Why 4s Fills Before 3d (And Why It Flips!)
The question that trips up so many Period 4 configurations: we fill 4s before 3d, yet we take 4s away first. Here is why the order swaps, and how to get every transition-metal ion right.
When 3d is empty, 4s is lower
For potassium and calcium the 3d subshell is empty, and 4s lies just below 3d in energy. So the electrons go into 4s first: K is [Ar] 4s1 and Ca is [Ar] 4s2. This is why Period 4 opens by filling 4s.
Once 3d fills, the order swaps
From scandium onwards, electrons enter 3d. Those 3d electrons repel the 4s pair outward, so 3d drops below 4s. In a filled transition-metal atom the 3d electrons now sit lower than the 4s ones, the reverse of where we started.
That is why 4s leaves first
Because 4s is now the higher, outermost subshell, it is emptied first on ionisation. Iron [Ar] 3d64s2 loses its two 4s electrons to give Fe2+ [Ar] 3d6, then a 3d electron for Fe3+ [Ar] 3d5.
Practise it yourself
Build the configuration for any Period 4 atom or ion, including the Cr and Cu exceptions, in the interactive ChemBank simulation.
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