Using a mass spectrometer, physicists have determined the mass of a hydrogen atom to be 1.660538921(73) × 10-27 kilograms, where the parenthetical digits are not known with complete certainty.

This is precisely the difference between the new, smaller, dimension of the proton, the nucleus of the hydrogen atom, and the value ... as Planck's constant or the mass of the electron can only ...

Generally, transition metal catalysts are essential for the reductive hydrogen atom transfer reaction, which is also known as the transfer hydrogenation reaction or the borrowing-hydrogen reaction.