All subjects · Chemistry · WCH11 (Unit 1)

Orbitals and electron configurations

Electrons occupy shells, sub-shells and orbitals. Electron configurations up to krypton (Z = 36) explain the layout of the Periodic Table.

Orbitals

An orbital is a region where there is a high probability of finding an electron. Each holds up to 2 electrons with opposite spins. s orbitals are spherical; p orbitals are dumbbell shaped, three per sub-shell at right angles.

Sub-shells: s (1 orbital, 2 electrons), p (3 orbitals, 6), d (5 orbitals, 10).

Filling order

Electrons fill the lowest energy sub-shell first: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. Within a sub-shell, orbitals fill singly with parallel spins before pairing (shown with arrows in boxes).

Examples: Na 1s2 2s2 2p6 3s1; Fe 1s2 2s2 2p6 3s2 3p6 3d6 4s2 (or [Ar] 3d6 4s2).

Two exceptions and ions

Chromium is [Ar] 3d5 4s1 and copper is [Ar] 3d10 4s1. For s- and p-block ions add or remove electrons from the outer sub-shell: Na+ is 1s2 2s2 2p6; Cl- is 1s2 2s2 2p6 3s2 3p6.

Blocks

The Periodic Table is divided into blocks by the sub-shell being filled: s block (Groups 1 and 2), p block (Groups 13 to 18), d block (transition elements). Elements in the same group have the same outer electron configuration, which is why they react alike.

Worked example

Write the full electron configuration of sulfur (Z = 16) and of the sulfide ion.

  1. 16 electrons: 1s2 2s2 2p6 3s2 3p4
  2. S2- has 18 electrons: add 2 to 3p

Answer: S: 1s2 2s2 2p6 3s2 3p4; S2-: 1s2 2s2 2p6 3s2 3p6

Key idea: Fill 1s 2s 2p 3s 3p 4s 3d 4p; single occupancy before pairing; Cr and Cu take 4s1; groups share outer configurations.

Check yourself

  1. What is the electron configuration of potassium (Z = 19)?
    • 1s2 2s2 2p6 3s2 3p6 3d1
    • 1s2 2s2 2p6 3s2 3p6 4s1
    • 1s2 2s2 2p6 3s2 3p7
    • 1s2 2s2 2p6 3s2 3p5 4s2
    Show answer

    1s2 2s2 2p6 3s2 3p6 4s1. 4s fills before 3d.

  2. What is the configuration of chromium (Z = 24)?
    • [Ar] 3d4 4s2
    • [Ar] 3d6
    • [Ar] 4s2 4p4
    • [Ar] 3d5 4s1
    Show answer

    [Ar] 3d5 4s1. Chromium is an exception: a half-filled 3d sub-shell is more stable.

  3. What is the maximum number of electrons a p sub-shell can hold?
    Show answer

    6. Three orbitals x 2 electrons = 6.

  4. What shape is an s orbital?
    • Dumbbell
    • Ring
    • Spherical
    • Tetrahedral
    Show answer

    Spherical. s orbitals are spheres centred on the nucleus.

  5. In which block is bromine?
    • p block
    • s block
    • d block
    • f block
    Show answer

    p block. Br is [Ar] 3d10 4s2 4p5: its highest energy electron is in a p sub-shell.

  6. Which species has the configuration 1s2 2s2 2p6?
    • Mg
    • Cl-
    • Mg2+
    • Ca2+
    Show answer

    Mg2+. Mg (12 electrons) loses 2 to give 10 electrons.

  7. How many unpaired electrons does a nitrogen atom have?
    Show answer

    3. N is 1s2 2s2 2p3; the three 2p electrons occupy separate orbitals singly.

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Last updated 7 November 2026. Spotted a mistake? Tell us.