Orbital Explorer

by ACAD Online Tuition Center

Electron configuration

Full configuration
Noble-gas (shorthand) notation

Periodic table tap any element

s-blockp-blockd-blockf-blockAufbau exception

Blocks are coloured by the subshell the last electron enters in Aufbau order.

Watch the orbitals fill

Aufbau filling order lowest n + l first; ties → lower n

Orbital box diagram arranged in filling order

Each box is one orbital (labelled with its ml value). = spin up (ms = +½), = spin down (ms = −½). The newest electron glows. Convention used: orbitals fill left to right from ml = −l, first electron spin-up — some textbooks use the opposite order.

Shell distribution Bohr-style model (follows the animation)

    Unpaired electrons & magnetism ground-state atom

    Orbital details ground state

    The rules and notation behind this tool

    Aufbau principle (Madelung n + l rule)

    Electrons occupy the lowest-energy subshell available. Subshells fill in order of increasing n + l; when two subshells have the same value, the one with lower n fills first. That is why 4s (4 + 0 = 4) fills before 3d (3 + 2 = 5), and 3d before 4p (both 5, but 3 < 4).

    Hund's rule of maximum multiplicity

    In a set of equal-energy (degenerate) orbitals, electrons occupy empty orbitals singly with parallel spins before any orbital gets a second electron. This reduces electron–electron repulsion and maximises exchange energy.

    Pauli exclusion principle

    No two electrons in an atom can have the same four quantum numbers (n, l, ml, ms). So one orbital holds at most two electrons, and they must have opposite spins (+½ and −½).

    Notation conventions used

    • Superscript = number of electrons in that subshell (e.g. 2p6); subshell letters s, p, d, f are written upright, as IUPAC recommends.
    • Noble-gas core in square brackets: [Ar] 3d6 4s2.
    • “By shell” lists subshells by n then l (the style of NIST/IUPAC tables and NCERT). “Filling order” lists them in Aufbau order (common in many textbooks). Both describe the same atom.
    • Empty subshells are omitted (Pd is [Kr] 4d10, not 4d10 5s0).

    The 20 exceptions

    Ground-state configurations here come from atomic spectroscopy (as tabulated by NIST). Twenty elements up to Lr differ from the simple Aufbau prediction: Cr, Cu, Nb, Mo, Ru, Rh, Pd, Ag, La, Ce, Gd, Pt, Au, Ac, Th, Pa, U, Np, Cm and Lr. Aufbau is a useful approximation — the real orbital energies shift as electrons are added. Configurations for Z ≥ 104 are theoretical predictions.

    Good to know for exams

    • When atoms form cations, electrons leave the highest n first: Fe → Fe2+ loses its 4s electrons, giving [Ar] 3d6.
    • Spin-only magnetic moment μ = √(n(n+2)) BM, where n = unpaired electrons (most used for transition-metal ions).
    • Keyboard: Space play/pause, ← → step, [ ] previous/next element, / search.