Question

What is the similarity in the electronic configuration of Hydrogen , Lithium and Potassium ?

Answer 1

The outer most electrons of these elements are enters into 's' orbital only and these elements have only one electron in their outermost orbital.
Hydrogen: 1s1
Lithium: 1s2 2s1
Potassium: 1s2 2s2 2p6 3s2 3p6 4s1