Electron configuration
Pick any element or ion. Watch the boxes fill.
Element or ion
Type a symbol, a name or an atomic number, add a charge such as Fe3+ or Cl-, or tap an element in the table. Orbitals fill from m = −l up, spin +½ first.
FeIron, Z = 26, 55.845 u
[Ar] 3d6 4s2
In full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s²
- Blockd block
- Period4
- Group8
- Valence8
- Unpaired4
- MagneticPara
Last electron 3d
- n3shell
- l2d subshell
- m−2orbital
- s−½spin down
Pick an element
Period 1
Period 3
Period 4
Period 5
Pick an ion
The rules behind it: aufbau principle, quantum numbers. Same subject: organic conversions, name reactions, oxidation numbers and redox, bonding and shapes, balance equations, mole calculator.
Follow the arrows
Lower (n + l) fills first. For equal (n + l), lower n first.
- 1s
- 2s
- 2p
- 3s
- 3p
- 4s
- 3d
- 4p
- 5s
- 4d
- 5p
- 6s
- 4f
- 5d
- 6p
- 7s
- 5f
- 6d
- 7p
Electronic configuration
How do you write the electronic configuration of an element?
Fill the subshells in order of increasing (n + l): 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d and so on. When two subshells have the same (n + l), the one with the lower n fills first. Each s holds 2 electrons, p holds 6, d holds 10 and f holds 14. Then write the subshells in order of n, for example iron is [Ar] 3d⁶ 4s².
Why are chromium and copper exceptions?
A half-filled or completely filled d subshell is extra stable, because of its symmetry and its larger exchange energy. So chromium is [Ar] 3d⁵ 4s¹ instead of 3d⁴ 4s², and copper is [Ar] 3d¹⁰ 4s¹ instead of 3d⁹ 4s².
How do you write the configuration of an ion?
Start from the neutral atom. For a positive ion, take electrons away from the highest n first, so 4s empties before 3d: Fe is [Ar] 3d⁶ 4s² and Fe³⁺ is [Ar] 3d⁵. For a negative ion, add electrons in the usual filling order: Cl⁻ is [Ne] 3s² 3p⁶.
Which convention does the tool use for the quantum numbers of the last electron?
Within a subshell the orbitals fill from m = −l up to m = +l, each first with spin +½, and then again from m = −l with spin −½. Some books fill from m = +l downwards; the n and l values are the same either way, only m and s can differ.
How do you find the number of unpaired electrons?
Draw the boxes of the partly filled subshell and put one electron in each box before pairing any (Hund’s rule). The single arrows are the unpaired electrons. An atom or ion with unpaired electrons is paramagnetic; with none it is diamagnetic.