Shells, Sub-shells and Orbitals

Electron shells are labelled by the principal quantum number n. The first shell contains an s sub-shell; the second contains s and p; the third contains s, p and d. Each sub-shell contains orbitals, described at this level as regions where an electron is likely to be found, rather than fixed paths.

Each orbital holds at most two electrons with opposite spins. The s, p, d and f sub-shells contain 1, 3, 5 and 7 orbitals, giving capacities of 2, 6, 10 and 14 electrons. The ground state is the lowest-energy electron arrangement.

Key terms

• principal quantum number (n)

• shell

• sub-shell

• orbital

• ground state