We don’t usually use molecular formulas to refer to organic compounds because they give so little information. Instead, we use one of the following representations:

expanded molecular formula
a large molecular formula where each individual carbon is shown and is bonded to its adjacent symbols; brackets indicate side branches; for example, the EMF of 2,2-dimethylbutane is CH3C(CH3)2CH2CH3
structural model
a three-dimensional model, either the ball and stick model or the space-filling model; see page 11 of the textbook for examples
complete structural diagram
a two-dimensional representation of the compound that shows every atom and the bond lines between them
condensed structural diagram
like complete diagrams, but hydrogen atoms don’t have to be shown individually: write CH3 instead of C with —H three times
line structural diagram
the most compact representation; there is an implied carbon atom at any point where the line ends or bends (vertices and termini), and hydrogen atoms are inferred
Complete, condensed, and line structural diagrams for methylbutane