Electron Configuration Calculator

Electron configuration is the notation that describes how electrons are distributed among an atom's orbitals in its ground (lowest-energy) state, written as a sequence of subshells (like 1s, 2s, 2p) each followed by a superscript number of electrons. For example, oxygen's electron configuration is 1s2 2s2 2p4, showing 8 total electrons distributed across three filled or partially filled subshells.

The Aufbau principle (German for building-up) states that electrons fill atomic orbitals starting from the lowest available energy level and moving upward, one at a time, respecting the Pauli exclusion principle and Hund's rule. It is combined with the n+l rule (Madelung rule) to predict the correct order in which subshells fill as atomic number increases.

The n+l rule states that subshells fill in order of increasing (n + l) value, where n is the principal quantum number and l is the angular momentum quantum number (0 for s, 1 for p, 2 for d, 3 for f). When two subshells have the same n+l sum, the one with the lower n fills first. This is why 4s (n+l = 4+0 = 4) fills before 3d (n+l = 3+2 = 5), even though 3 is a smaller shell number than 4.

Noble-gas shorthand notation abbreviates a long electron configuration by replacing the electrons that match a smaller noble gas's full configuration with that noble gas's symbol in square brackets. For example, iron's full configuration 1s2 2s2 2p6 3s2 3p6 3d6 4s2 becomes the shorthand [Ar] 3d6 4s2, since argon's configuration (1s2 2s2 2p6 3s2 3p6) accounts for iron's first 18 electrons.

Chromium (expected 4s2 3d4) is actually 4s1 3d5, and copper (expected 4s2 3d9) is actually 4s1 3d10, because a half-filled (5 electrons) or fully-filled (10 electrons) d-subshell provides extra stability from more favourable electron exchange energy, which outweighs the small energy cost of moving one electron out of 4s. These are the two most commonly taught Aufbau exceptions.

Yes. Similar half-filled or fully-filled d-subshell exceptions occur for niobium, molybdenum, ruthenium, rhodium, and silver in period 5, and for platinum and gold in period 6, all following the same half-filled or fully-filled stability pattern. This calculator applies these common d-block exceptions. Some rarer exceptions exist deeper in the lanthanide and actinide series that this simplified calculator does not model.

An s subshell holds a maximum of 2 electrons, a p subshell holds up to 6, a d subshell holds up to 10, and an f subshell holds up to 14. These maximums come from the number of orbitals in each subshell (1, 3, 5, and 7 respectively) multiplied by 2 electrons per orbital, following the Pauli exclusion principle.

This calculator computes the ground-state configuration for a neutral atom given its atomic number. For a cation, you can approximate its configuration by looking up the neutral atom with (Z - charge) electrons, though real ion configurations sometimes remove electrons from a different subshell than a naive electron count would suggest (for example, transition metal cations lose s electrons before d electrons).

Electron configuration is written notation (like 1s2 2s2 2p4) showing how many electrons occupy each subshell. An orbital diagram is a visual representation using boxes or lines for individual orbitals with arrows showing electron spin, which additionally shows how electrons are distributed among degenerate orbitals within a subshell according to Hund's rule. Configuration notation alone does not show this individual-orbital detail.

The note tells you whether the element you looked up follows the standard n+l filling order exactly, or whether it is one of the known d-block exceptions (chromium, copper, and similar elements) where the true ground state differs from the naive prediction. This distinction is a common source of confusion and exam questions in introductory chemistry.

Yes, the input is not case-sensitive, so fe, Fe, and FE all resolve to iron. You can also simply type the atomic number instead of the symbol; both input styles are accepted interchangeably.