Build the electron configuration
Pick a period-4 atom or ion below, then add electrons to 4s and 3d to build it. Watch 4s sit below 3d while empty, then rise above once 3d fills. Ions must be built as the neutral atom first, then ionised.
higher energylower energy
4p
core
Pick an element or ion below to start.
Drag the electron onto a level, or click 4s or 3d, to add it.
Period 4, pick an atom
or an ion (build the atom, then ionise)
Energy gaps are schematic, drawn to show the order, not to scale. You are marked on the final ground-state configuration, not the order in which you add the electrons.
First In, First Out
4s fills before 3d, yet on ionisation 4s empties before 3d. Same electrons, opposite order, because the levels swap once 3d is occupied.
They overlap
Normally shell n = 3 is below n = 4, but 4s and 3d are very close and overlap, which is why 4s can dip below 3d while empty.
Why 4s rises
Once 3d is occupied, those 3d electrons sit closer to the nucleus and repel the 4s electrons outward, raising 4s above 3d. So 4s comes off first.
Cr and Cu exceptions
A 3d⁵ or 3d¹⁰ set is extra stable, so Cr is 3d⁵4s¹ and Cu is 3d¹⁰4s¹, not the "expected" 3d⁴4s² / 3d⁹4s².