(N/A) Based on the number of other carbon atoms attached to a specific carbon atom,it is classified as primary $(1^{\circ})$,secondary $(2^{\circ})$,tertiary $(3^{\circ})$,or quaternary $(4^{\circ})$.
$1^{\circ}$ (Primary) Carbon: $A$ carbon atom attached to no other carbon atom (as in methane) or to only one other carbon atom (as in ethane).
$2^{\circ}$ (Secondary) Carbon: $A$ carbon atom attached to two other carbon atoms.
$3^{\circ}$ (Tertiary) Carbon: $A$ carbon atom attached to three other carbon atoms.
$4^{\circ}$ (Quaternary) Carbon: $A$ carbon atom attached to four other carbon atoms.
Examples:
| Structure | Forms of Carbon |
| :--- | :--- |
| $H_3C-CH(CH_3)-CH_2-CH_3$ ($2$-Methylbutane) | Terminal $CH_3$ groups are $1^{\circ}$. The $CH$ carbon is $3^{\circ}$ as it is attached to $3$ other carbons. The $CH_2$ carbon is $2^{\circ}$ as it is attached to $2$ other carbons. |
| $C(CH_3)_4$ ($2$,$2$-Dimethylpropane) | The four terminal $CH_3$ groups are $1^{\circ}$. The central carbon is $4^{\circ}$ as it is attached to $4$ other carbon atoms. |