The atomic mass number (A) of an atom represents the total number of protons and neutrons in its nucleus. On the other hand, the atomic number (Z) of an atom represents the number of protons in its nucleus.
In the case of argon, the atomic mass number is 40, which means it has a total of 40 particles (protons + neutrons) in its nucleus. However, the atomic number of argon is 18, indicating that it has 18 protons in its nucleus.
Since the atom is electrically neutral, the number of electrons surrounding the nucleus must also be equal to the number of protons. In the case of argon, it has 18 electrons orbiting the nucleus.
The difference between the atomic mass number and the atomic number is the number of neutrons in the nucleus. In this case, argon has 40 - 18 = 22 neutrons.
It's important to note that while the atomic mass number of argon is 40, it does not mean that all argon atoms have an exact mass of 40 atomic mass units (amu). The atomic mass number represents the weighted average of the masses of all the naturally occurring isotopes of argon, taking into account their relative abundances.