What Are Isotopes? - Definition, Types & Examples

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  • 0:00 What Are Isotopes?
  • 1:05 Isotopes of Hydrogen
  • 2:00 Isotopes of Carbon
  • 3:15 Types of Isotopes
  • 4:30 Types of Radioactive Isotopes
  • 6:30 Lesson Summary
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Lesson Transcript
Nissa Garcia

Nissa has a masters degree in chemistry and has taught high school science and college level chemistry.

Expert Contributor
Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

Not all atoms of an element are identical - atoms of the same element can have different numbers of neutrons. These different versions of the same element are called isotopes. In this lesson, we will discuss the examples and types of isotopes.

What are Isotopes?

Let's imagine a pair of identical twins. These twins have the same temperament, and since they're identical, it is very hard to tell them apart unless you examine them closely. When it is time for their annual physical, the twins need to step on a weighing scale, and when they do, one weighs slightly more than the other. In terms of chemistry, we can say that these twins are like isotopes of each other.

Atoms and elements are made of protons, neutrons and electrons. The nucleus is made of protons and neutrons, and the electrons surround the nucleus, as shown in the illustration below. The sum of the number of protons and the number of neutrons is equal to the atomic mass.

The Atom

In a given element, the number of neutrons can be different from each other, while the number of protons is not. These different versions of the same element are called isotopes. Isotopes are atoms with the same number of protons but that have a different number of neutrons. Since the atomic number is equal to the number of protons and the atomic mass is the sum of protons and neutrons, we can also say that isotopes are elements with the same atomic number but different mass numbers.

Let us take a look at an example.

Isotopes of Hydrogen

Isotopes of Hydrogen

The three are all isotopes of hydrogen. As you can see, they have the same atomic number, or number of protons, (number at the lower left of the element) but different atomic masses (number at the upper left of the element).

The number of neutrons can be calculated by calculating the difference between the atomic mass and atomic number. We can see that for the isotopes of hydrogen, they have varying number of neutrons. For protium, the number of neutrons is zero; for deuterium, the number of neutrons is one; and for tritium, the number of neutrons is two.

Going back to our comparison with identical twins, we can say that these three isotopes of hydrogen are like identical triplets of each other - they may appear to be identical outside, but they are different inside, and they also have different names.

Isotopes of Carbon

A very popular element, carbon, also has isotopes. There are three isotopes of carbon: carbon-12, carbon-13 and carbon-14. The numbers that are after the carbon refer to the atomic mass.

Isotopes of Carbon

The most common and abundant isotope of carbon is carbon-12. Looking at the percentages below each carbon isotope, we see that almost 98.9% of the carbon that is found is in the form of carbon-12. The least abundant form of carbon is carbon-14, with an abundance of less than 0.0001%. If we calculate the number of neutrons for each carbon isotope, we can see that they differ from each other. For carbon-12, we have 6 neutrons; for carbon-13, we have 7 neutrons; and for carbon-14, we have 8 neutrons.

You may notice if we look at the atomic masses of elements in the periodic table that they are rarely ever whole numbers, just like for carbon where the atomic mass is 12.011. This is because the atomic mass of carbon is based on the average atomic masses of its isotopes and the abundance of each isotope.


Types of Isotopes

There are two main types of isotopes, and these are radioactive isotopes and stable isotopes. Stable isotopes have a stable combination of protons and neutrons, so they have stable nuclei and do not undergo decay. These isotopes do not pose dangerous effects to living things, like radioactive isotopes.

They are typically useful when performing experiments in the environment and in the field of geochemistry. These isotopes can help determine the chemical composition and age of minerals and other geologic objects. Some examples of stable isotopes are isotopes of carbon, potassium, calcium and vanadium.

Radioactive isotopes have an unstable combination of protons and neutrons, so they have unstable nuclei. Because these isotopes are unstable, they undergo decay, and in the process can emit alpha, beta and gamma rays.

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Additional Activities

Modeling Isotopes

We learned in this lesson that isotopes are different forms of the same element. We can model the isotopes of various elements using coins. Let's say that protons are dimes and neutrons are pennies and a small cup or glass is the nucleus. Some key things to remember about isotopes are:

  1. The mass number is the sum of protons and neutrons (p + n).
  2. The atomic number is the quantity of protons in an atom.

Let's look at an example on how to model isotopes using coins. Hydrogen-1, or H-1, has an atomic mass of 1. It has 1 proton and no neutrons. To make this model we can put 1 dime in the cup to represent H-1.

Isotopes to create:

  1. H-3
  2. Li-7
  3. B-11
  4. C-14
  5. O-18

Follow-Up Questions

  1. How many dimes are in the H-3 isotope model?
  2. How many pennies are in the Li-7 isotope model?
  3. If you were to calculate the monetary value of the B-11 isotope model, what would it be?
  4. How many protons and neutrons are in C-14?
  5. What is the ratio of pennies to dimes in the O-18 isotope model?
  6. Models have shortcomings because they can't perfectly represent what they are modeling. In our penny and dime isotope models, can you list at least three shortcomings?


  1. 1
  2. 4
  3. $0.56 (5 dimes, 6 pennies)
  4. 6 protons, 8 neutrons
  5. 10:8
  6. Protons and neutrons have roughly the same mass and size. Pennies and dimes do not have the same mass nor the same size. Protons and neutrons are spherical in shape, while pennies and dimes are disk-shaped. The nucleus where protons and neutrons are located is spherical, while a cup that represents the nucleus is not.

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