Question

why one mole of hydrogen molecules is heavier than one mole of hydrogen atoms​

Answer 1

However, one mole of hydrogen atoms has a mass of 1 gram while one MOLE of hydrogen molecules has a mass of 2 grams. Recall that two hydrogen atoms bind to make a hydrogen molecule. The same can be said of one mole of any other substance where two atoms combine to make a simple molecule.

this is your answer I hope it will help you.