1.3.3—Order of increasing energy of the sub-shells

Build a ground-state configuration by filling available sub-shells from lower to higher energy. Shell number alone does not give the complete order because sub-shell energies overlap at higher levels; in particular, 4s is filled before 3d for a neutral atom.

Use a fixed method: count the electrons, follow the supported increasing-energy sequence, place no more than the allowed number in each sub-shell, and check that the exponents total the required electron count. A shorthand configuration uses a preceding noble-gas core where appropriate.

For potassium, the argon core is followed by 4s¹, so its shorthand configuration is [Ar] 4s¹ rather than [Ar] 3d¹. The 4s/3d overlap is the reason the simple numerical shell order is not a safe filling rule.

Do not transfer neutral-atom filling order unchanged to a transition-metal ion: when such ions form, electrons are removed from 4s before 3d. Keep filling order, ion formation and special exceptions distinct; orbital capacities are covered by the neighbouring sub-shell objective.