While this concept is simple, it is important as all of organic chemistry is based on Carbon. Carbon needs 4 atoms in its last shell. This means that at any point, it has to be bonding exactly four times.
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These are some examples of how this octet configuration can be reached.
1
Single bond with Hydrogen atoms.
2
Single bond with two Hydrogen atoms and 1 double bond with a Carbon atom.
3
Single bond with a Hydrogen atom and a triple bond with a Carbon atom.
4
Double bond with an Oxygen atom and a Carbon atom.
The ability of Carbon atoms to form chains and ring structure molecules.$^1$
For example, Heptane($\text{C}7\text{H}{16}$)
Each carbon atom is single bonded with two other Carbon atoms and two Hydrogen atoms. The Carbon atoms at the end of each chain are single bonded with one Carbon atom and three Hydrogen atoms.