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.

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.

Oh I see now, thanks!

The blurb is stating that Rutherford's model of the atom, which was highly successful at the time, had a limitation when it came to explaining the difference in masses between hydrogen and helium atoms.

To understand this, let's break it down step by step:

1. Rutherford's model: Ernest Rutherford proposed a model of the atom in 1911, often referred to as the planetary or nuclear model. According to this model, atoms have a dense, positively-charged nucleus at the center, with negatively-charged electrons orbiting around it in fixed paths.

2. Hydrogen atoms: A hydrogen atom consists of one proton in its nucleus and one electron orbiting around it.

3. Helium atoms: A helium atom consists of two protons in its nucleus and two electrons orbiting around it.

4. Mass comparison: The observation in the blurb is that even though the number of protons in a hydrogen atom is half that of a helium atom, the mass of a hydrogen atom is only 1/4 of a helium atom.

The reason for this observation lies in the fact that both protons and neutrons in the nucleus contribute to the atomic mass of an atom. Protons have a positive charge, while neutrons have no charge. Electrons, on the other hand, have a negligible mass compared to protons and neutrons, so they don't significantly affect the overall atomic mass.

In the case of hydrogen, with only one proton and no neutrons, its atomic mass is primarily determined by the mass of the single proton. Therefore, the atomic mass of hydrogen is relatively low compared to helium.

In helium, with two protons and usually two neutrons in the nucleus, the combined mass of the protons and neutrons contributes significantly to the overall atomic mass. As a result, helium has a higher atomic mass compared to hydrogen.

So, in summary, Rutherford's model didn't consider the contributions of neutrons to the atomic mass, which is why it couldn't explain why a hydrogen atom has significantly less mass than a helium atom, despite having fewer particles in its nucleus.