁰4s¹, not what the rule predicts. Roughly twenty elements break the pattern, mostly where a half-filled or completely filled d subshell is worth more than the promotion costs. This page generates the configuration from the rule and then applies the documented exceptions, so you can see which is which.

Filling order is not removal order

4s fills before 3d but empties first too. Iron(II) is [Ar]3d⁶, not [Ar]3d⁴4s². The orbital energies shift once the d subshell starts to populate, so the order that governed filling no longer applies to ionisation — a point almost every introductory course glosses over.

Models, and where they stop

Slater's rules, the Kapustinskii equation and Pauling's ionic-character expression are all empirical fits, not derivations. They were built to reproduce measured numbers with arithmetic simple enough to do by hand, and they succeed at that within perhaps five or ten per cent. Where a page here quotes one, it says which — a figure from a fitted rule and a figure from a Born-Haber cycle are not the same kind of thing, even when they agree.