Electronegativity. The covalent bond

If atoms are linked by shared electron pairs, the bond is called covalent. Between identical atoms (H₂, Cl₂, O₂, N₂) the shared pair sits in the middle — the bond is nonpolar; between atoms whose electronegativities differ slightly (HCl, H₂O, NH₃, CH₄) the pair shifts towards the atom with higher EN — the bond is polar and the atoms carry partial charges (δ⁺, δ⁻). Each shared pair is written as one line: H–H, O=O, N≡N, H–Cl; the number of shared pairs equals the valency of the element. In a donor–acceptor bond one atom (the donor) supplies the shared pair from its own unshared pair and the other (the acceptor) supplies an empty orbital: NH₃ + H⁺ → NH₄⁺, H₂O + H⁺ → H₃O⁺. The bond formed is no different from an ordinary covalent bond. Noble gases are single atoms because their outer shell is already complete.