Why does the graph of atomic size have peaks at the alkali metals and valleys at the noble gases

Atomic size decreases from left to right in te periodic table.

Explanation:

With increase in effective nuclear charge without a corresponding increase in the number of shells across the period, electrostatic interaction between electrons and the nucleus increases with the net effect that atomic size decreases towards the right of the periodic table. For this reason, there are peaks at the larger alkali metals and valleys at the smaller noble gases.