1. James “Jimmy Neutron” Chadwick
He discovered the neutron.

2. The Evidence that declared the previous model wrong
The atomic mass and the atomic number did not match up.
For instance, Helium had an atomic mass of four but its atomic number was two. There seemed to be no connection between the two, which left a discrepancy.
It appeared that there were other particles within the nucleus other than protons, but it was not a possibility that the particles could be electrons; therefore, the old model of an atom had to be incorrect.

3. Chadwick’s Experiments
Chadwick’s data came from an experiment performed by German scientists Bothe and Becker in 1930.
In this experiment, a sample of beryllium was bombarded with alpha particles, producing a radiation they assumed to be gamma rays, an assumption supported by the fact that the radiation was neutrally charged.
In 1932, however, Iréne and Frédérick Joliot-Curie repeated the experiment with the addition of a block of wax which the radiation struck.
A Geiger counter measured the number of protons dislodged by the radiation, indicating that this radiation penetrated much further than gamma rays.
Seeing the results, James Chadwick realized that this could be proof of a new type of neutral particle.
He calculated that this new particle would have approximately the same mass as a proton, and proved that it was neutrally charged by passing them between a pair of positively and negatively charged plates, which did not affect the particle’s path.
James Chadwick had demonstrated the existence of the neutron.

4. Before the Neutron
Before James Chadwick demonstrated the existence of the neutron, atoms were thought to consist of a single, massive, positively-charged nucleus orbited by electrons. There were several issues with this model, as several other scientists had already pointed out.
The biggest failure of the model was that it did not account for the fact that certain elements had atomic masses which were higher than would be expected if the charge to mass ratio of the nucleus was consistent.
The leading theory at the time was that the nucleus of an atom was made up of protons, all of which had identical charges and masses.
This clearly could not fit the experimental data, however, so another type of particle had to be included.
Ernest Rutherford proposed the the missing particles were actually a closely-bonded proton and electron.
It was James Chadwick, a scientist who had worked with Rutherford in the past, who suggested the existence of a third type of particle, which he called the neutron.

5. How James Chadwick Remodeled the Atom
The previous atomic model, containing only a positively charged nucleus surrounded by electrons, was flawed.
James Chadwick remade the atomic model, fixing said flaws.
James’ fix to the flaws was adding a neutral particle, a neutron, to the atom.
This addition to the model accounted for the generally higher atomic mass than the atomic number.
James Chadwick's new model both fixes errors in the previous models and incorporates his new evidence which he, alongside Ernest Rutherford, discovered.

6. Interesting “tidbits” about James Chadwick
Chadwick was a prisoner of war in Germany during World War I
Jimmy Neutron is named after the famous scientist
He led the British mission in the Manhattan Project
Chadwick interned at a camp in Berlin during the war
Chadwick detonated the first atomic bomb in 1945
James Chadwick won the Nobel Prize

7. Citations