Q: Argon atoms have a mass number of 40 but a relative atomic mass of 39.948. Why is this so?
Answer: Because argon has three stable isotopes that contribute to the atomic mass of the element.
The atomic mass of an element is best thought of as the weighted average of the atomic masses of its stable isotopes.
But as you might know, the authenticity of a chemical element is purely defined by the number of protons in an atom’s nucleus – this is recognised as the atomic number.
An argon atom will have protons in its nucleus.
However, the number of neutrons an atom can have while retaining its chemical identity can vary greatly.
The atom you mentioned in your question is argon-40, an argon isotope with a mass number equal to 40. This implies that it will also have
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 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
avg. atomic mass = ∑i(isotope×abundance)
Finally, the average atomic mass of a chemical element is determined by the atomic masses of its naturally occurring isotopes.