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7H-xanthine is the neutral xanthine tautomer with the indicated hydrogen attached to nitrogen7, while the two oxo groups occupy positions2 and6. ChEBI describes this form in CHEBI:48517 and relates it to 9H-xanthine. Both forms have molecular formula ${\mathrm{C}}_5{\mathrm{H}}_4{\mathrm{N}}_4{\mathrm{O}}_2$.^[ref]

The change from the 9H description to the 7H description relocates an existing hydrogen; it does not add another hydrogen or produce a 7+ ion. Thus an atom inventory, elemental analysis or formula-based molecular-mass calculation cannot distinguish the two forms. Their bonding and hydrogen placement must be specified as well.

A population example shows why a tautomer-specific name is more information than a generic formula. In an invented sample containing 1 millimole total xanthine, suppose an independently determined equilibrium assigned 30% to the 7H form and 70% to the 9H form. The amounts would be 0.3 and 0.7 millimole, respectively, but every molecule in this two-form model would still have the same elemental formula. These fractions are illustrative assumptions, not reported xanthine measurements.

The indicated hydrogen position also differs from protonation state: removing a proton would change the hydrogen inventory and net charge. Keeping these descriptions separate prevents an interconversion between tautomers from being mistaken for simple acid dissociation.

A tautomer is one of the readily interconvertible constitutional isomers related by tautomerism. In a proton-transfer example, the forms retain the same atom inventory and net charge but differ in where a hydrogen is attached and in associated bond orders. A molecular formula alone therefore cannot identify which tautomer is meant.^[ref]

Xanthine supplies a concrete example. ChEBI distinguishes 7H-xanthine, with the indicated hydrogen on nitrogen7, from 9H-xanthine, with it on nitrogen9. Both neutral forms have formula ${\mathrm{C}}_5{\mathrm{H}}_4{\mathrm{N}}_4{\mathrm{O}}_2$, and the curated records explicitly identify them as tautomers. The atom inventory is identical while the indicated hydrogen placement differs.^[ref]

Interconvertibility does not mean equal abundance. For an invented dilute two-tautomer equilibrium $A\rightleftharpoons B$ with $[B]/[A]=0.01$, the fraction in form $B$ is $0.01/(1+0.01)=1/101$, about0.99%. This is a bookkeeping example, not a measured xanthine ratio. Solvent, temperature and interactions with other molecules can affect which form is favored.

A tautomer is not merely a resonance drawing: atom attachments change between tautomers. It is also not automatically a different protonation state, because gaining or losing a proton changes the atom inventory or charge rather than just relocating an existing hydrogen.