The atomic mass number and atomic number of an element are two distinct properties that describe different characteristics of the atom.
The atomic number of an element, represented by the symbol Z, indicates the number of protons found in the nucleus of an atom. It defines the identity of the element and determines its position on the periodic table. In the case of argon (symbol Ar), its atomic number is 18, which means an argon atom contains 18 protons in its nucleus.
On the other hand, the atomic mass number, represented by the symbol A, refers to the total number of protons and neutrons present in the nucleus of an atom. Since the atomic mass number includes both protons and neutrons, it is typically a larger value than the atomic number.
In the case of argon, with an atomic mass number of 40, it means that a typical argon atom contains 18 protons (atomic number) and 22 neutrons (atomic mass number minus atomic number). The sum of protons and neutrons gives the atomic mass number.
It's important to note that atomic mass numbers are not always whole numbers because they are based on the average mass of the isotopes of an element, which have different numbers of neutrons. The atomic mass on the periodic table is usually a weighted average of the masses of all the naturally occurring isotopes of an element. In the case of argon, its atomic mass on the periodic table is approximately 39.95, which reflects the average mass of all the isotopes of argon found in nature.