The trend for the ionic radius as the atomic number increases in Period 2 is that it decreases.
In general, the ionic radius of an element is the distance between the nucleus and the outermost stable electron shell of an atom or ion. As the atomic number increases, the number of protons in the nucleus also increases, which creates a greater positively charged force (the positively charged protons) pulling the electrons closer to the nucleus. This causes the radius of the outermost stable electron shell to decrease, and therefore the ionic radius also decreases.
In Period 2, the atomic number increases from Z = 3 (Lithium) to Z = 10 (Neon). The ionic radius of Li+ is 0.76 Å and the ionic radius of Ne2+ is 0.43 Å, Therefore, the ionic radius decreases as the atomic number increases within Period 2.