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Nucleon is collectively known particles namely protons and neutrons. The electrons exists outside the nucleus. Since an atom is electrically neutral, the proton number in the nucleus is exactly equal to the number of electrons. This number is given by the symbol Z (Atomic number). It is the number of protons in an atom. Mass number is the total number of protons and neutrons in a atom. Lets see more about nucleon here.

What is a Nucleon?

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Nucleons are the subatomic particles like protons and neutrons that exists in the nuclei of atoms. But electron is not a nucleon. Each nucleus has at least one or more nucleons. Hence, these nucleons are in clusters which are surrounded by one or more electrons.

Nucleon Number

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Nucleon number is also known as mass number or atomic mass number. It is the total number of protons and total number of neutrons in a atom. It is given by,
Nucleon number (A) = Atomic number (Z) + Neutron number (N)

Mass of Nucleon

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The mass of a nucleon depends entirely on which nucleus it is in. The individual nucleons has more mass in lighter nuclei compared to heavier nuclei. The nucleon in hydrogen nuclei have more mass than in iron nuclei. Hence, its the binding energy that pulls the mass down. The graph shows mass per nucleon decreases and is least for a nucleon in an iron nucleus. Iron holds its nucleons more tightly than any other nucleus does. Beyond iron, the trend reverses itself as the proton mass increases for an increasing atomic number for the rest of the list of elements.
Mass per Nucleon
Mass per nucleon versus atomic mass

Nuclear binding energy

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Its known that nucleon are nothing but the protons and neutron. The nucleus is made of these particles. In a nucleus, the mass is always less than sum total of each individual masses of the nucleon. The nuclear binding energy is given by
Nuclear binding energy = $\Delta$ mc2
where, $\Delta$ m is the rest mass and c is the speed of light.


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Its all known matter theory that atoms are the smallest particles of matter. These tiny particles are made of still smaller particles which are the sub-atomic or elementary particles. These elementary particles are made of extremely small particles called quarks. Quarks are the group of subatomic particles that interact with strong force. These quarks are not experimentally proved but the idea was suggested by two American physicists Murray Gell Mann and George Zweig.
There are six flavors of quark namely
  • Up and down
  • Charm and strange
  • Top and bottom.

Their charges are +2/3, -1/3, -1/3, +2/3, -1/3 and +2/3 to that of electric charge. Each of these quarks have their own antiparticle known as antiquark.


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It's already discussed that quarks are the fundamental particles. There is another type of fundamental particles that do exist known as leptons. Leptons exists independent of each other and hence do not get affected with strong force. There are three families of leptons each has a negatively charged particle and a corresponding neutrino. Each of these six leptons has an antiparticles hence their total number is 12. The six leptons are
  • Electron and electron neutrino
  • Muon and muon neutrino
  • Tau and tau neutrino.


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Hadron is a composite particle of quarks which are the only particles subject to strong nuclear force whereas other particles like leptons are insensitive to it. There are almost 350 hadrons which are totally unstable. They are classified into two types:
  • Baryons
  • Mesons.