LiH is an ionic compound with a high melting point (680°C). The structure of BeH2 (in the solid state) is that of a polymer; it is a covalent compound. The molecules B2H6 and CH4 are nonpolar. In contrast, NH3, H20, and HF are all polar molecules in which the hydrogen atom is the positive end of the polar bond. Of this group of hydrides (NH3 H20, and HF), only HF is acidic in water.