Electron Configuration
How the electrons of an atom or ion are arranged: the full configuration, the noble-gas shorthand, valence and unpaired electrons, with the orbital boxes filled one by one. Including the real exceptions such as chromium and copper.
The Three Rules
Aufbau: electrons fill the subshells from low to high energy, in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d … (follow the diagonals). Pauli: one orbital (box) holds at most two electrons, with opposite spins. Hund: within a subshell, electrons first spread over the empty boxes one by one before they pair up.
A subshell has room for 2 (s), 6 (p), 10 (d) or 14 (f) electrons. The outermost electrons, the valence electrons, decide how an element reacts, which is why elements in one column of the periodic table behave alike.
Filling the Orbitals, Step by Step
FeKey Ideas
With ℓ = 0, 1, 2, 3 for s, p, d, f. That is why 4s (4 + 0) fills before 3d (3 + 2).
A positive ion loses its outermost electrons first: for transition metals the 4s electrons go before the 3d ones. A negative ion gains electrons in the normal filling order.
A half-filled or completely filled d subshell is extra stable, so one 4s electron moves to 3d. The calculator uses the measured ground states.
Sources & Verification
- NIST, Periodic Table: Atomic Properties of the Elements (NIST SP 966) — measured ground-state configurations of the neutral atoms.
- N. N. Greenwood & A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, 1997 — Ch. 2 (the Aufbau principle and its exceptions).
- IUPAC, Periodic Table of the Elements (2022) — group 3 = Sc, Y, Lu, Lr; La–Yb and Ac–No as the f-block.
Frequently Asked Questions
How do you write an electron configuration?
Fill the subshells from low to high energy in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s … until all electrons are placed; an s subshell holds 2, p 6, d 10 and f 14 electrons. Oxygen has 8 electrons: 1s² 2s² 2p⁴. The small number after each subshell is the number of electrons in it.
What is the electron configuration of iron?
Iron (26 electrons) is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶, or in noble-gas shorthand [Ar] 3d⁶ 4s². Hund's rule spreads the six 3d electrons over the five boxes, so four of them stay unpaired, which is why iron is magnetic.
What is the noble-gas shorthand?
The inner electrons are written as the noble gas that has exactly that configuration, in square brackets. Sodium, 1s² 2s² 2p⁶ 3s¹, becomes [Ne] 3s¹. What is left after the core are the outer electrons that matter for chemistry.
Why are chromium and copper exceptions?
A half-filled or completely filled d subshell is extra stable. Chromium is therefore [Ar] 3d⁵ 4s¹ instead of 3d⁴ 4s², and copper [Ar] 3d¹⁰ 4s¹ instead of 3d⁹ 4s². Molybdenum, silver, gold and some others behave the same way; the calculator uses the measured configurations.
How do you find the electron configuration of an ion?
For a positive ion, remove electrons from the outermost shell first: for transition metals the 4s electrons go before the 3d ones, so Fe³⁺ is [Ar] 3d⁵ and not [Ar] 4s² 3d³. For a negative ion, add electrons in the normal order: Cl⁻ is 1s² 2s² 2p⁶ 3s² 3p⁶, the same as argon.
How many valence electrons does an element have?
For main-group elements: the electrons in the outer shell, which equals the group number for groups 1 and 2, and the group number minus 10 for groups 13 to 18. Oxygen (group 16) has 6, chlorine 7. For transition metals the ns and (n−1)d electrons are usually counted: iron has 8.