noun Physics, Chemistry.
the form of molecular hydrogen in which the nuclei of the two hydrogen atoms contained in the molecule have spins in opposite directions.
Compare orthohydrogen.

1925–30; para-1 + hydrogen Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
parahydrogen (ˌpærəˈhaɪdrədʒən)
chem Compare orthohydrogen the form of molecular hydrogen (constituting about 25 per cent of the total at normal temperatures) in which the nuclei of the two atoms in each molecule spin in opposite directions

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