A-Level Trends in the elements across a period and down a group: worked solution

Question

Account for why the Pauling electronegativity of sodium (0.93) differs from that of chlorine (3.16).

Electronegativity is the ability of an atom to attract the bonding pair of electrons in a covalent bond. Na and Cl are in the same period (period 3), so shielding by inner shells is similar, but Cl has 17 protons versus Na's 11, i.e. a much greater nuclear charge, and Cl has a smaller atomic radius. The bonding electrons therefore experience a stronger effective nuclear attraction with Cl, giving it a high electronegativity (3.16), whereas Na's weaker nuclear pull and larger radius give a low value (0.93).