The Elusive Neutron
Predecessors to its Discovery
Irene Joliot-Curie & Frederic Joliot
Joseph John Thomson
Through a series of collision experiments conducted between 1909 and 1914, the New Zealand physicist Ernest Rutherford and his students Hans Geiger and Ernest Marsden not only made a breakthrough in atomic physics but established the beginning of what is often taught as the model of an atom in schools today . Rutherford shot a beam of alpha particles (helium nuclei stripped of their electrons) created through the process of alpha decay of an element at a speed of about 20,000 km/s at a thin gold foil. He expected most of the particles to pass through the foil. However, to his surprise, a small number of alpha particles underwent large deflections, with some actually rebounding back!
The experiment showed that most of the a-particles would pass through the gold foil as if there were no obstacles present, but a few collided on very hard objects and very small objects, resulting in back scattering. Rutherford was quoted as saying, "It was as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you" . Rutherford concluded that most of the mass inside an atom is concentrated in a minute space called the atomic nucleus. By analyzing the scattering data, he was able to deduce the diameter of an atomic nucleus to be approximately 10^(-14)m, which is around one hundred-thousandth of an atom’s diameter. He inferred that the atomic nuclei carried positive electric charges and therefore repelled alpha particles. This model of Atomic structure by Rutherford revealed that atoms, previously thought to be fundamental and indestructible, also had other subatomic internal structures besides the electron. Thus, JJ Thomson's plum-pudding model was disproved, leading to Rutherford's model of the atom. 
In 1913, Niels Bohr proposed improvements to Rutherford's atomic model. For this reason, the planetary model of the atom is sometimes called the Rutherford-Bohr model. Bohr added the idea of fixed orbits or energy levels for the electron traveling around the nucleus; they are confined into clearly define, quantized orbits and can jump between these but can not freely spiral inward or outward in intermediate states. This model allowed for the idea that electrons can become "excited" and move to higher energy levels for brief periods of time. 
By the 1920s, Rutherford had speculated on whether an electron could combine with a proton to create a small neutral object smaller than that size of a hydrogen atom (or in the nucleus). In 1931, Walther Bothe and Herbert Becker in Germany found that if the very energetic alpha particles emitted from polonium fell on certain light elements (specifically beryllium, boron, or lithium) an unusually penetrating radiation was produced -- radiation with no electrical charge but with great penetrating power. The next year, in Paris, Irèene Joliot-Curie & Frederic Joliot showed that if the unknown radiation fell on a hydrogen-containing compound such as paraffin, it ejected protons of very high energy. In this setup, the penetrating radiation was initially interpreted to be gamma rays, known to be high-energy photons. While this was initially consistent with the assumed gamma ray nature of the new radiation, detailed quantitative energy analysis of the unknown radiation made it increasingly difficult to reconcile with the energy available by the target compounds or from the gamma radiation. 
At the turn of the 20th century, the popular atomic model was that of J.J. (Joseph John) Thomson - scientist who discovered the electron in 1897. His model of the atom, known as the "plum-pudding" model, suggested a solid atom with negatively charged particles (electrons) evenly distributed throughout a generally positive “field” or distributed evenly throughout the mass of the atom. This model was later proved incorrect when Ernest Rutherford showed that the positive charge is concentrated in the nucleus of the atom. For his theoretical and experimental inventigations on the conduction of electricity by gases, Thomson was awarded the Nobel Prize in Physics in 1906.
The Early Atomic Model
Bohr and Joliot-Curie