Question

Argon atoms have a mass number of 40, but a relative atomic mass of 39.948 why is this so?

Answer 1

Because argon has three stable isotopes that contribute to the element's atomic mass.

Explanation:

An element's atomic mass is best thought of as the weighted average of the atomic masses of its stable isotopes.

As you know, the identity of a chemical element depends exclusively on the number of protons an atom has in its nucleus - this is known as atomic number.

In this case, you know for a fact that any atom that has `19` protons in its nucleus will be an atom of argon.

However, the number of neutrons an atom can have and still retain its chemical identity can vary quite significantly.

The atom you listed in the question is actually argon-40, an isotope of argon that has a mass number equal to `40`. This implies that it will also have

`color(blue)("mass number" = A = Z + "no. of neutrons")`

`"no. of neutrons" = A - Z = 40 - 18 = 22`

This isotope of argon has `22` neutrons in its nucleus. But this is just one isotope of argon. Argon has a total of `24` isotopes, out of which three are stable and contribute to its average atomic mass.

Each stable isotope has an abundance associated with it. The average atomic mass of the element argon will be

`color(blue)("avg. atomic mass" = sum_i("isotope"_i xx "abundance"_i))`

The atomic masses of argon's stable isotopes and their respective abundances can be found here:

https://en.wikipedia.org/wiki/Isotopes_of_argon
http://education.jlab.org/itselemental/iso018.html

So, let's calculate the average atomic mass of argon

`"avg. atomic mass" = "35.96755 u" xx 0.003336 + "37.96273 u" xx 0.000629 + "39.96238 u" xx 0.996035`

`"avg. atomic mass" color(white)(x) = color(white)(x) "39.948 u"`

As a conclusion, the average atomic mass of a chemical element depends on the atomic masses of its naturally occuring isotopes.