Why is the mass number an integer but the relative atomic mass is not?

1 Answer
Jul 22, 2018

Answer:

Well the mass number refers to a given #"nuclide..."#

Explanation:

A nuclide is a specific isotope, which of course has specific #Z_"the atomic number"#, and a SPECIFIC number of neutrons. The sum of both these numbers is necessarily precise AND integral (why so?.. because we cannot have half a neutron or two thirds a proton, these are discrete fundamental particles).

On the other hand, the #"relative atomic mass"# is the weighted average of the isotopic masses compared to the mass of a #""^12C# isotope, whose nucleus contains 6 protons (necessarily, why so), and 6 neutrons.

By way of example, the element boron has #Z=5#, and has an isotopic distribution of #20% # #""^10B#, and #80% # #""^11B#, and their weighted average is #10.81#, which is the relative atomic mass, and the mass we would use if we calculated the formula mass of a boron-containing compound. And clearly this mass number CAN BE non-integral.

And so the #"relative atomic mass"# is DIMENSIONLESS...and given the isotopic distribution common to most elements, it is NON-INTEGRAL. As a tip read the Periodic Table beside you now. This should be available in most exams of chemistry and fyziks, and a bit of study now will pay big dividends...