In the later part of the 18th century and the beginning of the 19th century the structure of the atom was not completely understood. After the discovery of the negatively charged particle that is called the electron and the discovery of the positively charged particle that is called the proton, the atom was assumed to be in a state of evenly distributed electrons and protons in a sphere of the atom due to its neutral structure.
With the advent of instrumentation, an experiment was done on certain atoms in which alpha particles that are positively charged helium ions are used to bombard atoms at rest. The result of this experiment showed that alpha particles were deflected from the atom by a certain angle. This result led to the suggestion that there is a positively charged entity that repelled the alpha particles. This was the first experiment that led to the discovery of the atomic nucleus.
Today it is known that the atom is composed of a nucleus which contains particles that are called protons and neutrons. Circulating around this nucleus are also electrons which revolve in circular motion around the nucleus in definite orbits that are characterized by a specific amount of energy that depends on the orbit distance from the nucleus.
Quantum mechanics predicts the behavior of electrons and the particles inside the nucleus. The nucleus is composed of small particles that are called protons and neutrons. These particles are not the fundamental particles on the subatomic level. Both protons and neutrons are composed of yet smaller particles that are called quarks and leptons. The energy that is required to produce a quark is much higher than the energy required to split the nucleus.
Quarks themselves are not fundamental particles. They are composed yet of smaller particles that are called baryons. These particles require still higher energy for their production much higher than the production of the quarks.
Protons in the nucleus are subject to a repulsive electric force which tends to disintegrate the nucleus. There is obviously another type of force which maintains the integrity of the nucleus and prevents it from its disintegration. This force is called the nuclear strong force. It is responsible for balancing the repulsive electric force between the protons. The potential that represents the energy of holding the protons and the neutrons in the nucleus was modeled by a japanese scientist that this potential bears his name and is called the Yukawa potential after his name. This potential formula can be derived from the relativistic quantum mechanical equation called the Klein-Gordon equation. By solving the time independent Klein-Gordon equation one can get the Yukawa potential which depicts the behavior of the nuclear potential energy as a function of the distance r between the nucleons.
Quantum mechanics can model the energy of a proton in a nucleus of the atom through the application of an infinitely high potential barrier which represents the nucleus. The particle in a box with infinitely high energy walls can fairly depicts the behavior of a proton and a neutron in the nucleus. Also quantum tunneling effect can predict the radioactive decay of protons and neutrons from radioactive metals such as uranium. The quantum tunneling effect predicts that there is a finite probability that a particle in a box with infinitely high walls will succeed to penetrate the wall and exit the box.
Another force in nature which is called the nuclear weak force is responsible for the disintegration of neutrons into protons and electrons in addition to a neutrino particle. Some particles that are emitted from the nucleus during radioactive decay have antiparticles which have the opposite charge. An example is the positron which is the antiparticle of the electron with a positive charge. The neutrino particle also has antiparticle that is called the anti-neutrino .