Uploaded September 2024 | Updated September 2026, 2 weeks ago
When we compare the average atomic mass and the mass number, it's important to understand what each term represents. Let's use chlorine as an example. Chlorine exists mainly as two isotopes: chlorine-35 and chlorine-37. The mass number of an atom is simply the sum of its protons and neutrons. For chlorine-35, this means 17 protons plus 18 neutrons, giving us a mass number of 35. For chlorine-37, the 17 protons plus 20 neutrons give us a mass number of 37. These mass numbers are always whole numbers because they count the actual particles in the atom.
App used in video: https://phet.colorado.edu/en/simulations/isotopes-and-atomic-mass
However, when you look at the periodic table, the atomic mass of chlorine is not a whole number—it's about 35.45. This value is the average atomic mass, which accounts for the different isotopes of chlorine and their relative abundances. In a sample of chlorine, you'll have a mix of chlorine-35 and chlorine-37 atoms. The average atomic mass is calculated by taking into account how much of each isotope is present in nature and averaging their masses. Because this is an average, and because isotopes don't occur in equal amounts, the average atomic mass ends up being a decimal.
So, while the mass number is specific to an individual atom and is a whole number, the average atomic mass reflects the weighted average of all the isotopes of an element found in nature, which is why it's typically a decimal. This distinction helps us understand both the specific characteristics of individual atoms and the overall nature of an element as it appears in the world.
When we compare the average atomic mass and the mass number, it's important to understand what each term represents. Let's use chlorine as an example. Chlorine exists mainly as two isotopes: chlorine-35 and chlorine-37. The mass number of an atom is simply the sum of its protons and neutrons. For chlorine-35, this means 17 protons plus 18 neutrons, giving us a mass number of 35. For chlorine-37, the 17 protons plus 20 neutrons give us a mass number of 37. These mass numbers are always whole numbers because they count the actual particles in the atom.
App used in video: https://phet.colorado.edu/en/simulations/isotopes-and-atomic-mass
However, when you look at the periodic table, the atomic mass of chlorine is not a whole number—it's about 35.45. This value is the average atomic mass, which accounts for the different isotopes of chlorine and their relative abundances. In a sample of chlorine, you'll have a mix of chlorine-35 and chlorine-37 atoms. The average atomic mass is calculated by taking into account how much of each isotope is present in nature and averaging their masses. Because this is an average, and because isotopes don't occur in equal amounts, the average atomic mass ends up being a decimal.
So, while the mass number is specific to an individual atom and is a whole number, the average atomic mass reflects the weighted average of all the isotopes of an element found in nature, which is why it's typically a decimal. This distinction helps us understand both the specific characteristics of individual atoms and the overall nature of an element as it appears in the world.
