“Same atoms; distinct connectivities:
Two isomers’ key descriptivities.
With ‘parts being equal,’
Second structure’s a sequel
To first, in atomic vicinities.”
The 18 April 2025 limerick continued the week’s focus on scientific vocabulary, this time via a chemistry-specific term: “isomer.”
“Same atoms; distinct connectivities: /
Two isomers’ key descriptivities.”
For students taking Organic Chemistry, a common early learning goal involves the classification of two given molecules as structural isomers or stereoisomers (or neither).
Structural isomers are also called constitutional isomers; these are molecules with the same molecular formulas (same number of each type of atom) that differ in the order that the component atoms are connected. In stereoisomers, both the molecular formulas and the connection orders are the same, but the three-dimensional arrangements of the atoms differ.
Each of the major classifications here involves several more subcategories (i.e., tautomers, enantiomers, etc.), each of which takes much time to visualize and learn. Learning these nuances is generally enough work that we do not further delve into the etymologies of the terms, but those can be nonetheless fascinating.
“With ‘parts being equal,’ /
Second structure’s a sequel /
To first, in atomic vicinities.”
In his 1959 book Words of Science and the History Behind Them, Isaac Asimov explains that, in 1830, chemist Jöns Jakob Berzelius noted the increasing number of situations in which new compounds were being discovered that had the same molecular formulas but different properties. Berzelius suggested the name isomers to distinguish these cases from one another: iso from the Greek for “equal,” and mer from the Greek for “parts.”
The poem celebrates the fact that while the numbers of component atoms in two isomers would be the same, the “atomic vicinities” within those isomers would differ.
As with much chemistry vocabulary, “isomer” was a word I had used for many years without truly considering its origin, until last April’s poetic routine.
Chemphasis 