When d block elements undergo chemical reactions along with the electrons in the outermost shell, the electrons in the penultimate d subshell also take part. In the d block elements, electrons are not lost in the same order as they are filled up i.e. Electrons are first lost from the outermost s subshell.
The difference in energy between the outer most s subshell and the penultimate d subshell is very small. Hence under suitable conditions the electrons in d subshell also take part in chemical reactions and show variable oxidation states.
Most of the coloured salts are compounds of transition elements. The colour is due to the presence of transition element ions in these compounds.
The subshell electronic configuration of Fe is 1S2 2S2 2P6 3S2 3P6 3d6 4S2
| Compound | Oxidation state of Fe | Subshell electronic configuration of the ions of Fe |
|---|---|---|
| Fecl2 | +2 | 1S2 2S2 2P6 3S2 3P6 3d6 |
| Fecl3 | +3 | 1S2 2S2 2P6 3S2 3P6 3d5 |