Final answer:
The average atomic mass for an element is the weighted average mass of the atoms in a naturally occurring sample of the element. The mass number, on the other hand, represents the total number of protons and neutrons within an atom. In the case of chlorine, its average atomic mass is 35.45, while its main isotopes have mass numbers of 35 and 37.
Step-by-step explanation:
The average atomic mass for an element is the weighted average mass of the atoms in a naturally occurring sample of the element. It is calculated by taking into account the masses of the different isotopes of the element and their relative abundances. The mass number, on the other hand, represents the total number of protons and neutrons within an atom. It is specific to each isotope of an element.
For example, if we take the element chlorine, it has two main isotopes: chlorine-35 and chlorine-37. Chlorine-35 is more abundant, so it contributes more to the average atomic mass of chlorine. Therefore, the average atomic mass of chlorine is 35.45 atomic mass units. This is different from the mass number of each isotope: chlorine-35 has a mass number of 35, and chlorine-37 has a mass number of 37.