Number of electrons increases as we move from left to right in a period. The last element in each period is the noble gas. The atomic radius of noble gases are measured with the help of vander Waal’s radius as noble gases do not form molecules. On the other hand the size of other elements are measured by covalent radii. As van der Waal’s radii are larger than covalent radii hence the size of noble gases are comparatively large.
