Hydrogen atoms and Helium atoms both have two protons in their atomic centers, but different numbers of neutrons. The atomic mass of all isotopes within a family of atoms is always the same. The atomic number of all isotopes within a family of atoms is always the same. Atoms of the same element can sometimes share two electrons. Atoms of different elements can sometimes share electrons.
Atoms share electrons so as to fill energy levels.
Only eight electrons can occupy an one orbital. Two electrons shared between two atoms creates a force or bond that holds the atoms together. More than two electrons are sometimes shared between atoms. Covalent bonds can only form between atoms that can ionize. Ionization of a hydrogen atom produces a proton.