Isotope Definition: Lesson for Kids

Instructor: 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.

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.

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
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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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