Electron configuration of ions

When transition metals form ions they will lose electrons from the 4s suborbital first even though this orbital is filled first.

Example:
Chromium atoms and ions
[Cr] 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d5
[Cr2+] 1s2,2s2,2p6,3s2,3p6,3d5