Concept preview

What you will be able to do

1. 01Describe energy levels, orbitals and subshells
2. 02Apply the aufbau filling order and orbital capacities
3. 03Write electron configurations and identify valence electrons

When a hydrogen atom is energised it emits only certain colours — a line spectrum — which Bohr explained by proposing that an electron can have only specific energies. An energy level is a specific amount of energy an electron in an atom can have. The levels are numbered $n = 1, 2, 3, \dots$, with $n=1$ closest to the nucleus and lowest in energy; the higher the level, the farther from the nucleus and the more energy it holds.