**Covalent Bond**

A covalent bond is a type of chemical bond formed by the sharing of electron pairs between atoms. It occurs between nonmetal atoms, as they have high electronegativity and tend to gain electrons. Covalent bonds are characterized by the sharing of electrons, which allows both atoms to achieve a stable electron configuration.

**Examples of Covalent Bonds**

a) NH3 (Ammonia):
In ammonia, nitrogen (N) forms three covalent bonds with three hydrogen (H) atoms. Each hydrogen atom shares one electron with the nitrogen atom, resulting in a full outer shell for nitrogen and a stable configuration for both atoms.

b) CF4 (Carbon Tetrafluoride):
In carbon tetrafluoride, carbon (C) forms four covalent bonds with four fluorine (F) atoms. Each fluorine atom shares one electron with the carbon atom, resulting in a full outer shell for both carbon and fluorine.

c) HF (Hydrogen Fluoride):
In hydrogen fluoride, hydrogen (H) forms a covalent bond with fluorine (F). The fluorine atom shares one electron with the hydrogen atom, resulting in a full outer shell for both atoms.

**Non-Example: LiF (Lithium Fluoride)**

LiF is not an example of a covalent bond but rather an example of an ionic bond. An ionic bond is formed between a metal and a nonmetal, where the metal loses electrons to become a cation, and the nonmetal gains electrons to become an anion. In the case of LiF, lithium (Li) loses one electron to become Li+, while fluorine (F) gains one electron to become F-. The electrostatic attraction between these oppositely charged ions forms an ionic bond.

**In Summary**

Lithium fluoride (LiF) is not an example of a covalent bond because it involves the transfer of electrons between a metal (Li) and a nonmetal (F), forming an ionic bond instead. The other options listed (NH3, CF4, and HF) are all examples of covalent bonds, as they involve the sharing of electron pairs between nonmetal atoms.