A polar covalent bond is a covalent bond in which the electrons are not shared equally between the two atoms. This can occur when the two atoms have different electronegativities, meaning that one atom attracts electrons more strongly than the other. In the case of NO2-, the oxygen atoms have a higher electronegativity than the nitrogen atom, so the electrons are pulled more towards the oxygen atoms. This creates a partial negative charge on the oxygen atoms and a partial positive charge on the nitrogen atom.