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Why do ethylene and propylene have the same empirical formula?

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representations of molecular structure — molecule Several methods of representing a molecule's structure. In Lewis structures, element symbols represent atoms, and dots represent electrons surrounding them. A pair of shared electrons (covalent bond) may also be shown as a single dash. The ball-and-stick model better illustrates the spatial arrangement of the atoms. For aromatic compounds, the Kekulé structure is common, in which each bond is represented by a dash, carbon atoms are implied where two or more lines meet, and hydrogen atoms are usually omitted. Bond-line formulas, similar to the Kekulé structure, are often used for complex nonaromatic organic compounds. Sugars are often drawn as Fischer projections, in which the carbon “backbone” is drawn as a straight vertical line, with carbon atoms implied where horizontal lines intersect the vertical one.

Ethylene and propylene have the same empirical formula, ${CH}_{2}$ , because they are both composed of carbon and hydrogen atoms in a 1:2 ratio.1

An empirical formula represents the simplest whole-number ratio of the elements present in a compound. While their molecular formulas—which state the actual number of atoms in each molecule—differ ( $C_{2} H_{4}$ for ethylene and $C_{3} H_{6}$ for propylene), both substances share the same relative proportion of constituent atoms.1