Why does the size of atoms decrease while moving across from left to right in a period within the Periodic Table?

In general, this is true -- atomic radius decreases going across the rows, and increases going down the columns of the Periodic Table.  As more protons, which are positively charged, are added to an atom's nucleus, the overall charge pulls all the orbiting electrons closer, so the more protons, the smaller the atomic radius, but only up to a point -- eventually, as the nucleus gets bigger, more electrons orbit around it, and the size increases. At the fourth row,  electrons start filling in the outer shells in a more complicated pattern.  See the first link for a graphical representation of atomic radii for each element: