The lowest possible number of protons an atom can have is one. The proton is a subatomic particle with a positive charge, and it defines the atomic number of an element. Each element on the periodic table is characterized by the number of protons in the nucleus of its atoms. For example, hydrogen, the lightest and most basic element, has one proton, while helium has two protons, and so on. So, one proton is the minimum requirement for an atom to be considered a distinct element.