The smallest element in Period 2 is Fluorine. As we go across a period, from left to right, the atomic number increases. We are adding one proton to the nucleus and one electron to the valence shell. The force of attraction between the nucleus (positive) and the valence electrons (negative) also increases from left to right. That's why the atomic radius decreases from left to right, and the elements on the right side of the periodic table are smaller than the elements on the left.
Answer: Fluorine