In the box provided explain why living organisms
contain more hydrogen atoms than any other
atoms, yet 65% of a typical organism's mass is
oxygen

Living organisms  contain more hydrogen atoms than any other  atoms, yet 65% of a typical organism's mass is  oxygen because hydrogen contains 16 times less mass than oxygen.

Explanation:

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.