Isotopes
Isotopes are atoms of one element with different numbers of neutrons.
About this topic
Isotopes are atoms of the same element with different numbers of neutrons, thus different mass numbers: carbon-12, carbon-13 and carbon-14. They have the same chemical properties because they have the same number of electrons, but slightly different physical properties such as mass and, for some, radioactivity.
Most elements occur naturally as a mixture of isotopes, and the average atomic mass on the periodic table is a weighted average of them. For example, chlorine is about 75 percent chlorine-35 and 25 percent chlorine-37. The calculation is (35 x 0.75) + (37 x 0.25) = 35.5. This explains why the table lists 35.45 and not a whole number.
Some isotopes are unstable and radioactive. Carbon-14 is used to date ancient objects, iodine-131 treats thyroid disease, cobalt-60 sterilises medical equipment, and uranium-235 fuels nuclear reactors. Hydrogen has three well-known isotopes: protium, deuterium and tritium.
A frequent misconception is that isotopes are different elements; they are not, because the proton count is the same. Also, isotopes differ in neutrons, not protons or electrons. Remember to write them as name-mass number, such as uranium-238.
Key ideas
- Same protons, different neutrons
- Carbon-12 and carbon-14
- Atomic mass is a weighted average
Quick check
Use cases: where Isotopes helps
- Homework: review the key idea behind Isotopes before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Isotopes without notes.
- Visual learning: play with the interactive lab to see how Isotopes changes when you adjust its inputs.
- Teaching a friend: explain Isotopes in your own words, then compare your version with the key ideas.
Chemistry explains medicine, food, cleaning products, fuels and building materials.
Frequently asked questions
What is Isotopes?
Isotopes are atoms of one element with different numbers of neutrons.
What will I learn in Isotopes?
Same protons, different neutrons. Carbon-12 and carbon-14. Atomic mass is a weighted average.
Is Isotopes part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Isotopes lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Isotopes?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.