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. Methane is an isotope of carbon. Water molecules are smaller than molecules of oxygen gas. Hydrogen and oxygen share the electrons equally in the covalent bonds of a water molecule. A polar molecule is one that is most stable at the poles of reaction.