Question

if manganese has a valency of 2...then why does it form mno2 when it reacts with oxygen
mean while mg2+o2=mgo
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Answer 1

because oxygen also has valency of -2.
here magnesium gives 2 electrons and oxygen accepts the 2 electron
hence compound is formed

Answer 2

your answer matevalency of a element is constant and solely depends on the no. Of valence electrons and in the case of magnesium it has an electronic configuration of 2,8,2..So the no. Of valence electrons are 2 and hence the valency is 2..

It is the oxidation state that changes and in this case (MgO),oxygen being electronegative has an oxidation state of -2 and that of magnesium is +2.