Electron Configuration Calculator

Electron configuration is the distribution of electrons in an atom across orbitals (subshells), written as 1s2 2s2 2p6... etc. Each subshell designation is: principal quantum number n, subshell letter (s, p, d, f), and superscript electron count. The configuration determines chemical reactivity and bonding behavior.

Aufbau (German for "building up") is the principle that electrons fill orbitals from lowest to highest energy. The filling order is: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p. Note that 4s fills before 3d because at typical atomic sizes, 4s is slightly lower in energy.

Chromium (Z=24) is [Ar] 3d5 4s1 instead of the expected [Ar] 3d4 4s2, and copper (Z=29) is [Ar] 3d10 4s1 instead of [Ar] 3d9 4s2. Both exceptions arise because half-filled (d5) and fully filled (d10) d subshells have extra stability from exchange energy. The energy gained by achieving these configurations exceeds the cost of leaving 4s with one electron.

Count the electrons in the highest occupied principal shell (the largest n). For a main-group element that means the electrons in the outermost s and p subshells; chlorine, [Ne] 3s2 3p5, has 2 + 5 = 7 valence electrons. Transition metals also count their outermost d electrons, which is why they show several oxidation states. Valence electrons determine bonding and reactivity, so elements in the same column share a valence count and similar chemistry.

Noble gas or condensed notation abbreviates the filled inner shells by replacing them with the symbol of the preceding noble gas in brackets. Iron's full 1s2 2s2 2p6 3s2 3p6 4s2 3d6 becomes [Ar] 4s2 3d6, since argon supplies the first 18 electrons. It keeps the focus on the chemically active outer electrons and is faster to write for heavier elements.