Find the valency of Iron by electronic configuration

You have studied about the electronic configuration on the basis of K, L M and N shells.
These shells further have sub shells s, p, d and f and it is easier to find the valency using the electronic configuration based on these sub shells.
Also, these sub shells are stable when they are fully or half filled.
Like p sub shell is stable when it has 3(half filled) or 6 (fully filled) electrons.
The electronic configuration of iron is 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d6
Thus the valence electron is 4s2, 3d6
Iron has +2 valency when it loses its 4s electron and +3 valency when its two 4s electron and one p electron.
On basis of this we say, the valency of iron is +2 and +3
You will study in detail about these sub shells in higher classes.
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The number of valence electrons in the transition metals is somewhat different than main group elements. As you go from left to right across the periodic table, the electrons added to the transition metals go into the d-orbitals. However, because the energy of the 4s orbital is lower than the 3d orbital (and the 5s is lower than the 4d, etc.), the 4s orbital fills first. Therefore the electron configuration of iron for instance is [Ar]4s23d6.

Because the valence electrons are defined as the electrons in the outermost or highest energy shell, for iron, that would be the 4th shell. So the 6 electrons in the 3d orbital don't count. Only the 2 electrons in the 4s orbital count since they are in the 4th shell. Most transition metals thus have 2 valence electrons (although some, such as chromium, only have one because of exceptions to the filling rules -- the configuration of chromium is [Ar]4s13d5).

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