Carbon is small and has four valence electrons; these factors combine into a unique structure that allows it to easily make a chain of carbon atoms. It creates covalent bonds — the strongest bonds between atoms.
A covalent bond is one where atoms share electrons to form a bond. This type of bond is stronger than an ionic bond where electrons are donated to another atom. Valence electrons are carried by an atom in the outer shell, which can be shared with another atom.
Because of carbon's four valence electrons, it can even bond with itself two or three times. These bonds are all covalent bonds, creating a strong chain of atoms. Because each bond has eight valence electrons, it also fulfills the octet rule, which establishes that a molecule containing certain elements (such as carbon or nitrogen) is more stable if it has eight valence electrons.
Similarly, carbon also bonds well with other atoms. In fact, scientists know of at least 10 million compounds that include carbon, according to the Jefferson Lab. The study of these compounds is what is known as organic chemistry. In fact, carbon is the element found with the most frequency in living things on Earth, which is why scientists say life is carbon based. Carbon is essential for life on Earth.