Electron arrangement

Electrons in an atom are arranged in shells (energy levels) around the nucleus. The first shell holds up to 2 electrons and the next two up to 8 each, so sodium is 2,8,1. The outer shell decides how an element reacts and which group of the periodic table it is in. At A-level and in CAPS each shell is split into sub-shells (s, p and d) that fill in order of energy, written like 1s² 2s² 2p⁶ 3s¹.

Key facts

  • Shells fill from the inside out: up to 2 electrons in the first shell, then 8, then 8 (for the first 20 elements).
  • A neutral atom has as many electrons as its atomic number.
  • The number of occupied shells is the period. The number of outer electrons gives the group: groups 1–7 and 0 in GCSE numbering, or 1, 2 and 13–18 in the IUPAC numbering.
  • Ions gain or lose electrons to get a full outer shell, like a noble gas: Na⁺ is 2,8 and Cl⁻ is 2,8,8.
  • Sub-shells: s holds 2 electrons, p holds 6 and d holds 10. They fill in the order 1s 2s 2p 3s 3p 4s 3d 4p.
  • Each orbital holds 2 electrons with opposite spins, and electrons fill separate orbitals in a sub-shell before they pair up. Transition metal atoms lose their 4s electrons first when they form ions.

Worked examples

Example 1

Write the electron arrangement of magnesium (atomic number 12). Which group and period is it in?

  1. Magnesium has atomic number 12, so a neutral atom has 12 electrons.
  2. Put 2 in the first shell, 8 in the second and the last 2 in the third. 2 + 8 + 2 = 12
  3. Three occupied shells: period 3. Two outer electrons: group 2.

Answer: 2,8,2: group 2, period 3

Example 2

Write the electron arrangement of the oxide ion, O²⁻. (Oxygen has atomic number 8.)

  1. An oxygen atom has 8 electrons: 2,6. It has 6 outer electrons (the crosses).
  2. The charge is 2−, so it has gained 2 electrons (the dots). 8 + 2 = 10
  3. So O²⁻ is 2,8: a full outer shell, the same arrangement as neon.

Answer: 2,8

Example 3

Write the full electron configuration of iron (atomic number 26) and of the Fe²⁺ ion.

  1. Fill the sub-shells in order of energy: 1s² 2s² 2p⁶ 3s² 3p⁶ holds 18 electrons. 2 + 2 + 6 + 2 + 6 = 18
  2. 4s is lower in energy than 3d, so it fills next (20 electrons), and the last 6 go into 3d. 18 + 2 + 6 = 26
  3. In 3d the electrons go one into each orbital before they pair up, so iron has 4 unpaired electrons.
  4. Fe²⁺ has lost 2 electrons, and the 4s electrons are lost first: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶.

Answer: Fe: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶; Fe²⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶