Carbon's atomic number is 6 and it's most likely to form

covalent bonds. Why is this significant to living organisms?

Ability of carbon to form four covalent bonds allow it to form various biomolecules that are the compounds of carbon with a variety of functional groups.

Explanation:

Carbon accounts for more than half the dry weight of cells. A carbon atom form single bonds with hydrogen atoms, and both single and double bonds with oxygen and nitrogen atoms. It has four valence electrons and can form very stable four carbon-carbon single covalent bonds. Alternatively, two carbon atoms also can share two or three electron pairs to form double or triple bonds.

Covalently linked carbon atoms in biomolecules form linear chains, branched chains, or cyclic structures. Therefore, the ability of carbon atoms to form covalent bonds allow it to form the carbon skeletons for biomolecules to which other functional groups are added to confer specific chemical properties on the biomolecules. No other atom can form molecules of diverse sizes and shapes carrying a variety of functional groups. For example, fatty acids are the chain of hydrocarbon with the COOH group. Most of the carbohydrates are represented as (CHO) n.