# What Is the Atomic Nucleus? Properties and Composition

The atomic nucleus is the small central part of the atom, with a positive electric charge. The nucleus concentrates most of the atom's mass.

The primary subatomic particles in the nuclei of atoms are protons and neutrons or nucleons (except hydrogen nuclei that contain 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 the atomic number. The mass number is the total number of protons and neutrons.

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

## What Particles Are Found in the Nucleus of an Atom?

An atom's nucleus comprises sub-particles called nucleons that can be of two types: protons and neutrons.

The atomic nucleus concentrate almost all the mass of an atom. The rest of the pack is distributed among the electrons. However, the electrons weigh very little than the neutrons and protons.

The atomic nuclei's diameters are in the range of 1.75 fm for hydrogen, equivalent to a single proton's diameter to about 15 fm for the heavier atoms, such as uranium.

The following numbers describe the atomic nucleus:

• Atomic number, Z, which is equal to the number of protons.

• The number of neutrons is represented by N.

• The mass number, A = Z + N, is equal to the number of nucleons (protons plus neutrons).

### How Are Protons Held Together?

Neutrons are neutral, and protons have a positive charge. Protons would tend to separate from each other, but a strong force holds them together.

The nuclear force that holds nucleons together is the energy obtained in nuclear fusion reactions. By breaking these force bonds, a loss of mass is experienced, which is converted into energy according to Albert Einstein's theory: E = mc2

A magic number is a number of nucleons such that they are arranged into complete shells within the atomic nucleus. As a result, atomic nuclei with a 'magic' number of protons or neutrons are much more stable than other nuclei.

The de-excitation of a nucleon from one excited level or state to another of lower energy and the decay of radioactive isotopes can generate gamma rays.

The alpha particles ( α ) are nuclei completely ionized, i.e., without the container corresponding electrons, helium -4 ( 4 He). These nuclei are made up of two protons and two neutrons. As it lacks electrons, its electric charge is positive.

## What Are the Atomic Models?

Atomic models are theories that have been developed throughout history. The different models' objective is to define the structure of an atom is like from a scientific point of view.

The first proposal on the atomic nucleus's internal structure was elaborated in 1808 by the English chemist John Dalton. According to Dalton's proposal, all matter is made up of indivisible and invisible atoms. At that time, for Dalton, the existence of the atomic nucleus was unknown.

The first model of the atomic structure appeared in 1897 through Joseph John Thomson. Thomson was the first to identify the electron, which had a negative charge.

Through this discovery, he deduces that if atoms have a neutral charge, there must be positively charged particles that counteract the electrons (it would be in proton). Thus, depending on the number of electrons, it must be the same amount of protons.

Ernest Rutherford discovered the atomic nucleus in 1911, which is the base of Rutherford's model: “The positively charged particles and most of the mass of an atom was concentrated in an extremely small volume.”

In 1913 Niels Bohr postulated that electrons rotate, generating an electron cloud at high speeds around the atomic nucleus, charged with kinetic energy. The orbiting electrons are arranged in various circular orbits, which determine different energy levels.

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Published: January 15, 2013
Last review: October 12, 2020