Isotope Definition: Lesson for Kids

Lindsy Frazer

Dr. Frazer has taught several college level Science courses and has a master's degree in Human Biology and a PhD in Library and Information Science.

Expert Contributor
Gretchen Graef

Gretchen has a Ph.D in Materials Science and Engineering. She has been an engineer, technical writer, and a teacher teaching physics, chemistry, biology, and mathematics.

Just because two atoms are of the same element does not mean they are identical. Find out how two atoms of the same element can differ, what isotopes are, how they are named and the different types in this lesson. Updated: 07/10/2020

What is an Isotope?

Did you have an apple today? How about yesterday? A lot of the apples you've eaten may all look the same - red and round - but when you cut them apart, you'll see they're all slightly different.

For example, let's say one apple has two seeds, a second apple has four seeds, and a third apple has six seeds. But even though each one has a different number of seeds, they are all the same thing: red delicious apples!

These apples are a lot like the isotopes found in chemistry. Isotopes are atoms with the same number of protons and electrons, but a different number of neutrons.

The number of protons in an atom can tell you what element it is. Atoms with one proton are atoms of the element hydrogen, just like all atoms with two protons are atoms of the element helium.

Changing the number of protons an atom has changes what element it is. But, just like changing the number of seeds in an apple doesn't change it into an orange or a banana, changing the number of neutrons in an atom does not change the element or its properties. Since an element's atomic number is based on the number of protons its atoms contain, all isotopes of an element have the same atomic number.

What is Atomic Mass?

Apple seeds weigh about half a gram each. So, apples with more seeds weigh more than apples with fewer seeds. Adding or subtracting seeds from an apple doesn't change much about an apple except for how much it weighs.

Just like apples, changing the number of neutrons in an atom changes the mass of the atom. The mass of an atom, also known as its atomic mass, is calculated by adding the number of protons and neutrons in the atom.

Atomic mass = number of protons + number of neutrons in an atom

How Are Isotopes Named?

Do you have a nickname, or do people sometimes shorten your name? Maybe you have a friend named William, but everyone calls him Bill. Each isotope has a long formal name and a shorter nickname, just like William. But both of these names are based on the isotope's mass.

For example, aluminum is an element with many isotopes. Each isotope of aluminum (Al) is named based on its mass. Check out the formal and short names for the aluminum isotopes in this diagram.

Aluminum has two names

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Additional Activities

Radioactive Decay

Stable isotopes resist change, but not all isotopes are stable. Unstable isotopes are called radioisotopes. They break down to isotopes that are resistant to change. When they break down, they emit either particles or pure energy. The three types of radiation are named for the first three letters of the Greek alphabet and are:

Alpha emission: Consist of two protons and two neutrons.

Beta emission: These are high speed electrons. They have more energy than alpha particles.

Gamma radiation: This is pure energy that is emitted from decay. It has the highest energy of all. Gamma rays can even pass through lead.

Uses for Radioisotopes

Radioisotopes are used for determining when something was formed. Carbon-14 is used to determine when a living organism died because, while it lived, Carbon-14 that decayed to Carbon-12 was replaced. When it died, decay occurred without replacement. The rate of decay is constant and known. The half-life of Carbon-14 to Carbon-12 is the time it takes for one half of the Carbon-14 to decay and is 5,730 years. If one half of the carbon has decayed, the organism died around 6,000 years ago. Other isotopes that decay slower are used to date older items. Uranium-238 and Potassium-40 have half-lives of over one billion years. The oldest rock on Earth found so far was in Quebec, Canada. It was dated at 4 billion years old. The oldest item that came from space is the Murchison meteorite that fell in Australia in 1969. It was dated in January of 2020. Scientist found that this famous meteorite contained silicon carbide particles that are over 7 billion years old. That is older than the Earth!

Check for Understanding

  1. (Remember from the lesson) An isotope has the same number of _____ and _____, but a different number of _____.
  2. Gamma rays have _____ energy than alpha or beta particles.
  3. A _____ isotope is one that resists change.
  4. Explain why Carbon-14 it not a good isotope to choose for dating rocks that are very old. Write your answer in one complete sentence.


  1. electrons and protons (either order), neutrons
  2. higher, greater
  3. stable
  4. Give credit for any reasonable answer that mentions the relative half-lives of Carbon-14 versus other isotopes. Deduct credit for not writing a complete sentence or sentences. Example answer: Carbon-14 is not a good choice because of its small half-life is compared to the age of very old rocks. The half-life of Carbon-14 is small compared with the half-life of Potassium-40 or Uranium-238.

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