Hi, Delyla--great question!
As you move from left to right across a period in the periodic table, electronegativity increases. This basically means that the atom wants to draw its electrons closer to itself as you move from left to right across the period. Effective nuclear charge would be stronger as you move from left to right across the period as well, and this means that the positive charge experienced by each electron in the atom increases (electrons are negatively charged, so you can think of this as how strongly it's attracted to the net positive charge). Electronegativity and effective nuclear charge are different but go hand-in-hand and both refer to how attracted and close the electrons are to the nucleus of the atom, where the protons are.Thus, the atomic radii will decrease because of the stronger nuclear charge.
Let me know if you need any clarification!