Metals generally lose electrons when they react to form cations, that is ion with a positive charge. As you might expect the electrons are lost from valance shell e.g.
Similarly for non-metal atoms, these will gain electrons to form ions with a negative charge, that is anions, e.g.
In each of the examples above the ion formed has a noble gas
electronic configuration, that is np6.
With transition metals the formation of cations does not quite go as you would expect. When we were working out the electron arrangements for the transition metals using the AUFBAU principle we filled up the sub-shells in the order: .................4s 3d that is the 4s sub-shell was filled before the 3d sub-shell.
However the 4s and the 3d sub-levels are very close in energy. After the 4s sub-level is filled it actually becomes higher in energy than the 3d sub-level. This means that when we remove electrons from a transition metal to form the cation, we remove the 4s electrons before the 3d electrons. For example:
Iron forms two common ions, Fe2+and Fe3+. The electronic configuration of these ions is shown below:
The 2 electrons which are lost come from the 4s sub-level and NOT the 3d sub-level.
This time the iron atoms lose 3 electrons. Two of these electrons come from the 4s sub-level and one electron comes from the 3d sub-level. It is worth noting that the Fe3+ ion has a half-filled 3d sub-level, this will make it more stable than the Fe2+ion.
Remember one of the transition metals with a slightly unexpected electronic configuration is chromium;
it has a half-filled d sub-shell and 1
electron in the 4s sub-shell.
So the electronic configuration of the Cr3+ ion is: