To figure out which element of these is the smallest, i.e. has the smallest radius, we can refer to a periodic table for the location of each element.
The general trend for atomic radii is that the radius
From this trend, to see which of these elements is the smallest, you must determine which element is farthest right and/or the farthest up in the table. This element is nitrogen (
Just as the question at the bottom of the image proposes, why is this the general trend?
As you go down a group, the outer electrons are successively farther from the nucleus due to the atomic orbitals increasing in size (they're more likely to be found farther from the nucleus). Each successive period includes orbitals that have a larger space for the electrons to occupy, so to speak.
As you go right across a period, there are successively more electrons and protons for each element you come across (one more proton and one more electron for each). Coulomb's law states that the attractive electric force between two bodies (in this case, the protons) increases as the charges increase. Since the magnitude of the charges increases as you go across a period (more charged bodies going across a period), the attractive force will be larger in magnitude, and thus the positive nucleus pulls with greater force the electrons toward it (decreasing the distance between the electrons and protons, and hence, the size).
These two points illustrate a general description of this trend.