Potassium is more reactive than sodium and both metals are found in the same group. The atomic radius of potas

Question

larger than that of sodium
the same as that of sodium
smaller than that of sodium
variable

Answer 1

The larger the radius, the more weakly are the outer electrons held in any atom. K atoms are larger than Na atoms, and the K's outer electron is easier to remove, which makes it more reactive.

Answer 2

In fact your statement is wrong, sodium is many times more reactive than potassium. It is easy to verify this just letting drop a small quantity of each, sodium immediately explodes in a flame burst, potassium as well but in a more slowly and moderate way.
Besides considering atomic radii you also have to consider the energy level of the outermost electron.