Na has a lower ie1 than mg, but mg has a lower ie2 than na because: the radius of na is smaller than that of mg. ie1 increases from left to right across a period and ie2 decreases from left to right across a period. the radius of mg is larger than that of na. ie1 decreases from left to right across a period and ie2 increases from left to right across a period. ie1 increases from left to right across a period. however, because na has only one valence electron, ie2 for na involves the removal of a core electron − requiring significantly more energy.