Electron configuration, step by step

How to write them, how to read them, and why chromium and copper get to break the rules.

An electron configuration like [Ar] 3d6 4s2 is a complete address book for an atom's electrons. Learn the notation once and every configuration on this site — each element's detail card shows one — becomes instantly readable.

The building blocks: shells and subshells

Electrons around a nucleus occupy shells numbered 1, 2, 3… outward. Each shell contains one or more subshells, labeled with letters, and each subshell holds a fixed maximum of electrons:

• s — holds up to 2 electrons
• p — holds up to 6
• d — holds up to 10
• f — holds up to 14

A configuration is just a list of occupied subshells. 1s2 2s2 2p4 reads: two electrons in the 1s subshell, two in 2s, four in 2p — eight electrons total, which makes it oxygen.

The filling order

Electrons fill the lowest-energy subshells first (the aufbau, or "building-up," principle). The order isn't simply 1, 2, 3…, because subshell energies overlap between shells. The sequence to memorize — or read off the diagonal chart your textbook shows — begins:

1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d → 7p

The famous wrinkle: 4s fills before 3d. That's why potassium is [Ar] 4s1 rather than [Ar] 3d1, and why the transition metals appear only from scandium onward.

Noble-gas shorthand

Writing every subshell gets tedious fast, so chemists abbreviate: replace the longest leading stretch of a configuration with the noble gas it equals, in brackets. Sodium's full configuration 1s2 2s2 2p6 3s1 becomes [Ne] 3s1 — "neon's configuration, plus one more electron in 3s."

The shorthand isn't just shorter; it's smarter, because it separates the inert core electrons (the bracket) from the valence electrons (everything after) — and the valence electrons are the only ones that do chemistry. Glance at sodium's [Ne] 3s1 and you already know it behaves as a +1 metal.

Worked examples

• Hydrogen (1 electron): 1s1
• Carbon (6): [He] 2s2 2p2 — four valence electrons, the root of carbon's four bonds.
• Chlorine (17): [Ne] 3s2 3p5 — seven valence electrons, one short of a full shell: a classic halogen.
• Iron (26): [Ar] 3d6 4s2 — a partly filled d-subshell, the signature of a transition metal.
• Uranium (92): [Rn] 5f3 6d1 7s2 — an f-subshell filling: an actinide.

Configuration ↔ periodic table position

The table and the notation are two views of the same thing:

• The period number equals the outermost shell number.
• The group tracks the valence count — group 1 ends in s1, group 2 in s2, group 17 in p5, group 18 in p6 (a full shell).
• The block (s, p, d, f) names the subshell being filled — which is literally the table's shape, as covered in how to read the periodic table.

This means you can usually write a configuration straight from the table: walk from hydrogen to your element, reading each region you cross as a filled subshell.

The famous exceptions

Two rule-breakers appear on every exam: chromium is [Ar] 3d5 4s1 (not 3d4 4s2) and copper is [Ar] 3d10 4s1 (not 3d9 4s2). In both cases the atom promotes one 4s electron because a half-filled (d5) or filled (d10) d-subshell has extra stability, and the tiny energy gap between 4s and 3d makes the swap worthwhile.

The same logic echoes down the table — niobium, molybdenum, ruthenium, rhodium, palladium (an extreme case: [Kr] 4d10, no 5s electrons at all), silver, and gold all deviate similarly, and the f-block adds cases of its own (e.g. lanthanum, cerium, gadolinium). Don't memorize them all — know the Cr/Cu pair, understand the half-filled/filled-subshell reason, and recognize the pattern when you see it.

Practice with real data

Open the interactive table and quiz yourself: pick an element, predict its configuration from its position, then click the tile to check. Start with period 3 (no exceptions), graduate to the transition metals, and finish with the f-block. Every configuration on this site is also listed on the element's own page — for instance iron — alongside its other properties. For how those valence counts drive behavior across a whole column, continue to element families.