 Now that we have a handle on the electrons around the atom, let's take a quick look at the nucleus. For atoms to be neutral, the number of protons with a positive charge must equal the number of electrons with their negative charge. But mass spectrometers showed that atoms have more mass than the number of protons alone could account for. For example, carbon has six protons and six electrons, but its mass is just a tad more than the mass of 12 protons. In the 1920s, it was assumed that electron-proton pairs existed in the nucleus to account for the increase in mass without an increase in charge. But with the advances in quantum mechanics, it became clear that an electron couldn't exist in a volume as small as the nucleus. Lawrence Rutherford and James Chadwick proposed that a new particle, the neutron, must exist in the nucleus to account for the data. In 1932, Chadwick and others performed a series of experiments verifying his suggestion. They began by beaming alpha particles into beryllium. This produced a radiation that was not affected by applied electric fields. In other words, it was electrically neutral. At first this radiation was thought to be gamma rays, but when this new radiation was used to bombard a hydrogen-rich substance like paraffin, a proton radiation was produced. The energy acquired by those protons was measured and found to be more than a gamma ray could possibly impart to a proton. In fact, protons ejected from the paraffin on the right was equal to the energy of the radiation coming out of the beryllium on the left. The conclusion was that the particles hitting the paraffin were of the same mass and energy as the protons, but without any charge. At this point it was generally accepted that indeed the neutron had been discovered.