Question

why valency of iron is 2

Answer 1

iron has its atomic number as 26 so if we arrange it on the manner we arrange on the shells we will have the electronic configuration of 2,8,14,2 .as we have the valence electron as 2 it is better to have two electrons taken rather than adding many more.so it has a valency of  2.
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Answer 2

The valency of an element is the number of electrons it gains or loses or shares to attain the noble gas configuration. ... Also note that the 3d orbital has 1 electron pair, rest of the electrons are unpaired. Now, ironexhibits 2 valence states of +2 and +3. When iron loses the 2 4s electrons, it attains a valency of +2.