How to Calculate the Average Mass of Atoms (Animated)

Elements can exist as different isotopes in nature, which means they have the same number of protons but different numbers of neutrons.Let's look at chlorine as an example. Here we have Chlorine-35, with 17 protons and 18 neutrons.And here's Chlorine-37, which has the same 17 protons but 20 neutrons instead of 18.Both isotopes have the same atomic number because they have the same number of protons, but their mass numbers are different due to the different number of neutrons.Despite having different masses, both isotopes have identical chemical properties because they have the same number of electrons.This is because chemical reactions depend on the electron configuration, which is identical for all isotopes of an element.These isotopes occur naturally, but not in equal amounts. In the next section, we'll explore their natural abundance.To find the natural abundance of isotopes, we need to look at scientific data tables or the periodic table.For chlorine, we can find this information in detailed reference materials, which show us its two naturally occurring isotopes.The data shows that chlorine has two isotopes: Chlorine-35 and Chlorine-37. Let's look at their specific abundances.Scientific tables provide precise measurements of isotopic abundance. For chlorine, we can see the exact percentages.Chlorine-35 makes up seventy-five point seven seven percent of all naturally occurring chlorine.While Chlorine-37 accounts for the remaining twenty-four point two three percent.These precise percentages are crucial for calculating the average atomic mass of chlorine.Now that we know the abundance of each isotope, we can use these values to calculate chlorine's average atomic mass.Now that we understand isotopes and their abundances, let's learn how to calculate the average atomic mass.The formula has two main components: the mass of each isotope and its abundance in nature.Each isotope's mass is measured in atomic mass units, which we learned about earlier.The abundance must be expressed as a decimal, not a percentage.To convert from percentage to decimal, we divide by one hundred. For example, seventy-five point seven seven percent becomes zero point seven five seven seven.Let's break down the calculation process into clear steps.When we multiply mass times abundance, we're finding the weighted contribution of each isotope to the final average.In the next section, we'll use these concepts to calculate the actual average atomic mass of chlorine.Now let's calculate the average atomic mass of chlorine using our formula and the abundance values.We'll use these values: Chlorine-35 with mass 34.97 amu and abundance 75.77 percent, and Chlorine-37 with mass 36.97 amu and abundance 24.23 percent.First, let's calculate the contribution from Chlorine-35. Multiply 34.97 by 0.7577.Next, we'll calculate Chlorine-37's contribution. Multiply 36.97 by 0.2423.Now we add both contributions: 26.49 plus 8.96.Our calculated value of 35.45 atomic mass units matches exactly with chlorine's atomic mass shown on the periodic table.This calculated average mass of 35.45 amu represents the weighted average of all naturally occurring chlorine isotopes.