# What are the guidelines to read skeletal-line structures?

When we look at a skeletal representation, we know that each bend of the chain represents $C {H}_{2}$, a methylene unit. Where there are 3 $C - C$ bonds depicted, a $C - H$ or methine bond is specified. If there is an unsaturated bond, i.e. $C = C$, or $C = O$, or $C = N$, OR a ring junction, the unsaturation (the double bond! or ring) will be so represented, and the structure will bear 2 hydrogens less than the saturated structure (this is called 1 degree of unsaturation). An alkyne will bear 4 hydrogens less than the saturated structure; i.e. acetylene, $H - C \equiv C - H$, versus ethane, ${H}_{3} C - C {H}_{3}$ (i.e. acetylene has 2 degrees of unsaturation). For a saturated molecule (i.e. single bonds only), each junction of the bond represents a methylene unit, a $C {H}_{2}$ group. For a linear alkane with $n$ carbon atoms, there will be $2 n + 2$ hydrogen atoms - alkanes are saturated. Of course, a carbon chain will waggle all over the place; we usually draw them so as to maximize molecular symmetry. Why? Because it's simpler that way.