Avogadro's number (6.022 × 10^23) is a fundamental constant in chemistry and is defined as the number of atoms, molecules, or other particles in one mole of a substance. It is based on the concept that elements and compounds consist of discrete particles, such as atoms or molecules.
When we use Avogadro's number to relate the mass of a substance to the number of particles, we use the average atomic mass of the element. The average atomic mass takes into account the relative abundance of different isotopes of the element and their respective masses.
Isotopes are atoms of the same element that have different numbers of neutrons, thus varying in atomic mass. For example, carbon-12 (12C) and carbon-13 (13C) are isotopes of carbon with atomic masses of 12 atomic mass units (amu) and 13 amu, respectively.
While it is possible to work with a certain mass of a specific isotope, Avogadro's number and the concept of the mole are used to bridge the gap between the macroscopic scale (mass) and the microscopic scale (number of particles). By using the average atomic mass, which considers the isotopic distribution in a sample of an element, we can relate the mass of the sample to the number of particles present.
Avogadro's number allows chemists to make quantitative connections between the mass of a substance, the number of particles it contains, and other chemical quantities, facilitating calculations and understanding at the atomic and molecular level. It provides a consistent basis for working with different elements and compounds, regardless of their isotopic composition.