Electron Configuration Calculator
Enter an atomic number (1 through 118) to get the full electron configuration and noble gas shorthand notation. The calculator follows the aufbau principle, Hund's rule, and the Pauli exclusion principle, with corrections for known exceptions like chromium and copper.
Electron configuration describes how electrons are distributed among an atom's orbitals. It determines chemical behavior, bonding patterns, and placement on the periodic table.
The Aufbau Filling Order
Electrons fill orbitals from lowest to highest energy:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p
Each subshell holds a fixed number of electrons: s = 2, p = 6, d = 10, f = 14.
Three Key Rules
- Aufbau Principle -- electrons fill the lowest-energy orbital first
- Pauli Exclusion Principle -- each orbital holds at most 2 electrons with opposite spins
- Hund's Rule -- electrons occupy empty orbitals in a subshell before pairing up
Worked Example: Iron (Z = 26)
Full configuration: 1s2 2s2 2p6 3s2 3p6 4s2 3d6
Noble gas shorthand: [Ar] 4s2 3d6
Iron has 8 valence-region electrons (2 in 4s and 6 in 3d), though only the 2 in 4s are considered "valence" for main-group counting.
Common Exceptions
Some elements break the pattern because half-filled or fully filled d subshells are extra stable:
| Element | Expected | Actual |
|---|---|---|
| Cr (24) | [Ar] 4s2 3d4 | [Ar] 4s1 3d5 |
| Cu (29) | [Ar] 4s2 3d9 | [Ar] 4s1 3d10 |
| Mo (42) | [Kr] 5s2 4d4 | [Kr] 5s1 4d5 |
| Ag (47) | [Kr] 5s2 4d9 | [Kr] 5s1 4d10 |
| Au (79) | [Xe] 6s2 4f14 5d9 | [Xe] 6s1 4f14 5d10 |