Ammonia, NH3, has a higher melting point than PF3, despite the fact that melting point increases as you move down a group of elements in the periodic table. Ammonia has a higher melting point because of the
Although, it is a fact that the melting point increases as you move down a group of elements in the periodic table, you cannot use this information for compounds since the type of bonds present in the compounds are another factor that would affect their properties. For this case, ammonia would have a higher melting point as compared to PF3 because of the presence of hydrogen bonding. PF3 only has dipole-dipole forces which is much weaker as compared to a hydrogen bond. Hydrogen bonding is a special type of dipole forces which is a result of the electronegativity of a hydrogen atom and a very electronegative atom.
