Both elements, sodium (Na) and potassium (K) lie in the Group 1 column of the Periodic Table, which contains the members of the Alkali Metals family. Members within a family, or column, of elements tend to have similar chemical properties. The similar properties among elements are what initially inspired Russian chemist Dmitri Mendeleev (1834-1907) to publish the Periodic Table in 1869.
Chemically, an atom of sodium and an atom of potassium both have one electron in their outermost orbits; this makes them reactive as both tend towards losing that electron. That loss creates positively-charged sodium and potassium ions (Na+ and K+), which are critical for nerve impulse propagation and muscle function. See more at links: