Answer: K is bigger in atomic size. The metallic character increases as the size increases.
Explanation:
Reactivity of a metal is defined as the tendency of an element to lose electrons. The metallic character increases as we move down a group and it decreases as we move across a period.
When the size of an element increases, the valence electrons gets away from the nucleus and the tendency of an element to loose electrons increases.
In a group, the size of an element increases because there is an addition of new shell and electron is added in that shell.
Thus for alkali metals, the metallic character increases from Lithium to sodium to potassium and so on.
