The mass of helium atoms are higher than predicted given the number of protons. Helium atoms have an actual mass of about 4 amu. In fact, most atoms seem to be too heavy. Lithium is known to have 3 protons, but a mass of 7 amu, carbon atoms have 6 protons and a mass of 12 amu. Rutherford proposed the idea that there could also be another particle in the nucleus with the protons. He thought these particles would be neutral and named them neutrons.

How does the idea of a neutron help address the puzzles about the protons in the nucleus and the mass of atoms?