When dealing with isotopes, we can talk about the most abundant isotope. Recall, the atomic mass of an element is an average mass of all its isotopes, and we're going to say that the most abundant isotope for an element is the one with a mass number closest to the atomic mass of the element. Remember, your mass number gives you the number of protons and neutrons together for a given isotope. So if we take a look here, we have our element, their atomic mass, the isotope symbols, and then we talk about their most abundant isotope. So for hydrogen, its atomic mass, according to the periodic table, is 1.008 amu and its isotope symbols are hydrogen. When it has a mass number of 2 it's called deuterium, and when it has a mass number of 3, it's called tritium. Alright. Now, remember, we said that the most abundant isotope is the one that has a mass number closest to the atomic mass. The atomic mass is 1. Here are our mass numbers, 1, 2, and 3. The one closest to the atomic mass is hydrogen-1. Its atomic mass number is 1 and it's closest to the atomic mass of 1.008.

Let's go to the next one, boron. Boron has two forms, boron-10 and boron-11. The atomic mass of boron is 10.81 amu. Looking at boron-10 and boron-11, which one has a mass number closest to this atomic mass? The answer would be boron-11 because 11 is closer to 10.81 than 10 is.

Then finally, we have sulfur. Sulfur has an atomic mass of 32.06 amu when you look on the periodic table. Alright? Now, some versions might show 32.07, but then remember, it's okay. It can either be 32.06 or 32.07, depending on which periodic table you are looking at. But that doesn't matter. Here is my atomic mass. Which mass number is closest to that 32.06? And we see that the answer is sulfur-32. 32 is closer to 32.06 than either 33, 34, or 36. Alright. So just remember, the most abundant isotope, to figure it out, look at what the atomic mass of the element is on the periodic table, look at the different mass numbers for all the isotopes, the one that has a mass number closest to that atomic mass is the most abundant isotope.