Question

why does atomic radius decreases as you move to the right across a period for main group elements but not for transition elements

Answer 1

In main group elements differentiating electron enters into valece shell.When atomic number increases nuclear atraction increases the valence electrons. So in period left to right atomic radius decreases.

In transition elements the differentiating electron enters into d-orbitals of the penultimate shell (n-1) shell . in any given transition series the nuclear charge increases as the atomic number increases. The (n-1)d electrons screen the outer electrons from the nuclear atraction. This means nuclear atractions on the outer electrons are feeble. As a consequence the atomic radius slightly decreases as we move from left to right in the transition series.