Question

Why is it necessary to use the average atomic mass of all isotopes, rather than the mass of the most commonly occurring isotope, when referring to the atomic mass of an element?

Answer 1

Why is it necessary to debit my account by `£7-49` instead of `£7-50`, even tho' the `"1 p"` difference is unimportant to me?

Explanation:

Why? Because accounting, and even more so chemistry, is a highly quantitative exercise. Mass is always conserved in a chemical reaction, and accurate masses, which are the weighted averages of the isotopic masses, are necessary to demonstrate this conservation of mass. For the lighter elements, hydrogen, carbon, and oxygen, this is not so important in that there is normally only the ONE isotope. But for the heavier elements, especially where the isotopic distribution is large, and we get fractional atomic masses, we must account for the isotopic distribution.

PS If my account were debited `£7-50` instead of the `£7-49` purchase price, I would raise merry hell, and demand satisfaction from my bank manager.