Answer:
Radius of Cobalt > Radius of Nickel
Explanation:
In general, unionized elements in series with the lower atomic number (#protons) will tend to have a larger radius than those with higher atomic numbers. Therefore Cobalt (Atomic No = 27) would have a slightly larger atomic radius than Nickel (Atomic No = 28). A rule to remember is as atomic number increases in a series of elements the numbers of protons and electrons increase which results in an increase in the nuclear attraction for electrons thereby decreasing the atomic radius of elements. The following periodic table of atomic radii (in picometers) will support the explanation.
http://ww1.igoscience.com/ Co => 152pm Ni => 149pm
