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physical object > natural object > particle > molecule > diatomic molecule > molecular hydrogen > para-hydrogen

para-hydrogen comparison table

Subject	is a kind of	is an instance of	has definition
molecular hydrogen	neutral particle	diatomic molecule	A molecule of hydrogen, discovered in interstellar space in 1970. H2 is a very hard molecule to detect. None of its transitions lie in the visible part of the spectrum. Second, being a symmetric homonuclear molecule, it does not have an electric-dipole rotation-vibration spectrum, and detection must be based on the weak quadrupole spectrum. Third, ultraviolet radiation is a very efficient dissociator of H2, so any H2 that survived would presumably be located inside very dense interstellar clouds. So far observations have borne out this supposition. Measurements of the region within about 1 kpc of the Sun suggest that H2 is about twice as abundant as atomic H.
para-hydrogen		molecular hydrogen	Molecular hydrogen in which the two protons of the diatomic molecule have opposite directions of spin. It is a lower energy state than ortho-hydrogen.

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