Why is the atomic mass of Argon not a whole number?

The atomic mass of Argon is not a whole number because it is a weighted average of the masses of all its naturally occurring isotopes. Argon has three main isotopes: ³⁶Ar, ³⁸Ar, and ⁴⁰Ar. The atomic mass takes into account the relative abundance of each isotope, leading to a value of 39.95 u.