Electron configuration
Full and noble-gas configurations, orbital diagrams and unpaired electrons for any element or ion, including Cr, Cu and transition-metal ions.
Type a symbol, name or atomic number. Add a charge for an ion, like Fe3+, O2-, Cl- or iron(III).
Writing electron configurations
An electron configuration lists the occupied subshells (1s, 2s, 2p…) with the number of electrons in each as a superscript. Three rules build it. The Aufbau principle: fill subshells from lowest energy up, in the order 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p. The Pauli exclusion principle: an orbital holds at most two electrons, with opposite spins, so s holds 2, p holds 6, d holds 10 and f holds 14. Hund’s rule: within a subshell, electrons spread out one per orbital before any pair up.
The noble-gas shorthand replaces the inner electrons with the symbol of the previous noble gas in brackets. Iron, 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶, becomes [Ar] 4s² 3d⁶.
Ions. For an anion, add electrons in the normal order. For a cation, remove electrons from the highest shell number (n) first. This is why Fe²⁺ is [Ar] 3d⁶, not [Ar] 4s² 3d⁴: the 4s electrons leave before the 3d ones.
Order. Some textbooks write subshells in filling order (4s before 3d); others group them by shell (3d before 4s). Both are correct. Use the toggle to match your course.
Exceptions. Chromium and copper are the two most tested: Cr is [Ar] 4s¹ 3d⁵ and Cu is [Ar] 4s¹ 3d¹⁰, because half-filled and filled d subshells are extra stable. The tool uses measured ground states for every element.
Worked examples
Oxygen (8 electrons). Fill 1s with 2, 2s with 2, and put the last 4 in 2p: 1s² 2s² 2p⁴, or [He] 2s² 2p⁴. In the orbital diagram, the first three 2p electrons go into separate boxes and the fourth pairs up, leaving 2 unpaired electrons, so oxygen is paramagnetic. It has 6 valence electrons (2s² 2p⁴).
Iron(III), Fe³⁺ (23 electrons). Start from neutral iron, [Ar] 4s² 3d⁶. Remove the two 4s electrons first, then one 3d electron: [Ar] 3d⁵. All five 3d electrons are unpaired.
Frequently asked questions
What is the difference between valence electrons and the electron configuration?
The configuration lists every electron. Valence electrons are only the ones in the outermost shell, the highest n. For main-group elements that’s the s and p electrons of the last shell: chlorine, [Ne] 3s² 3p⁵, has 7. For transition metals, the outer s and the d electrons both take part in bonding, so the count depends on the reaction.
Why does 4s fill before 3d but empty first?
In a neutral atom, 4s is slightly lower in energy than 3d at the point where it fills, so it fills first. Once the 3d subshell holds electrons, 3d drops below 4s, so ionization removes 4s electrons first. That’s why transition-metal ions such as Fe²⁺, Cu²⁺ and Zn²⁺ have no 4s electrons.
What does paramagnetic mean?
An atom or ion with one or more unpaired electrons is paramagnetic and is weakly pulled into a magnetic field. If every electron is paired, it is diamagnetic. The tool counts unpaired electrons from the orbital diagram.
What does isoelectronic mean?
Two species are isoelectronic if they have the same number of electrons and the same configuration. Na⁺, Mg²⁺, F⁻, O²⁻ and Ne all have 10 electrons: 1s² 2s² 2p⁶.
How do I write the configuration with superscripts in my homework?
Use the Copy button. It copies the configuration with real superscript characters (like 2p⁴) that paste into Word, Google Docs and most homework sites.