Alright, so now that we've covered the basics of the atom, in this video we're going to introduce isotopes. All atoms of the same element will have the same number of protons because changing the number of protons, by definition, changes the element. However, all atoms of the same element do not necessarily have the same number of neutrons. The number of neutrons can vary between atoms of the same element, leading us to isotopes. Isotopes are defined as atoms of the same element that vary only in the number of neutrons and have the same atomic numbers, meaning they have the same number of protons. Since changing the atomic number or the number of protons results in a change of the element, all isotopes will have the same atomic number or the same number of protons but differ in the number of neutrons. This means that they will have different mass numbers, which is the total number of protons and neutrons. Varying numbers of neutrons means varying mass numbers.
Recall that we not only defined the atomic number and mass number in our previous lesson but also defined the atomic mass. The atomic mass is similar to the mass number but differs in that it is the average mass of all the isotopes of a particular element. Now let's look at our example to understand the atomic mass of carbon's three isotopes. Shown below are the three isotopes of the carbon atom. One thing to note about these three isotopes is that they all have the same exact number of protons, which is 6, defining their atomic number. Additionally, all three scenarios here have the same exact number of electrons, which also does not vary between these scenarios. What differs between these three scenarios is the total number of neutrons in the nucleus: 6, 7, and 8 neutrons respectively. This variance makes them isotopes, atoms of the same element that vary only in the number of neutrons.
To calculate the mass number for each isotope, we total the number of protons and neutrons. For the first isotope, combining 6 protons and 6 neutrons gives us a mass number of 12. This isotope, referred to as carbon-12, can be abbreviated with the mass number 12 above the atomic number 6. For the second isotope, combining 6 protons and 7 neutrons results in a mass number of 13, thus carbon-13. Similarly, the third isotope, with 6 protons and 8 neutrons, has a mass number of 14, making it carbon-14. These isotopes also differ in abundance; the first isotope comprises about 99% of all carbon atoms, with the remaining 1% consisting of the other two isotopes. Because of this distribution, the mass of the rarer isotopes impacts the atomic mass minimally.
The atomic mass shown considers 100% of these carbon atoms; since 99% are carbon 12, the average is extremely close to the mass number of the most abundant isotope, resulting in an atomic mass of 12.011. The 0.011 comes from the tiny percentage of isotopes that are slightly heavier than 12. This concludes our introduction to isotopes, and we will practice this concept as we move along through our course. I will see you all in our next video.
