Question

Why is hf a covalent molecule even though h and f have high electronegativity difference?

Answer 1

HF (electronegativity difference 1.9 on the Allred-Rochow scale) and SiF4 (2.36) both have electronegativity differences around 2. But they are, in fact, molecular compounds that are gases at room temperature.

Meanwhile, CaS has an electronegativity difference ( ΔEN ) of 1.40. It forms a cubic crystal just like sodium chloride, has a high boiling point, conducts electricity when molten, and generally behaves like any other ionic compound. But... the average electronegativity ( EN¯ ) of HF is 3.15; that of SiF4 is 2.92; and that of CaS is 1.74. The combination of ΔEN and EN¯ puts the first two in the covalent region, and the third in the ionic region, of the van Arkel-Ketelaar bond-type triangle.