Question

Although Rutherford's model was highly successful, scientists realized that it was incomplete. For example, hydrogen atoms contain one proton, and helium atoms contain 2, yet a hydrogen atom has only 1/4 the mass of a helium atom.

I'm not comprehending what my textbook is trying to state in this short blurb.

Answer 1

It's trying to tell you (and not doing a great job of it) that the He atom must contain something not in the H atom. The H atom has 1 proton and the He atom has 2. Since the He atom weighs 4 times as much as the H atom, it must contain something else that contributes to the mass. In about 1932 the neutron was discovered and that's the magic answer. H atom has 1 proton with mass 1. He is 2 protons and 2 neutrons with a mass of 4.00.

Answer 2

Oh I see now, thanks!