
One of two or more compounds with the same formula but different arrangements of atoms.An isomer is one of two or more compounds that share the same molecular formula but differ in the arrangement of atoms, leading to distinct physical and chemical properties. This structural diversity is fundamental in chemistry, as isomers can have vastly different behaviors, from toxicity to reactivity. In astrophysics, isomers of molecules like hydrogen cyanide are found in interstellar space, where their relative abundances reveal the temperature and history of star-forming regions.
The concept was introduced in the 1830s by Jöns Jacob Berzelius, who coined the term 'isomerism' after observing that compounds like silver fulminate and silver cyanate had the same composition but different properties. The structural explanation came later with Kekulé and Couper in the 1850s.
Isomers arise because atoms can connect in different sequences (structural isomers) or adopt different spatial orientations (stereoisomers). The energy differences between isomers are often small, so external conditions like temperature or radiation can drive transitions. In space, the ratio of isomers is set by the local environment, acting as a thermometer or a probe of chemical pathways.
Isomer analysis is used in pharmaceutical development to ensure drug efficacy and safety, and in astrochemistry to interpret spectral lines from telescopes, revealing the conditions in molecular clouds.
Nuclear isomers, which are metastable states of atomic nuclei, can store energy for hours or even years, and their sudden decay can trigger powerful gamma-ray flashes, as observed in cosmic events.













