Why is HF not ionic even though the difference in electronegativities of hydrogen and fluorine is 1.9? Why is it considered a polar covalent bond?
The Correct Answer and Explanation is:
Hydrogen fluoride (HF) is not ionic despite the electronegativity difference of 1.9 between hydrogen and fluorine because the bond still involves the sharing of electrons rather than a complete transfer of electrons, which is characteristic of ionic bonds.
In an ionic bond, one atom completely donates an electron to another, forming positive and negative ions. This typically occurs between metals and nonmetals where the electronegativity difference is greater than 2.0, and one atom has a very low ionization energy while the other has a very high electron affinity.
In HF, hydrogen has an electronegativity of approximately 2.1, while fluorine has an electronegativity of about 4.0. The difference is 1.9, which falls within the range generally considered polar covalent rather than fully ionic. In a polar covalent bond, the electrons are shared unequally between the atoms. Fluorine, being much more electronegative, pulls the shared electrons closer to itself, creating a partial negative charge (δ−) on fluorine and a partial positive charge (δ+) on hydrogen. However, there is no full electron transfer to form distinct ions, which is the hallmark of an ionic bond.
Furthermore, the physical properties of HF also support its classification as polar covalent. HF exists as discrete molecules in the gaseous and liquid states and exhibits hydrogen bonding, a strong type of dipole interaction common in polar covalent molecules. Ionic compounds, on the other hand, typically form crystalline solids at room temperature and conduct electricity when molten or dissolved in water.
Thus, despite the relatively high electronegativity difference, HF is best described as having a polar covalent bond because of the shared nature of the electrons and the molecular behavior of the compound.
