Question

explain why living organisms contain more hydrogen atoms than any other atoms, yet 65% of a typical organism’s mass is oxygen.

Answer 1

The reason is that oxygen atoms have a greater mass than hydrogen atoms by 16 times. This is because hydrogen has only one proton and no neutrons, so, its atomic number and mass is 1. While oxygen has 8 protons and 8 neutrons, so its atomic number is 8 and has a mass of 16.
Therefore it takes 16 hydrogen atoms to equal the mass of one oxygen atom.