Average Atomic Mass
The atomic number of an element defines the element's identity and signifies the number of protons in the nucleus of one atom. For example, the element hydrogen (the lightest element) will always have one proton in its nucleus. The element helium will always have two protons in its nucleus.
Atoms Are Collections of Isotopes
Atoms of the same element can, however, have differing numbers of neutrons in their nucleus. For example, stable helium nuclei exist that contain either one or two neutrons, but both atoms have two protons. These different types of helium atoms have different masses (3 or 4 atomic mass units), and they are called isotopes. For any given isotope, the sum of the numbers of protons and neutrons in the nucleus is called the mass number. All elements exist as a collection of isotopes.
Calculating Average Atomic Mass
The average atomic mass of an element is the sum of the masses of its isotopes each multiplied by its natural abundance (the decimal associated with percent of atoms of that element that are of a given isotope. When data are available regarding the natural abundance of various isotopes of an element, it is simple to calculate the average atomic mass. For helium, there is approximately one isotope of Helium-3 for every million isotopes of Helium-4; therefore, the average atomic mass is very close to 4 (4.002602).
In another example, chlorine consists of two major isotopes, one with 18 neutrons (75.77 percent of natural chlorine atoms) and one with 20 neutrons (24.23 percent of natural chlorine atoms). The atomic number of chlorine is 17 (it has 17 protons in its nucleus). To calculate the average mass, first convert the percentages into fractions (that is, divide them by 100). Then calculate the mass numbers. For example, the chlorine isotope with 18 neutrons has an abundance of 0.7577 and a mass number of 35. To calculate the average atomic mass, multiply the fraction by the mass number for each isotope, then add them together.
Average atomic mass = f1M1 + f2M2 + ... + fnMn
Average atomic mass of chlorine = (0.7577 * 35) + (0.2423 * 37) = 35.48
Whenever we do mass calculations involving elements or compounds (combinations of elements), we always use average atomic masses.