Why does the atomic radius of an atom decrease as you go across a period?

The atomic radius is often defined as being one half the distance between the two nuclei in a homonucleur diatomic molecule. In general, there is a decrease in atomic radius as you go across a period. This is because atoms in the same period share the similarity of having their valence electrons in the same sub-shell. The difference that occurs however is that there is an increase in atomic number, and thus nuclear charge. The increase in nuclear charge means a higher attraction is exerted from the nucleus, pulling the electrons closer to the nucleus and thus decreasing the size of the atom.