What is an isotope in simple terms?

An isotope is a version of an element that has the same number of protons but a different number of neutrons in its nucleus. This means isotopes of the same element have different masses but are still the same element.

What is an isotope used for in science?

Isotopes are used to date rocks and fossils, to trace how substances move in the body or environment, and in medical imaging and treatments. Their different masses and stability make them useful tools for measuring and tracking changes.

How can you tell if two atoms are isotopes of each other?

Two atoms are isotopes of the same element if they have the same number of protons but different numbers of neutrons. If the proton counts are different, they are different elements, not isotopes.

What is an isotope and how is it written?

An isotope is written by naming the element and its mass number, such as carbon-12 or chlorine-35. It can also be written in nuclear notation, for example carbon-12 as 12 over 6 next to C, where the top number is the mass number and the bottom number is the atomic number.

What Is an Isotope? Definition and Examples

Isotope explained for kids

Think of an element like a type of toy, such as a building block. Isotopes are like versions of that same block that are a little heavier or lighter inside, but still fit with the same sets. They act like the same element in most ways, but their mass is different.

Isotope examples

Example 1 (middle school level): Carbon isotopes

Carbon is an element that has 6 protons in every atom. That 6 is its atomic number, and it never changes for carbon.

However, carbon atoms can have different numbers of neutrons:

Carbon-12 has 6 protons and 6 neutrons, for a mass number of 12.
Carbon-14 has 6 protons and 8 neutrons, for a mass number of 14.

Carbon-12 and carbon-14 are isotopes of carbon. They are both carbon, because they both have 6 protons, but they have different masses because they have different numbers of neutrons.

Example 2 (high school level): Writing isotope notation for chlorine

Chlorine is an element with 17 protons in each atom, so its atomic number is 17. Two common isotopes of chlorine are chlorine-35 and chlorine-37.

For chlorine-35:

Atomic number = 17 (so 17 protons).
Mass number = 35 (protons + neutrons).
Number of neutrons = 35 − 17 = 18 neutrons.

This isotope can be written as Cl-35 or in nuclear notation as ³⁵₁₇Cl.

For chlorine-37:

Atomic number = 17 (17 protons again).
Mass number = 37.
Number of neutrons = 37 − 17 = 20 neutrons.

This isotope can be written as Cl-37 or ³⁷₁₇Cl. These are isotopes because they are both chlorine (same 17 protons) but have different numbers of neutrons and different mass numbers.

Going deeper with isotopes

Isotopes matter in chemistry and physics because the number of neutrons affects an atom's mass and stability. Some isotopes are stable, which means they do not change over time, while others are unstable, or radioactive, and slowly change into other elements by giving off energy and particles.

In chemical reactions, which are processes where atoms rearrange to form new substances, isotopes of the same element usually behave almost the same, because chemical behavior mostly depends on the number of protons and the arrangement of electrons. However, the different masses of isotopes can affect how fast reactions happen or how substances move and spread.

Isotopes are also used as tools in science. For example, carbon-14 is used to estimate the age of once-living materials, and certain isotopes are used as tracers to follow where atoms go in the body or in the environment.

Common mistakes about isotopes

Mistake: Thinking isotopes are completely different elements because their mass numbers are different, such as believing carbon-12 and carbon-14 are different elements.
Correction: Isotopes are the same element as long as they have the same number of protons; only the number of neutrons and the mass change.
Mistake: Believing isotopes must always be radioactive, for example assuming any isotope with a name like carbon-14 is dangerous.
Correction: Some isotopes are stable and not radioactive at all; only unstable isotopes are radioactive.
Mistake: Confusing atomic number with mass number, such as saying chlorine-35 has 35 protons because of the 35 in its name.
Correction: The atomic number tells the number of protons, and it is fixed for each element; the mass number is protons plus neutrons and can change from one isotope to another.

How isotopes connect to everyday life

Isotopes show up in many real-world situations your child may hear about. Smoke detectors in homes often use a radioactive isotope to sense smoke. Doctors use certain isotopes in small, controlled amounts to take images of organs. News stories about dating fossils or ancient objects often mention carbon-14, an isotope of carbon used to estimate age.

Related chemistry terms to explore

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What is an isotope?