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“The beauty of a living thing is not the atoms that go into it, but the way those atoms are put together.”

-Carl Sagan

What are Isotopes?

You can think of isotopes as atomic "siblings." They are part of the same family (same number of PROTONS) but they are unique in that each isotope has a different number of NEUTRONS in its nucleus.

The MASS NUMBER of any ion is the sum of its PROTONS and NEUTRONS. Istotopes have the same ATOMIC numbers (number of protons) but different MASS numbers (number of neutrons).

Notice how all three isotopes of carbon have 6 protons, but each one has a different number of neutrons.

Why do we have isotopes?

Isotopes are still a bit mysterious. I have never found a great, definitive explaintation of why they exists. Scientists agree neutrons help stabilize the nucleus of the atom. Think about it...a close packed group of protons (all with a + charge) would repel each other so neutrons act like little charge buffers. But why an element can be made up of so many different isotopes and why some elements have more stable isotopes than others? Still kind of a mystery. When we talk about radioactive decay you'll also learn that isotopes can break down and turn into other isotopes or even other elements!

These resources will help aquaint you with isotopes:

How do you write the symbol for isotopes?

You've already seen how to write the name of an isotope: (Element Name) - (Mass Number of Isotope) like Carbon-12, Carbon-13, and Carbon-14.

To write the symbol for an isotope you just add a few things to the the basic symbol for an element.

isotope symbols

The LOWER number represents the number of PROTONS. Notice that is the same for all isotopes because if you change the number of protons, you have changed the element. All isotopes of an element will always have the same number of protons.

The UPPER number represents the MASS NUMBER, which is number of protons PLUS number of neutrons. This is unique for every isotope. Magnesium 24 has 12 neutrons, Magnesium-26 has 14 neutrons and Magnesium-30 has 18 neutrons. All Magnesium isotopes have 12 protons.

How much do you need to know about isotopes?

Think Ahead and Beyond: How medicine uses istopes.