Describe and explain the trend in atomic radii across the periodic table

Atomic radii decreases in size, moving from left to right in the periodic table.
This is because atoms on the right hand side of a period have a larger number of protons. So, there is a greater positive charge for atoms on the right hand side, like Neon with which to attract negative electrons. This causes the electrons to be pulled in with more force towards the nucleus, decreasing the radius.