Isotopes: Same Element, Different Mass

Core Idea

Isotopes are atoms of the same element that have different numbers of neutrons. Because they have the same number of protons, isotopes are the same element and behave almost identically in chemical reactions. However, they have different masses because of the extra or fewer neutrons. The mass number of an atom is the total count of protons plus neutrons. For example, carbon-12 has 6 protons and 6 neutrons, while carbon-14 has 6 protons and 8 neutrons — both are carbon, but they have different masses.

How It's Best Learned

Build model atoms (using beads or drawings) of two isotopes of the same element, like carbon-12 and carbon-14. Count the protons and neutrons in each to see that the proton count stays the same while the neutron count changes. This makes the concept concrete and visual.

Common Misconceptions

• Isotopes are not different elements — they have the same atomic number and nearly identical chemical behavior.
• "Isotope" does not mean "radioactive" — most isotopes are perfectly stable (like carbon-12 and carbon-13). Only certain isotopes are radioactive.
• The mass number is not the same as the atomic number — mass number = protons + neutrons, while atomic number = protons only.
• The atomic mass shown on the periodic table is not a whole number because it is a weighted average of all naturally occurring isotopes of that element.

Explainer

You know that the number of protons defines which element an atom is — change the protons and you change the element. But what about neutrons? It turns out that atoms of the same element can have different numbers of neutrons, and these variations are called isotopes.

Take carbon as an example. Every carbon atom has exactly 6 protons — that is what makes it carbon. But the number of neutrons can vary. Carbon-12 has 6 protons and 6 neutrons (6 + 6 = 12). Carbon-13 has 6 protons and 7 neutrons (6 + 7 = 13). Carbon-14 has 6 protons and 8 neutrons (6 + 8 = 14). All three are carbon. All three behave almost identically in chemical reactions. The only difference is their mass, because neutrons add weight.

The number after the element name is called the mass number. It equals the total count of protons plus neutrons in the nucleus. The mass number is different from the atomic number, which counts only protons. So carbon always has an atomic number of 6, but its mass number can be 12, 13, or 14 depending on the isotope.

A common misunderstanding is that "isotope" means "radioactive." In reality, most isotopes are completely stable. Carbon-12 and carbon-13 are stable isotopes that make up almost all the carbon on Earth. Carbon-14, on the other hand, is radioactive — its nucleus is unstable and slowly breaks apart over time. Scientists use the steady decay of carbon-14 to date ancient artifacts and fossils, a technique called radiocarbon dating. But the radioactivity is a property of specific unstable isotopes, not of isotopes in general.

If you look at the periodic table, you will notice that the atomic mass listed for each element is usually not a whole number. For example, carbon's atomic mass is about 12.01, not exactly 12. That is because the listed mass is a weighted average of all the naturally occurring isotopes of that element, accounting for how common each isotope is. Carbon-12 is by far the most abundant, so the average is very close to 12, but the small contributions of carbon-13 push it slightly above 12. This averaging explains why atomic masses on the periodic table often have decimal values.