On the periodic table, the number of valence electrons in atoms increases left to right across a period. what effect does this have on the ionic radii of the elements?

The radii of elements decreases from left to right, as the number of valence electrons increases. This occurs because of the increasing positive-negative attraction between the protons and the electrons (remember that protons are added from left to right as well). Group 1 elements have the largest radius of their period, while Group 18 have the smallest of their period.