Avogadro's number, denoted by the symbol "Nₐ," is a fundamental constant in chemistry and physics. It represents the number of atoms or molecules in one mole of a substance. The value of Avogadro's number is approximately 6.022 × 10^23 particles per mole.
Avogadro's number is a constant because it is based on the fundamental nature of matter and the definition of the mole. The mole is defined as the amount of substance that contains the same number of entities (atoms, molecules, ions, etc.) as there are atoms in exactly 12 grams of carbon-12. By definition, one mole of any substance contains Avogadro's number of particles.
When calculating the average atomic mass of an element, we use the concept of weighted average, taking into account the abundance of each isotope and its corresponding atomic mass. The average atomic mass is calculated based on the relative abundance of isotopes in nature.
Using a certain mass of a specific isotope would not be appropriate for calculating average atomic mass because it does not reflect the overall composition of the element found in nature. Elements typically exist as a mixture of isotopes with different abundances. Therefore, to determine the average atomic mass, we consider the weighted average of the masses of all isotopes present, taking into account their relative abundances.
By using the average atomic mass, which incorporates the abundance of each isotope, we obtain a value that represents the overall atomic mass of an element as it occurs naturally.