Describe how the atomic radius changes as you read from left to right across a row. Why does this happen?

Across a row from left to right, the atomic radii are decreasing.

Across a row in the Periodic Table, atoms have the same number of shells.  Increase in nuclear charge makes the nucleus attract the electrons in the outermost shell more strongly.  This effect outweighs the increasing repulsion between more electrons.  Thus, the atomic radii are decreasing across a row.