In chemistry and physics, a nucleon is a proton or a neutron in the atomic nucleus. In contrast, there are free protons and neutrons, which aren’t considered to be nucleons. Protons have a net positive electrical charge, while neutrons are electrically neutral. So, nucleons in an atomic nucleus have a net positive charge.
Mass Number and Atomic Mass
The sum of the number of protons and neutrons (nucleons) is the mass number (A) of an atom. In fact, sometimes this value is called the nucleon number. Differences between mass numbers of the same element identify that element’s isotope, which differ only by the number of neutrons they contain.
The electron mass is negligible compared to the masses of protons and neutrons, so atomic mass is the sum of the masses of the nucleons.
Each nucleon consists of three subatomic particles called quarks. A proton consists of two up quarks and one down quark, while a neutron is composed of one up quark and two down quarks. Each up quark has an electric charge of +2/3, while a down quark has a charge of -1/3.
Proton and neutron masses are similar. A proton has a mass of 1.6726×10−27 kg or 938.27 MeV/c2. The neutron mass is 1.6749×10−27 kg or 939.57 MeV/c2, which makes it about 0.13% heavier than a proton.
Nucleon Interactions in the Nucleus
Protons repel each other because they have like electric charges, but all nucleon attract each other due to the strong interaction. The strong interaction is more powerful than electric attraction or repulsion, but it acts over a very short range. When nucleons attract one another, they bind via the strong nuclear force. As in chemical bond formation between electrons, the binding of nucleons also releases energy called nuclear binding energy. One consequence of nuclear binding is that the sum of the masses of the protons and neutrons used to make an atomic nucleus is greater than the mass of the resulting nucleus. This is called mass defect. Also, breaking a proton or neutron free of the nucleus requires an input of energy.
Atom diagrams typically depict protons and neutrons as separate spheres randomly crammed together to form a nucleus. In reality, nucleons are partially delocalized. In fact, particle physicists consider protons and neutrons in the nucleus to be two nucleon states, rather than separate entities. The two states form an isospin doublet. Neutrons can be converted into protons, and protons can be converted in neutrons.
Antiprotons and antineutrons are the antimatter particles corresponding to protons and neutrons. An antiproton consists of two up antiquarks and one down antiquarks, while an antineutron consists of one up antiquark and two down antiquarks. Antimatter atoms contain nuclei made up of antinucleons.
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