Why does Li form covalent bond unlike other alkali which forms ionic bond?

Answer 1

Atomic number of Lithium is three. It has got two electrons in the first orbit. The third electron is free and in the outermost orbit. The atomic number of Sodium is 11 and that of potassium is 19. Sodium and potassium one electron in the outermost orbit. This outermost electron can be easily donated to have stable configuration. That is like Neon and Argon, respectively. So sodium and potassium can conveniently give up the last electron, rather than taking seven electrons to complete the outermost orbit. When sodium gives an electron, it loses about ten percent of charge. If potassium loses an electron, it loses about 5.5 percent charge. But if Lithium loses an electron, it loses 33.33 percent electrical charge. It will become grossly electrically positive in that case. Lithium feels uncomfortable to lose that much electrical charge. If Lithium forms the covalent bond, it does not have to become grossly electrically positive. Rather the electrical charge remains same, as the electrons are shared up in case of covalent bond. That is the reason as the why Lithium forms covalent bond, unlike other alkaline metals.

Answer 2

Lithium compounds with halogens, for example, have ionic bonds.