How do we find the number of neutrons in a nuclide, given #Z#, and the isotopic mass?

1 Answer
anor277
Jun 29, 2017

The number of neutrons equals the #"isotopic mass"# LESS the #"atomic number"#, #Z#.

Explanation:

To a first approximation, the mass of the nuclide is the number of massive nuclear particles, i.e. the #"number of protons"# (which number defines the atomic number, #Z#) and the #"number of neutrons"#, which number defines the isotope.

By way of example, the most abundant element in the universe is hydrogen, #Z=1#. All hydrogen nuclei have the one PROTON present in their nuclei. A small percentage of hydrogen nuclei contain ONE neutron, a massive, neutrally charged particle, to give the #"deuterium isotope"#, #""^2H#; a smaller percentage of hydrogen nuclei contain TWO neutrons, a massive, to give the #"tritium isotope"#, #""^3H#.

Most higher elements contain a number of isotopes, and their reported atomic mass is the weighted average of the individual isotopes.

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