- Oxygen would have the smallest radius.
Atomic radius is defined as one half of the distance between the nuclei of two atoms of the same element when the atoms are joined. The atomic radius decreases across periods because there are less electron shells to shield the attractive power of the nucleus. You add a proton each time you go across the period, making it have a more attractive power. The attractive power causes it to pull in the electron shells tighter, which decreases the radius. It increases down groups because each time you go down, a new energy level is added. This creates a new electron shell. The number of electrons outnumber the number of protons, which means the attractive force isn’t as great. This allows the shells to spread out more because it doesn't have the attractive force pulling it in, creating a larger radius.
Oxygen would have the smallest radius because it has the smallest number of electron shells. This causes the positive charge to be greater and pull the electron shells closer, therefore decreasing the radius. Sodium has a larger radius than oxygen because it has more electron shells and a smaller attractive force, allowing the shells to spread out. This causes a larger radius. Iodine has a larger radius than oxygen because even though it has a very strong attractive force, the number of electron shells outnumbers the attractive force. This means the radius for iodine is larger because the all the shells spread out.