Isotopes | Atoms, isotopes, and ions | High school chemistry | Khan Academy

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. Atoms 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. In 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. When 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. We'll start with carbon, which has an atomic number of six. In nature, carbon is mainly composed of two different isotopes. One of these isotopes has six neutrons, and the other isotope has seven neutrons.

Let's start with the isotope that has six protons and six neutrons. With 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 six written at its bottom left.

But for the isotope with six protons and seven neutrons, the mass number would be six plus seven, or thirteen. 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 six written at its bottom left.

Notice 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.

Let's return to our carbon isotopes example. For the carbon isotope that has six protons and six neutrons, we write that as C-12, or carbon 12. For the carbon isotope with six protons and seven neutrons, we write that as C-13, or carbon 13.

Let's wrap up this video with a quick recap. We 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. An atom's mass number can be described by the equation: mass number equals the number of protons plus the number of neutrons.

Lastly, we can describe isotopes using a couple of different notations, which are shown below.