Aluminium (Z=13) fills shells by the aufbau principle: 1s² 2s² 2p⁶ 3s² 3p¹ → Bohr shells K=2, L=8, M=3. Each shell's effective nuclear charge is computed with Slater's shielding rules (not a fixed value — it changes as electrons are removed):
Z_eff(n) = Z − S
S = 1.00·(e⁻ in n−2 or lower)
+ 0.85·(e⁻ in n−1)
+ 0.35·(other e⁻ in same n) [0.30 for 1s pair]
Shell radius follows the Bohr-model scaling r_n = a₀·n²/Z_eff (a₀ = 52.9 pm). Because M-shell electrons are poorly shielded from each other (0.35 each) but well shielded by the K+L core (1.00 each), Z_eff(M) rises sharply as 3p and then 3s electrons are stripped off — so the ion shrinks fast.
Ionisation energies shown are the real successive values for Al (CRC): IE₁=577.5, IE₂=1816.7, IE₃=2744.8, IE₄=11577 kJ/mol. The huge IE₄ jump — removing a K/L core electron instead of an M-shell one — is why aluminium stops at the Al³⁺ ion and never forms Al⁴⁺ in ordinary chemistry.
- Shell diagram — rings drawn at the computed r_n, radii to scale relative to each other.
- Orbital shapes — same electron count grouped by subshell (s = sphere, p = dumbbell lobes) instead of simple rings.
- Trend panel — real first-ionisation energies across period 3; note Al's IE₁ is lower than Mg's, because a shielded 3p electron is easier to remove than a filled 3s² pair — a genuine anomaly in the periodic trend, not a monotonic increase.