What causes atoms to share electrons equally with atoms, but unequally share with others?

Atoms only share electrons in covalent bonds. covalent bonds are only formed between nonmetals. there are two different types of covalent bonds; polar and nonpolar. in a polar covalent bonds electrons are shared unequally because one atom has a stronger force of attraction than the other (EX. H2O). In a nonpolar covalent bond, the electrons are shared equally because they're electronegativity is also equal (EX. Cl2).

Overall, the atoms electronegativity determines the way it shares electrons