Answer: Relative nuclear spin orientation of the two H nuclei (parallel vs antiparallel).
- A Relative nuclear spin orientation of the two H nuclei (parallel vs antiparallel)
- B The precisely measured H-H bond length within each form of the molecule
- C The overall H-H bond dissociation energy required to break the molecule apart
- D Their isotope composition, with the para form containing deuterium instead
Correct answer: A. Relative nuclear spin orientation of the two H nuclei (parallel vs antiparallel)
Explanation: ortho-H<sub>2</sub> has parallel proton spins (triplet); para-H<sub>2</sub> has antiparallel spins (singlet). They have slightly different physical properties.
Each water molecule's O-H bonds (covalent, solid) can hydrogen-bond (dashed) to neighbouring molecules, forming an extended network responsible for water's unusually high boiling point and density anomaly.
Concept context
The simplest and most abundant element in the universe. Study its unique position in the periodic table, isotopes (protium, deuterium, tritium), properties of water and hydrogen peroxide, and industrial uses.