Isomers were quite confusing for early chemists, before atomic theory was widely accepted, and people where thinking mostly in terms of proportions of equations, related: Section "Isomers suggest that atoms exist".
Exist because double bonds don't rotate freely. Have different properties of course, unlike enantiomer.
Bibliography:
Key exmaple: d and L amino acids. Enantiomers have identical physico-chemical properties. But their biological roles can be very different, because an enzyme might only be able to act on one of them.
Example:
- the three most table polymorphs of calcium carbonate polymorphs are:
Articles by others on the same topic
Isomers have the same molecular formula but different arrangements of their atoms. Constitutional isomers differ in connectivity; stereoisomers retain connectivity but differ in spatial arrangement.
For example, ethanol and dimethyl ether share but connect their atoms differently. Two enantiomers instead have mirror-related spatial structures. Formula, connectivity, and stereochemistry therefore encode different levels of chemical identity.