Electron Configuration Calculator

Electrons fill atomic orbitals based on the Aufbau principle, the Pauli exclusion principle, and Hund's rule. The Aufbau principle dictates that electrons occupy the lowest available energy orbitals first, following the Madelung rule (n + l order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, etc.). Each orbital type holds a maximum capacity: s-orbitals hold 2, p-orbitals hold 6, d-orbitals hold 10, and f-orbitals hold 14 electrons. Noble gas shorthand simplifies this sequence by substituting the core electron configuration with the symbol of the nearest preceding noble gas in brackets.

Let us determine the electron configuration for sulfur (S), which has an atomic number (Z) of 16. This means a neutral sulfur atom contains 16 electrons. Following the orbital filling sequence, we distribute the 16 electrons as follows: First, the 1s orbital takes 2 electrons (1s²). Next, the 2s orbital takes 2 electrons (2s²). The 2p orbital fills its maximum of 6 electrons (2p⁶). Then, the 3s orbital takes 2 electrons (3s²). At this point, we have used 2 + 2 + 6 + 2 = 12 electrons. We have 4 electrons remaining, which go into the 3p orbital (3p⁴). Thus, the full expanded electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁴. For the noble gas notation, we look at the preceding noble gas, neon (Ne), which accounts for 1s² 2s² 2p⁶. Replacing those core electrons with [Ne], we get the shorthand configuration: [Ne] 3s² 3p⁴.

Always double-check your total electron count against the atomic number of the element for neutral atoms. Remember the notable exceptions in the d-block, such as chromium and copper, where half-filled or fully filled subshells provide extra stability, causing an electron to shift from the ns to the (n-1)d orbital. When dealing with ions, remember to add electrons for anions and remove electrons from the highest principal quantum number (n) orbital first for cations, rather than strictly reversing the filling order.