Which compounds are formed by sharing of electrons? 

Covalent compounds 

The correct answer is option 3. Covalent compounds.

Covalent Compounds: In covalent bonding, atoms share electrons to achieve a stable electron configuration. This occurs primarily between nonmetal atoms, as they have similar electronegativities and tend to attract electrons equally. When two atoms come close together, their orbitals overlap, allowing the electrons to be shared between them. This sharing creates a bond known as a covalent bond.

Characteristics of Covalent Compounds:

• They typically form between nonmetal atoms.
• Covalent bonds are formed by the sharing of electrons.
• Covalent compounds often exist as discrete molecules.
• They tend to have lower melting and boiling points compared to ionic compounds.
• They are often insoluble in water.
• Covalent compounds can be gases, liquids, or solids at room temperature, depending on their molecular structure and intermolecular forces.

Examples of covalent compounds include water \((H_2O)\), methane \((CH_4)\), carbon dioxide \((CO_2)\), and ammonia \((NH_3)\). In these compounds, atoms share electrons to complete their outer electron shells, resulting in the formation of stable molecules.