When an alkaline earth metal, m, reacts with oxygen the formula of the compound produced will be
a. m₂o
b. mo
c. m₂o₃
d. mo₂
e. m₃o₄

D as the oxygen has gained to electrons from the magnesium in the reaction.

The answer is M2O

why?

the assumed valency of M should be 1+ ( because it reacts with oxygen and the valency is not provided)

the valency of oxygen is O2-

thus, by cris cross technique, we get M2O