Concept:
Chemical bonds are formed in different ways. Ionic bonds form by transfer of electrons, while covalent bonds form by sharing of electrons.
Step 1: In a covalent bond, two atoms share one or more pairs of electrons.
Step 2: This sharing helps both atoms complete their valence shell.
Step 3: Transfer of electrons forms ionic bonds.
Step 4: Presence of free electrons is related to metallic bonding.
Therefore, covalent bond is formed due to:
\[
\boxed{\text{Sharing of electrons}}
\]