Electrons aren’t scattered randomly around the nucleus — they’re organised into shells, then subshells, then orbitals. This three-level structure is the skeleton every electron configuration is built on.
📘 What you need to know
Electrons occupy principal energy levels (shells), numbered by the principal quantum number (n).
Each shell holds up to 2n² electrons: n=1 → 2, n=2 → 8, n=3 → 18, n=4 → 32.
Shells split into subshells — s, p, d, f — with energy rising in the order s < p < d < f.
Subshells contain orbitals: s has 1, p has 3, d has 5, f has 7. Each orbital holds a maximum of 2 electrons.
Subshells fill from lowest energy first (the Aufbau principle) — note 4s fills before 3d.
Shells (principal energy levels)
The main energy levels are the shells, numbered by the principal quantum number (n). The lower the n, the closer the shell to the nucleus and the lower its energy.
Each shell can hold a fixed maximum, following the rule 2n²:
n = 1 → up to 2 electrons
n = 2 → up to 8 electrons
n = 3 → up to 18 electrons
n = 4 → up to 32 electrons
Check it: for n = 3, 2n² = 2 × 3² = 2 × 9 = 18. The formula always gives the shell’s capacity.
Subshells (s, p, d, f)
Each shell is divided into subshells, labelled s, p, d and f. Within a shell their energies rise in the order s < p < d < f. Each subshell holds a set number of orbitals, and each orbital holds up to 2 electrons:
Each subshell holds a set number of orbitals, and each orbital takes 2 electrons — giving the maximum for each subshell.
Orbitals and their shapes
An orbital is a region where an electron is most likely to be found. Each holds a maximum of 2 electrons, and orbitals have definite 3D shapes:
s orbitals are spherical, and get bigger as the shell number increases.
p orbitals are dumbbell-shaped, and come in threes (px, py, pz) pointing along the three axes at right angles to each other.
s orbitals are spherical; the three p orbitals are dumbbells along the x, y and z axes. (The shape of d orbitals is not required for IB.)
Filling order — the Aufbau principle
Electrons fill the lowest-energy subshells first. This is the Aufbau principle, and it gives an atom its most stable arrangement, the ground state.
The energy order mostly follows s < p < d < f, but there’s one famous quirk: the 4s subshell is slightly lower in energy than 3d, so 4s fills before 3d.
This 4s-before-3d point is worth burning into memory — it explains why potassium and calcium fill 4s, and it’s the key to writing configurations (and ions) for the transition metals later.
WORKED EXAMPLE
(a) How many electrons can the n = 3 shell hold? (b) How many orbitals are in a d subshell, and how many electrons can it hold? (c) Which fills first, 4s or 3d?
(a) Shell capacity = 2n²2 × 3² = 18 electrons(b) A d subshell has 5 orbitals5 orbitals × 2 = 10 electrons.(c) 4s fills first4s is slightly lower in energy than 3d.18 · d = 5 orbitals/10e · 4s before 3d
💡 Exam tip
Learn the subshell capacities cold: s = 2, p = 6, d = 10, f = 14.
The only ordering exception you need is 4s before 3d — everything else follows s < p < d < f.
Up next: Writing Electron Configurations — putting all of this together to write the full and shorthand configuration of any atom or ion.
Want this explained one-to-one?
Book a free session with an experienced IB Chemistry tutor and get your trickiest topics made simple.
