Where does isomer collide?

Two or more distinct compounds that share the same molecular formula, the same count of each kind of atom, but differ in how those atoms are connected or arranged in space, so they are separate substances with their own properties. The class covers constitutional isomers, which differ in the order of bonding, and stereoisomers such as cis-trans pairs and mirror-image enantiomers, which differ only in spatial arrangement; Moss's 1996 IUPAC recommendations Basic terminology of stereochemistry set out these categories. The referent here is a whole molecule built from the same atoms in a different pattern, a matter of chemical bonding and shape, and it is not a state of a single atomic nucleus.[1]

Unit

One of a set of compounds sharing a molecular formula but differing in structure

Recovery means

Some isomers interconvert by isomerization, but configurational stereoisomers such as enantiomers are fixed unless bonds are broken

Measured by

Distinguished by spectroscopy, chromatography, X-ray crystallography, and optical rotation for chiral pairs

A long-lived excited state of an atomic nucleus that has the same number of protons and neutrons as its ground state, so it is the same isotope of the same element, but holds extra energy and is slow to release it, sometimes surviving for seconds, hours, or far longer. The delay arises because reaching the ground state would require a large change in the spin of the nucleus, which traps the energy; Walker and Dracoulis's 1999 review Energy traps in atomic nuclei explains this, and technetium-99m, a nuclear isomer used in medical imaging, is a familiar example. The referent is one nucleus sitting at a higher energy level, a matter of nuclear structure measured in millions of electron-volts, not two different molecules made from the same atoms.[2]

Unit

One metastable nuclear energy level, marked by a small m after the mass number, as in Tc-99m

Recovery means

The isomer decays to its ground state by emitting a gamma ray or by internal conversion, on its own characteristic half-life

Measured by

Characterised by its half-life, the energy of the gamma rays it emits, and the spin difference from the ground state