A-Level How atoms bond and how that governs a substance's behaviour: worked solution

Question

Using ideas about electronegativity, account for why the bonding in NaCl differs from that in PCl5.

Bonding type is governed by the electronegativity difference between the atoms. In NaCl, sodium (a metal) has very low electronegativity and chlorine has high electronegativity, so the electronegativity difference is large. The bonding electron is transferred essentially completely from Na to Cl, forming Na+ and Cl- ions held by electrostatic attraction: the bonding is ionic. In PCl5, phosphorus and chlorine are both non-metals with relatively high and fairly similar electronegativities, so the electronegativity difference is small. The atoms share electrons rather than transferring them, forming covalent P-Cl bonds; the bonding is covalent.