The basis of everything related to nuclear energy lies in the atom, since nuclear technology is based on the use of the internal energy contained in atoms. For this reason, to understand how nuclear reactions occur (nuclear fission or nuclear fusion) it is useful to understand how an atom is structured.
An atom is the smallest constituent unit of ordinary matter that has the properties of a chemical element.
The atom is composed of a nucleus and one or more electrons linked to the nucleus. The nucleus is made of one or more protons and, typically, a similar number of neutrons; protons and neutrons are called nucleons.
The protons of the atomic nucleus are united by very strong bonds in which a large amount of energy is generated when they are broken or generated. Nuclear energy is based on the creation or breakage of these links.
The origin of the word atom comes from the Greek and means indivisible. The conception of indivisible comes from antiquity when it was believed that the atom was the smallest element that could exist. However, we now know that the atom is made up of still smaller particles: sub atomic particles.
The atom contains protons, neutrons and electrons, with the exception of hydrogen-1 and the hydrogen cation. In this case, hydrogen-1 does not contain neutrons, and the hydrogen or hydron cation does not contain electrons. The protons and neutrons of the atom are called nucleons, because they are part of the atomic nucleus.
Orbiting the electrons. The electron is the lightest particle of how many make up the atom. It has a negative electric charge, whose magnitude is defined as the elementary electric charge, and it is ignored if it has substructure, so it is considered an elementary particle. The mass of a proton is 1836 times greater than the mass of the electron. The charge of the proton is positive. The neutron has a mass 1839 times the mass of the electron. The neutron has a neutral electrical charge (neither positive nor negative).
Electron mass: 9.11 · 10 -31 kg
Mass of one proton: 1.67 · 10 -27 kg
Mass of one neutron: 1.69 · 10 -27 kg
The proton and the neutron are not elementary particles, but are a composite of other particles called quarks. Subatomic particles constitute a bound state of quarks u and d. A proton contains two quarks u and a quark d, while the neutron contains two d and a u, consistent with the charge of both. The quarks are held together by the strong nuclear force, mediated by gluons. In addition to these particles, there are other subatomic particles in the standard model: more types of quarks, charged leptons (similar to the electron), etc.
A quark is a fundamental particle collected in the standard model of particle physics. The quarks have electric charges equal to +2/3 and -1/3 respectively, with respect to the elementary charge.
The atomic nucleus
The atomic nucleus is the central part of the atom that is composed of nucleons joined together. The nucleon is a subatomic particle component of the nucleus, that is, a proton or a neutron. The mass number of an atom is the number of nucleons in its atomic nucleus.
The volume of the nucleus is approximately proportional to the total number of nucleons, the mass number. The nucleons are held together by the nuclear force, which is much more intense than the electromagnetic force at short distances, which allows to overcome the electrical repulsion between the protons.
An atom is constituted by a very dense central nucleus, which contains protons and neutrons, and by electrons that move around the nucleus at a relatively large distance.
The atoms of the same element have the same number of protons, which is called the atomic number and is represented by Z. The atoms of a given element can have a different number of neutrons: they are then said to be isotopes. The uranium enrichment action involves modifying the number of neutrons in a uranium atom to obtain a more unstable isotope to favor nuclear fission chain reactions. Both numbers together determine the nuclide.
The atomic nucleus can be altered by very energetic processes compared to chemical reactions. Unstable nuclei, such as uranium and plutonium used as a nuclear fuel, suffer disintegrations that can change their number of protons and neutrons emitting radiation. A heavy nucleus can be fissioned into other lighter nuclei in a nuclear reaction or spontaneously. By means of a sufficient amount of energy, two or more cores can merge into a heavier one, in this case, it would be a nuclear fusion reaction.
In atoms with a low atomic number, nuclei with a different amount of protons and neutrons tend to disintegrate into nuclei with more even, more stable proportions. However, for higher values of the atomic number, the mutual repulsion of the protons requires a larger proportion of neutrons to stabilize the nucleus.
The electron is a stable elementary particle with the smallest negative charge that exists in nature. This charge is called elementary charge, since any separable electric charge is composed of a whole number of them.
The electrons, of negative sign, are attracted by the protons, of positive sign in the atom through the electromagnetic force. Due to this electromagnetic force an external energy source is necessary to release them. The closer an electron is to the nucleus, the greater the attractive force, and the greater the energy necessary for it to escape.
Electrons tend to form a certain type of standing wave around the atomic nucleus. Each of these waves is characterized by an atomic orbital, a mathematical function that describes the probability of finding the electron at each point in space. The set of these orbitals is discrete, that is, it can be enumerated, as is proper in any quantum system. The electron cloud is the region occupied by these waves, visualized as a negative charge density around the nucleus.
Each orbital corresponds to a possible value of energy for the electrons, which are distributed among them. The principle of exclusion of Pauli prohibits that more than two electrons are in the same orbital. Transitions between different energy levels can occur: if an electron absorbs a photon with sufficient energy, it can jump to a higher level; also from a higher level you can end up at a lower level, radiating the rest of the energy into a photon. The energies given by the differences between the values of these levels are those observed in the spectral lines of the atom.