Electron Configuration Generator

Valence Electrons follows the group-number convention: electrons added since the preceding noble-gas core (here, argon, Z=18) count as valence until the element's own outermost p subshell begins filling. Iron's configuration is [Ar]3d6 4s2 -- no 4p electrons yet -- so all 8 electrons added since argon (the 4s2 and the 3d6) count as valence, matching iron's periodic-table group (group 8). Core Electrons is the remaining 18, exactly argon's electron count.

Yes. It applies all 20 experimentally-confirmed ground-state exceptions at Z <= 103, cross-checked against NIST's Atomic Spectra Database (the standard reference for measured atomic ground states and ionization energies): chromium, copper, niobium, molybdenum, ruthenium, rhodium, palladium, silver, lanthanum, cerium, gadolinium, platinum, gold, actinium, thorium, protactinium, uranium, neptunium, curium, and lawrencium. Chromium (Z=24) shows 4s1 3d5 and copper (Z=29) shows 4s1 3d10 -- the experimentally observed configurations, not the naive 4s2 3d4 / 4s2 3d9 that strict Aufbau filling alone would predict. Beyond Z=103, superheavy elements are short-lived and synthetic, so this calculator reports the theoretical Aufbau/Madelung order for them rather than a further hand-verified exception list.

It applies Hund's rule -- electrons spread singly across all orbitals in a subshell before any pairing -- to every partially-filled subshell in the ground-state configuration and sums the result, which is the standard way to count unpaired electrons for a multi-electron atom. This correctly handles elements with more than one simultaneously partially-filled subshell, such as gadolinium (4f7 5d1: 7 unpaired from the half-filled f subshell plus 1 from the d subshell, 8 total). It does not model relativistic or spin-orbit-coupling exceptions beyond the ground-state configuration itself.

It flags exactly seven atomic numbers -- 2, 10, 18, 36, 54, 86, and 118 -- where an electron shell closes completely at that Z under strict Aufbau filling. It does not attempt any broader chemical-reactivity judgment; it's purely a lookup against those seven noble-gas atomic numbers, so an element one proton away from any of them (like fluorine at Z=9 or sodium at Z=11) correctly reports false even though both are close neighbors of neon.

Atomic Number is rounded to the nearest whole number, then clamped to the periodic table's actual range of 1-118. A value of 0, a negative number, or a blank field returns hydrogen's configuration (1s1); a value of 119 or higher returns oganesson (Z=118) and reports Noble Gas? as Yes. The calculator does not raise an error for an out-of-range input, so a caller feeding it through the API or embed should validate the value first if it needs to distinguish "hydrogen" from "invalid input."