Electron arrangement · Grade 9
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.