Electron Configuration
For a main group element, the number of valence electrons that it may have depends on the electron configuration in a simple way. However, for a transition metal, the relationship is more complex.
For a main group element, a valence electron is defined as an electron that resides in the electronic shell of highest principal quantum number n. For example the electronic configuration of phosphorus (P) is 1s2 2s2 2p6 3s2 3p3 so that there are 5 valence electrons (3s2 3p3), corresponding to a maximum valence for P of 5 as in the molecule PF5; this configuration is normally abbreviated to (Ne) 3s2 3p3, where (Ne) signifies the core electrons whose configuration is identical to that of the noble gas neon.
However, this simple method does not work for a transition metal, which has incomplete d (i.e., 3d, 4d or 5d) subshells, whose energy is normally comparable with that of an s electron. Instead, a valence electron for a transition metal is defined as an electron that resides outside a noble-gas core. For example, manganese (Mn) has configuration 1s2 2s2 2p6 3s2 3p6 4s2 3d5; this is abbreviated to (Ar) 4s2 3d5, where (Ar) denotes a core configuration identical to that of the noble gas argon. In this atom, a 3d electron has energy similar to that of a 4s electron, and much higher than that of a 3s or 3p electron. In effect, there are possibly seven valence electrons (4s2 3d5) outside the argon-like core; this is consistent with the chemical fact that manganese can have an oxidation state as high as +7 (in the permanganate ion: MnO4-).
The further right of each transition metal series, the lower the energy of an electron in a d subshell and the less such an electron has the properties of a valence electron. Thus, although a nickel atom has, in principle, ten valence electrons (4s2 3d8), its oxidation state never exceeds four. For zinc (and succeeding elements), the 3d subshell is complete and a 3d electron is considered to be a core electron.
Because the number of valence electrons which actually participate in chemical reactions is difficult to predict, the concept of the valence electron is less useful for a transition metal than for a main group element; as mentioned already, the d electron count provides a more useful tool for understanding the chemistry of a transition metal.
Read more about this topic: Valence Electron