why is atomic radius of Mg is smaller than Na?

the atomic radius of Mg is smaller than Na

becoz as we move along a period from left to right there is an addition of extra electron in the shell of an atom, which increses the effective nuclear charge of the atom

increase in nuclear charge of atom means attraction of electrons towards the nucleus and hence the size of the atom decreases

Mg with 2 electrons in its outermost shell has got higher nuclear charge than Na with only 1 electron in its outermost shell

therefore atomic radius of Mg is smaller than Na

Mg has got higher nuclear charge effect than Na

due to which the force of attraction of electrons in Mg towards the nucleus increases which results in the contraction of the size of the Mg atom whereas Na has got only 1 electron in its outermost shell and experience less attractive force 

therefore Mg is smaller than Na 

I`m the modern periodic table as the atomic number increases to and element the ionisation energy also increase which led to the shrinking of the atomic size elements and it occurs to be small when that the atomic no increases so na with less at no has big atomic size and mg with more at no has smaller size.