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

Question

Olivia Anton · Accepted Answer

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

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#. The reported atomic mass of the majority of higher elements is the weighted average of the individual isotopes, since most elements contain multiple isotopes.

Jose Ayala · Answer

undefined The number of neutrons in a nuclide can be found by subtracting the atomic number (Z) from the isotopic mass (A). So, the number of neutrons (N) is calculated as N = A - Z.