This is most pronounced for protium -LRB- NUMBER@1 -RRB- vis-à-vis deuterium -LRB- NUMBER@2 -RRB- , because deuterium has twice the mass of protium .

This `` mass effect '' is large for protium -LRB- NUMBER@1 -RRB- and deuterium -LRB- NUMBER@2 -RRB- , because deuterium has twice the mass of protium .
