The average atomic mass and the mass number are two different concepts related to the mass of an atom, and they represent different aspects of an atom's composition.
The mass number of an atom refers to the total number of protons and neutrons in the nucleus of an atom. It is often denoted as a whole number and is specific to a particular isotope of an element. Isotopes are variants of an element that have the same number of protons (same atomic number) but differ in the number of neutrons. The mass number provides information about the total mass of an atom's nucleus.
On the other hand, the average atomic mass, also known as atomic weight, is the weighted average of the masses of all the naturally occurring isotopes of an element. Since different isotopes of an element have different numbers of neutrons, they also have different mass numbers. The average atomic mass takes into account the relative abundance of each isotope in nature and calculates a weighted average based on those abundances.
The average atomic mass is often expressed in atomic mass units (u) or unified atomic mass units (u), where 1 atomic mass unit is defined as 1/12th the mass of a carbon-12 atom. It is typically a decimal number due to the different isotopes and their varying abundance. The average atomic mass provides a more realistic and comprehensive representation of the mass of an element in nature, considering the different isotopes and their relative abundances.
In summary, the mass number refers to the sum of protons and neutrons in a specific isotope of an element, while the average atomic mass represents the weighted average mass of all the naturally occurring isotopes of an element, taking into account their relative abundances.