The modern periodic law was proposed by

In the given pairs, which element will have the bigger atom? Give reason for your choice. 

$\mathrm{Mg}$ (atomic number $12$) or $\mathrm{Cl}$ (atomic number $17$) 

In the given pairs, which element will have the bigger atom? Give reason for your choice. 

$\mathrm{Na}$ (atomic number $11$) or $\mathrm{K}$ (atomic number $19$)