Structural formula

A structural formula represents how chemical atoms in a molecule are connected. It adds information that a molecular formula alone does not provide. Depending on the notation, bonds and grouped atoms may be drawn explicitly or represented in a condensed structural formula.^[ref]

For ethanol, $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ can be unpacked as a carbon with three hydrogens, joined to a carbon with two hydrogens, joined to oxygen, joined to hydrogen. Counting the groups recovers ${\mathrm{C}}_2{\mathrm{H}}_6\mathrm{O}$, since $3+2+1=6$. That count is a useful check, but it discards the connectivity information that distinguishes ethanol from dimethyl ether, $\mathrm{C}{\mathrm{H}}_3\mathrm{OC}{\mathrm{H}}_3$.

A structural drawing also needs a clear convention about spatial meaning. A plain line usually represents a chemical bond, rather than a measured length on the page. Rotating the paper does not change the represented molecule. If a drawing is intended to specify a three-dimensional configuration, the additional conventions belong to stereochemistry; a connectivity formula should not silently be treated as complete molecular geometry.

A condensed structural formula writes connected groups of chemical atoms compactly, omitting some explicit bond strokes. It preserves interpretable connectivity information, unlike a molecular formula that merely totals the atoms. It is one way to represent a structural formula, whose purpose is to describe how atoms are connected.^[ref]

For an original counting example,can be unrolled as $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_3$. The two terminal groups supply two carbons and six hydrogens; four interior groups supply four carbons and eight hydrogens. Thus the molecular formula is ${\mathrm{C}}_6{\mathrm{H}}_{14}$, since $2+4=6$ and $6+8=14$. The parentheses abbreviate a connected repeated group, rather than introducing a separately floating molecule.

The compression must be interpreted with the chosen convention. A formula such as $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ specifies different connectivity from $\mathrm{C}{\mathrm{H}}_3\mathrm{OC}{\mathrm{H}}_3$ despite their common atom counts. Neither expression by itself gives every bond angle or a unique three-dimensional conformation; those questions involve molecular geometry and stereochemistry.