Question

What observation led chadwick (and rutherford) to conclude there must be something besides just the proton in the nucleus of atoms?

Answer 1

Rutherford used the gold foil experiment to disprove Thompson Plum Pudding model of the atom. Rutherford experiment showed that the center of the atom contains a positively charged nucleus and most of the atom’s mass lies in the center of the atom in a small volume. Later Chadwick proved that the nucleus of the atom also contains uncharged particles called neutrons.  

Answer 2

Rutherford discovered the nucleus, and Chadwick discovered the neutrons. Before their discoveries, the widely accepted theory was Thompson's where the electrons and protons are located all inside the nucleus. However, when Rutherford did his gold foil experiment, some of the cathode rays (negatively charged) were reflected back to the source. This is due to the presence of the dense nucleus where the neutron resides.