The Periodicity of Hydrides in Groups 4 to 7

Hydride is the name given to any hydrogen compound. There are three types of hydrides: ionic hydrides, covalent hydrides and transitional metal hydrides. Ionic hydrides consist of hydrogen and an electropositive metal. The hydrogen atoms form ionic bonds with an electropositive metal by acquiring an electron from the metal to complete its S-shell. Covalent hydrides however consist of a covalent bond between hydrogen atoms and electronegative metals (generally groups 4-7). In transitional metal hydrides, the …

…one lone pair of electrons which cannot satisfy all the hydrogen atoms. Hydrogen fluoride (HF) has the opposite issue. With hydrogen fluoride, there are not enough hydrogen atoms to satisfy the number of lone pairs of electrons as hydrogen fluoride has 3 lone pairs of electrons and one hydrogen bonded to it. Since ammonia has more hydrogen bonds as there are already hydrogen bonds within the molecule, ammonia has a higher boiling point than hydrogen fluoride.