Molecular Structure.
Atoms, electrons, neutrons and protons

Atomic nucleus

Atomic nucleus

The atomic nucleus is the small central part of the atom, with positive electric charge and in which most of the mass of the atom is concentrated. It was discovered by Ernest Ruthenford in 1911. After the discovery of the neutron, in 1932, the atomic nucleus model was quickly developed by Dmitri Ivanenko and Werner Heisenberg.

The main subatomic particles of the nuclei of atoms are protons and neutrons or nucleons (except that of ordinary or own hydrogen, which contains only one proton). The same chemical element is characterized by the number of protons in the nucleus that determines the total positive charge. This number is called an atomic number. The mass number is the total of protons and neutrons.

Nuclear physics is the scientific branch that is responsible for the study and understanding of the atomic nucleus, including the forces that unite it and its composition.

Properties of the atomic nucleus

Almost the entire mass of an atom is found in the atomic nucleus, with a very small contribution from the electron cloud because electrons weigh very little compared to neutrons and protons. Protons and neutrons come together to form the atomic nucleus through the nuclear force.

The force that holds the subparticles together in the nucleus of the atoms is the energy that is obtained in the reactions of nuclear fusion and nuclear fusion. When these bonds of force are broken, a mass loss is experienced that becomes energy according to Albert Einstein's theory. E = mc 2

The diameter of the core is in the range of 1.75 fm for hydrogen which equals the diameter of a single proton to about 15 fm for the heavier atoms, such as uranium.

Structure models of the atomic nucleus

The first proposal on the internal structure of the atomic nucleus was elaborated in 1808 by the English chemist John. According to Dalton's proposal, all matter is made up of indivisible and invisible atoms. At that time, there were no smaller particles; for Dalton the existence of the atomic nucleus was unknown.

The first model of the internal structure of the atom appeared in 1897 through Joseph John Thomson. Thomson was the first to identify the electron. Thompson defined the electron as a subatomic particle with a negative charge. Through this discovery he was able to deduce that if the atoms are presented with a neutral charge and have a negatively charged particle they would have to have at least one other charge of positive charge (it would be in proton)

Thus neither Dalton's atomic model nor Thomson's included any description of the atomic nucleus. The notion of atomic nucleus emerged in 1911 when Ernest Rutherford and his collaborators Hans Geiger and Ernest Marsden conducted a set of experiments using a beam of alpha radiation and very thin metal laminated sheets.

In 1913 Niels Bohr postulates that electrons rotate at high speeds around the atomic nucleus, charged with kinetic energy. The electrons are arranged in various circular orbits, which determine different energy levels. The electron can access a higher energy level, for which it needs to absorb energy. To return to its original energy level it is necessary that the electron emits the absorbed energy (for example, in the form of radiation).

Commonly there are two different models describing the atomic nuclei:

  • The model of the water drop
  • The layer model

Atomic model of liquid drop

The liquid drop model is one of the first models of the atomic nucleus structure. In this atomic model the nucleus as a fluid formed by neutrons and protons, with a repulsive internal electrostatic force proportional to the number of protons. The quantum mechanical nature of these particles appears through the Pauli exclusion principle. Pauli's principle of expulsion states that there can not be two nucleons of the same type in the same state. Thus, the fluid is actually what is known as a Fermi liquid.

This model of atomic nucleus does not try to describe the complex internal structure of the nucleus but only the energies of connection between neutrons and protons, as well as some aspects of the excited states of an atomic nucleus that are reflected in the nuclear spectra.

Atomic structure model of layers

The atomic core model of layers is a model that tries to capture part of the internal structure reflected both in the angular momentum of the nucleus, and in its angular momentum. In addition, the model aims to explain why nuclei with a number of nucleons (neutrons and protons) are more stable.

The explanation of the model is that the nucleons are grouped in layers. Each layer is formed by a set of quantum states with similar energies, the difference of internal energy between two layers is large compared to the variations of internal energy within each layer. Thus, given that the nucleons are fermions, an atomic nucleus will have the layers of lower energy full, so the nucleons can not fall to lower layers already full.

The layers here must be understood in an abstract sense and not as physical layers as the layers of an onion, in fact, the geometric shape of the space occupied by a nucleon in a given state of a layer interpenetrates with the space occupied by nucleons of other layers, analogous to how the electronic layers interpenetrate in an atom.

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Last review: August 30, 2017

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