Nucleus Definition in Chemistry

Learn About the Atomic Nucleus

An atomic nucleus consists of protons and neutrons held together by the strong nuclear force.
An atomic nucleus consists of protons and neutrons held together by the strong nuclear force. ALFRED PASIEKA/SCIENCE PHOTO LIBRARY / Getty Images

Nucleus Definition

In chemistry, a nucleus is the positively charged center of the atom consisting of protons and neutrons. It's also known as the "atomic nucleus". The word "nucleus" comes from the Latin word nucleus, which is a form of the word nux, which means nut or kernel. The term was coined in 1844 by Michael Faraday to describe the center of an atom. The sciences involved in the study of the nucleus, its composition, and characteristics are called nuclear physics and nuclear chemistry.

Protons and neutrons are held together by the strong nuclear force. Electrons, although attracted to the nucleus, move so fast they fall around it or orbit it at a distance. The positive electrical charge of the nucleus comes from the protons, while the neutrons have no net electrical charge. Nearly all the mass of an atom is contained within the nucleus, since protons and neutrons have much more mass than electrons. The number of protons in an atomic nucleus defines its identity as an atom of a specific element. The number of neutrons determines which isotope of an element the atom is.

Size of the Atomic Nucleus

The nucleus of an atom is much smaller than the overall diameter of the atom because the electrons can be distant from the atom's center. A hydrogen atom is 145,000 times larger than its nucleus, while a uranium atom is around 23,000 times larger than its nucleus. The hydrogen nucleus is the smallest nucleus because it consists of a lone proton.

It is 1.75 femtometers (1.75 x 10-15 m). The uranium atom, in contrast, contains many protons and neutrons. Its nucleus is about 15 femtometers.

Arrangement of Protons and Neutrons in the Nucleus

The protons and neutrons are usually depicted as compacted together and evenly spaced into spheres. However, this is an oversimplification of the actual structure.

Each nucleon (proton or neutron) can occupy a certain energy level and a range of locations. While a nucleus can be spherical, it may also be pear-shaped, rugby ball-shaped, discus-shaped, or triaxial.

The protons and neutrons of the nucleus are baryons composed of smaller subatomic particles, called quarks. The strong force has an extremely short range, so protons and neutrons must be very near to each other to be bound. The attractive strong force overcomes the natural repulsion of the like-charged protons.

Hypernucleus

In addition to protons and neutrons, there is a third type of baryon called a hyperon. A hyperon contains at least one strange quark, while protons and neutrons consist of up and down quarks. A nucleus that contains protons, neutrons, and hyperons​ is called a hypernucleus. This type of atomic nucleus has not been seen in nature, but has been formed in physics experiments.

Halo Nucleus

Another type of atomic nucleus is a halo nucleus. This is a core nucleus that is surrounded by an orbiting halo of protons or neutrons. A halo nucleus has a much larger diameter than a typical nucleus. Its also much more unstable than a normal nucleus. An example of a halo nucleus has been observed in lithium-11, which has a core consisting of 6 neutrons and 3 protons, with a halo of 2 independent neutrons.

The half-life of the nucleus is 8.6 milliseconds. Several nuclides have been seen to have a halo nucleus when they are in the excited state, but not when they are in the ground state.

References:

 M. May (1994). "Recent results and directions in hypernuclear and kaon physics". In A. Pascolini. PAN XIII: Particles and Nuclei. World Scientific. ISBN 978-981-02-1799-0. OSTI 10107402

W. Nörtershäuser, Nuclear Charge Radii of 7,9,10Be and the One-Neutron Halo Nucleus 11Be, Physical Review Letters, 102:6, 13 February 2009,