Isotopes

0:00- [Instructor] Every element is defined by the number of protons in its atoms, which is called its atomic number. So for example, every atom of potassium has 19 protons and every atom of cobalt has 27 protons. But what about neutrons? Well, an element doesn't always have the same number of neutrons in its atoms.

0:23Atoms of the same element with different numbers of neutrons are called isotopes. Because isotopes of an element have different numbers of neutrons, those isotopes also have different masses. Remember, most of the mass of an atom comes from the protons and neutrons in its nucleus.

0:42In fact, the sum of the number of protons and neutrons in an atom is called its mass number. We can write this in the form of an equation as follows, mass number equals the number of protons, plus the number of neutrons. Next, let's dive into isotope notation, or how we represent isotopes in writing.

1:05When describing different isotopes, you'll often see this notation where X is the chemical symbol, which is an abbreviation of the element's name, A is the mass number, the number of protons plus neutrons, and Z is the atomic number, the number of protons. Let's practice writing out the notation for different isotopes.

1:27We'll start with carbon, which has an atomic number of 6. In nature, carbon is mainly composed of two different isotopes. One of these isotopes has 6 neutrons and the other isotope has 7 neutrons. Let's start with the isotope that has 6 protons and 6 neutrons.

1:45With that information, we know that the mass number must be 12, which is the sum of the protons and neutrons. So the notation for this isotope of carbon would look like this. We have the chemical symbol C with the mass number 12 written at its top-left, and the atomic number 6 written at its bottom-left.

2:08But for the isotope with 6 protons and 7 neutrons, the mass number would be 6 plus 7, or 13. So the notation for the carbon isotope would be as follows. Here, we have the chemical symbol C with the mass number 13 written at its top-left, and the atomic number 6 written at its bottom-left.

2:33Notice that the chemical symbol and atomic number share the same information, the identity of the element, and are the same for both isotopes, but the mass number is different for each isotope depending on the number of neutrons. Another notation that is used for isotopes is the chemical symbol or the element's name, symbolized by X, followed by a hyphen, and the mass number, symbolized by A, so X-A.

3:04Let's return to our carbon isotopes example. For the carbon isotope that has 6 protons and 6 neutrons, we write that as C-12 or carbon-12. For the carbon isotope with 6 protons and 7 neutrons, we write that as C-13 or carbon-13. Let's wrap up this video with a quick recap.

3:29We know that elements are defined by the number of protons in their atoms, but atoms of the same element can have different numbers of neutrons. These are called isotopes. Because isotopes have different numbers of neutrons, they also have different mass numbers, which are the sum of the protons and neutrons in each isotope's atom.

3:51An atom's mass number can be described by the equation, mass number equals the number of protons, plus the number of neutrons. And lastly, we can describe isotopes using a couple of different notations, which are shown below.