Why Iron has Electron shell 2,8,14,2 and why it has valency 2+ instead of 2-

as iron has 26 as its atomic number so the distribution of elcrons shells is 2,8,14,2 because the no. of electrons in the shells are given by the formula 2n^2 so the no. of electrons in the 3^rd shell are 18 but acccording the bohr atomic configuration rules the valence shell cannot have more than 8 electrons so we cannot put 18 electrons in valence shell but 2 electrons so that it can form duplet in the valence shell to attain noble gas configuration to become stable.

 

As the no. of valence electrons = valency (in case of metals only) so when iron will loose the 2 electrons from its outrmost shell at that time the no. of electrons will become less than the no. of protons so it will have more positively charged particles.The sign on valency is +ve when the atom has excess of +vely charged paricles and -ve when the atom has negatively charged particles. as in case of Fe there are more +vely charged particles so it has valency as+2 but not-2.

 

@ ayushman 
as u have mentioned that the electronic configuration of  iron is 2,8,14,2 that is fully correct . 
and what u need to understand is valency is defined as the no. of electrons present in the outermost shell . 
and always element wants to be more stable . so, in this case iron will be stable if it loose 2 electrons rather than gaining 6 electrons and hold it in the valence shell . 
so, by loosing electron iron forms +2 valencies . 
HOPE IT CLEARS YOUR DOUBT ..