Question

Which of the following is a nuclear isomer of hydrogen?

• A. Ortho-H
• B. Para-H
• C. Both $A$ and $B$
• D. None of these

Answer 1

The correct option is C Both $A$ and $B$

Molecular hydrogen occurs in two isomeric forms, one with its two proton spins aligned parallel (orthohydrogen), the other with its two proton spins aligned antiparallel (parahydrogen). At room temperature and thermal equilibrium, hydrogen consists of approximately $75\%$ orthohydrogen and $25\%$ parahydrogen. The para form dominates at low temperatures $($approx. $99.8\%$ at $20$ K$)$.