The chemical element in the second period with the largest atomic radius is lithium, which is followed by beryllium, boron, carbon and nitrogen. The elements with the smallest atomic radius in this period are neon, fluorine and oxygen, respectively.
In each period, the atomic radius decreases from left to right across the periodic table. This is because each element from left to right in the period has one more proton in its nucleus. The increased number of protons leads to a more positive charge in the nucleus, which causes it to attract the atom's electrons more strongly. This draws them towards the nucleus and decreases the atom's radius.