How would you describe the trends in atomic radius of the alkali metals?
Atomic radius increases down the alkali metals group.
Every time we move down the group, the number of electron shells of the element increases by one. E.g. sodium has 3 electron shells, potassium has 4, rubidium has 5. Since the number of electron shells increase, the atom is getting larger and thus the atomic radius would get larger.
Furthermore, the increase in the number of electron shells increases the shielding effect. Basically, the inner electron shells of an atom "absorb" some of the positive pull of the protons in the nucleus. The strength of the positive pull decreases with each subsequent electron shell and thus the outer electrons are least effected. The more electron shells the atom has, the less the positive pull strength will be on the outermost electrons and thus the further away the electrons are able to be from the nucleus (as they are not being pulled as much.) This also increases the atomic radius.