Argon atoms have a mass number of 40, but a relative atomic mass of 39.948 why is this so?
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
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
#color(blue)("mass number" = A = Z + "no. of neutrons")#
#"no. of neutrons" = A - Z = 40 - 18 = 22#
This isotope of argon has
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:
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.