tautomer

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A molecule that can change into two forms that differ by a hydrogen and a double bond

noun
/ˈtɔːtəˌmɜr/

Specialized

: tautómero

Examples

Your example of a tautology is textbook perfect, Jim, appealing even at the level of chemistry, where we define tautomers.

Blog text (15)
A tautomer can switch between different forms, affecting the compound's chemical behavior.
In organic chemistry, a tautomer often consists of a keto and an enol form, which can interconvert.
Researchers studied how tautomers influence the reactivity of certain chemical compounds.

Surface Forms

tautomer singular

tautomers plural

Morphology

tautomer = tauto + mer

Formed from specialized Greek combining forms ('tauto-' + '-mer') used in technical chemical terminology, which are not recognizable to average learners and do not transparently convey the meaning.

Etymology

Tautomer comes from Greek parts tauto 'same' and mer 'part'. A tautomer is one of two forms of a molecule that have the same parts arranged in a different way—often by moving a hydrogen and a double bond—so the name helps you remember the forms are 'the same' but can change into each other.