Question

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

Answer 1

Argon:

• Argon is an element with an atomic symbol $\mathrm{Ar}$ and an atomic number $20$.
• The mass number of Argon is $40$.

Relative atomic mass:

• The stable isotopes of Argon are ${(}^{36}\mathrm{Ar},{}^{38}\mathrm{Ar},\mathrm{and}{}^{40}\mathrm{Ar})$.
• The abundance of ${}^{36}\mathrm{Ar}$ is $0.334\%$.
• The abundance of ${}^{37}\mathrm{Ar}$ is $0.0629\%$.
• The abundance of ${}^{40}\mathrm{Ar}$ is $99.604\%$.
• The average atomic mass of the element argon will be given as:

$\mathrm{Average}\mathrm{atomic}\mathrm{mass}=\sum \mathrm{i}(\mathrm{isotope}\mathrm{i}\times \mathrm{abundance}\mathrm{i})$

• $$\begin{gathered} \mathrm{Average}\mathrm{atomic}\mathrm{mass}=35.96755\mathrm{u}\times 0.003336+37.96273\mathrm{u}\times 0.000629+39.96238\mathrm{u}\times 0.996035 \\ \mathrm{Average}\mathrm{atomic}\mathrm{mass}=39.948\mathrm{u} \end{gathered}$$