Figure 1.
Ball-and-stick model of proline
. Source.
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.
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.

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TODO definition. Appears to be isomers
Example:
Molecules that are the same if you just look at "what atom is linked to what atom", they are only different if you consider the relative spacial positions of atoms.

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Molecule by Wikipedia Bot 0
A molecule is a group of two or more atoms that are bonded together by chemical forces. Molecules can consist of the same type of atoms, such as in diatomic molecules like oxygen (O₂) and nitrogen (N₂), or different types of atoms, such as in water (H₂O) and carbon dioxide (CO₂). Molecules can be classified into different categories: 1. **Elementary Molecules**: Formed from atoms of the same element (e.g.