Solution:

Valency is the combining capacity of an element. For metals, it is equal to the number of electrons in the outermost shell of their atoms, and for nonmetals it is eight minus the number of electrons in the outermost shell.

The carbon atom’s four valence electrons can be shared by other atoms that have electrons to share, thus forming covalent (shared-electron) bonds.

They can even be shared by other carbon atoms, which in turn can share electrons with other carbon atoms and so on, forming long strings of carbon atoms, bonded to each other like links in a chain.