1234CsNaMgAlSiHCNClOFpulls electrons weaklypulls stronglyH–H no gap: shared evenlyH–Cl gap 1.0: shared unevenly (polar)Na–Cl gap 2.2: handed over (ionic)
Electronegativity measures how hard an atom pulls on the electrons in a bond. Metals on the left pull weakly, and the atoms near a full shell, oxygen and fluorine above all, pull hardest. Two atoms with the same pull share their electrons evenly. A modest gap makes a polar bond, where the shared pair sits closer to one atom. A gap above about 1.7 means the stronger atom takes the electron outright, and the bond is ionic.