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A chemical structure specifies how chemical atoms are connected and, where relevant, how they are arranged in space. A molecular formula counts atoms; a structural formula supplies connectivity information. Molecular geometry and stereochemistry add spatial distinctions that cannot generally be recovered from the atom inventory alone.^[ref]

For a concrete comparison, ethanol and dimethyl ether both have molecular formula ${\mathrm{C}}_2{\mathrm{H}}_6\mathrm{O}$. Their condensed structural formulas arerespectively. In ethanol, the oxygen is bonded to one carbon and one hydrogen; in dimethyl ether, it is bonded to two carbon atoms. The same atom counts therefore describe different connectivities.

A drawing is a representation of a chemical structure, not necessarily a frozen picture of every atom's position. Rotating around a bond can change a molecule's spatial arrangement while preserving its connectivity. Conversely, two drawings that look different on the page can represent the same chemical structure. Establishing identity requires interpreting bonds and spatial conventions, not comparing the appearance of the diagrams alone.

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.

Dimethyl ether, also called methoxymethane, is an ether with an oxygen atom bonded to two methyl groups. Its structural formula is $\mathrm{C}{\mathrm{H}}_3-\mathrm{O}-\mathrm{C}{\mathrm{H}}_3$ and its molecular formula is ${\mathrm{C}}_2{\mathrm{H}}_6\mathrm{O}$. It is a constitutional isomer of ethanol: the atoms have the same inventory but different connectivity.^[ref]

The oxygen connectivity gives a useful molecular comparison. Ethanol contains an oxygen–hydrogen bond; dimethyl ether does not. Thus the two chemical structures present different possibilities for hydrogen bonding, even though weighing equal numbers of their molecules would not distinguish their common elemental formula.

Their physical identity can be tested by measurements beyond composition. NIST tabulates a normal boiling point near $248.3\mathrm{K}$, approximately $-24.9{}^{\circ }\mathrm{C}$, for dimethyl ether. This illustrates why a small carbon-containing molecule can be a gas under ordinary room conditions: phase behavior follows its interactions and external conditions, rather than molecular formula alone.^[ref]