it is isospin - In physics, and specifically, particle physics, isospin (isotopic spin, isobaric spin) is a quantum number

Separation of a mixture according its isotopic composition.

explain the mechanism of diffusion in the separation of isotopic mixtures

Elements with isotopic atoms? An isotope is the same form of an element, but with a different number of neutrons. An element with isotopes/"isotopic atoms" is simply an element with isotopes.

The most common isotopic mass of the phospate group, PO3, is 79. The second most common isotopic mass is incredibly rare, but is 78.

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Phosphate (PO4)3- has not an isotopic mass because it is not an isotope but it is an anion. The mass of this anion is 94,953 42.