Atoms tend to get smaller as you move from left to right across a group in the periodic table. The reason for this is that the positive charge of the nucleus is more strongly felt by the valence electrons as you move from left to right. Eg. Moving right from group 1 to group 7. The hydrogen anion H- only feels the positive charge of 1 proton, while the valence electrons in the F- anion feels the positive charge of 9 protons. Fluorine only has one more shell than hydrogen. The single shell which hydrogen has, compared to the two shells in fluorine, being in the end a less important factor for anion radius, compared with the effect of a charge of +9 attracting electron density towards itself. The hydrogen anion only has the force of one proton charge +1 drawing electron density towards itself. So although hydrogen only has 1 shell compared to the 2 in fluorine; the +9 positive charge in fluorine more than compensates for that disadvantage. The screening effect of the 2 electrons in the 1st electron shell in fluorine not sufficient to screen to a large enough extent the +9 charge felt by the valence electrons in a fluorine anion. Here it is seen that fluorine lies towards the right end of the first period. As atomic radius decreases from left to right fluorine (F) will have the smallest radius. The atomic radius of lithium (Li) is 167 pm, nitrogen (N) is 56 pm, chlorine (Cl) is 79 pm, fluorine (F) is 42 pm and the atomic radius of sulphur (S) is 88 pm.