For example, a helium atom has an atomic mass of 4, but an atomic number (or positive charge) of 2. Since electrons have almost no mass, it seemed that something besides the protons in the nucleus ...

Helium is notoriously hard to contain because, like hydrogen, it is a tiny little atom even by atomic standards ... into a power adapter and let it charge for up to one hour.

The relative charge of a nucleus is the same as its proton ... it becomes a negatively-charged ion A helium atom has two electrons in an energy level outside the nucleus. The atom is neutral ...