Electrons energy levels and atomic orbitals

Electrons in atoms occupy a strict hierarchy of shells, sub-shells and orbitals.

Each shell is identified by its principal quantum number (n) and contains sub-shells labelled s, p and d.

Sub-shells fill in order of increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p.

The 4s sub-shell fills before 3d because it is lower in energy; however, when transition metal cations form, the 4s electrons are removed before the 3d electrons.

Electronic configurations may be written in full (e.g. Fe: 1s²2s²2p⁶3s²3p⁶3d⁶4s²) or in shorthand using the previous noble gas (e.g. [Ar] 3d⁶4s²).

The arrangement of electrons follows two principles:

• The lowest-energy sub-shells fill first.
• Within a sub-shell, electrons occupy separate orbitals singly before pairing, in order to minimise inter-electron repulsion.

The electrons-in-boxes notation shows each orbital as a box and each electron as a spin arrow, with two paired electrons in the same box drawn with opposite spins as ↑↓.

The s orbital is spherical, while each of the three p orbitals is dumb-bell shaped and aligned along one of the x, y or z axes.

Key Definition A free radical is a species with one or more unpaired electrons, making it highly reactive.

At a Glance

These ideas underpin bonding, periodicity, transition metal chemistry, and reaction mechanisms throughout the course.