# What are skeletal formulas or skeletal structures (of organic compounds) ?

We know say that $\text{n-butane}$ is ${C}_{4} {H}_{12}$..we could write it long hand...${H}_{3} C - C {H}_{2} C {H}_{2} C {H}_{3}$...alternatively, we could write it as...
The second representation is the skeletal formula....the zig-zag represents a methylene unit, i.e. $C {H}_{2} C {H}_{2}$...and of course with more complicated structures we could insert heteroatoms, and carbocycles, and heterocycles appropriately.