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Nuclear Constituents

Thursday, March 31, 2022

Historical Postulates

Physicists have historically postulated that the nucleus of an atom contains a certain number of electrons, to balance the nuclear charge of an atom (+Z+Z) with the atomic mass number AA. However, this prediction does not match the experimental results from the nuclear magnetic moment (which shows that protons have spin 1/21/2).


The neutron was discovered as an nuclear constituent in 1932. In the proton-neutron model, atoms of atomic number ZZ have ZZ protons and AZA-Z neutrons, such that the atomic mass number is AA. Chemical properties of an element depend only on the atomic number ZZ.

Name Symbol Charge Mass Rest energy Spin
Proton pp +e+e 1.007276 u1.007276~\text{u} 938.28 MeV938.28~\text{MeV} 12\frac{1}{2}
Neutron nn +0+0 1.008665 u1.008665~\text{u} 939.57 MeV939.57~\text{MeV} 12\frac{1}{2}


Atoms of the same atomic number with different mass numbers are called isotopes. Isotopes with atomic number ZZ, mass number AA, and neutron number NN (N=AZN=A-Z) are written as


Since XX (a chemical symbol) and ZZ hold the same information, ZZ is not necessary. And with both the atomic number and mass number, NN is not necessary. Therefore, isotopes are compactly written as 1H^1H, 2H^2H, etc.