Final answer:
The element with the smallest atomic radius in group 13 (3a) is boron (B). Atomic radius decreases across a period, thus boron, being the first element in the group, has the fewest protons and smallest atomic radius.
Step-by-step explanation:
The symbol of the element in group 13 (3a) with the smallest atomic radius is boron (B). In general, atomic radius decreases as you move across a period from left to right on the periodic table due to the increase in the number of protons in the nucleus, which pulls the electrons closer. Boron is the first element in group 13, and therefore, it will have the smallest atomic radius compared to the other elements in the same group because it has the fewest protons.
When considering the elements given (Sr, Sn, Rb, In), we see that none of them belong to group 13. However, if we refer to the options given in another question (K, Kr, Ga, Ge), we can identify that Ga (gallium) is the element from group 13. Yet, gallium does not have the smallest atomic radius in group 13; boron does.
To summarize the trend of atomic size, we can note that atomic radius increases down a group and decreases across a period.