What Is an Isotope?
You already know that every atom of an element has the same number of protons — that number defines the element. But not all atoms of the same element are identical. Some atoms have extra neutrons in their nucleus, making them slightly heavier. These variants are called isotopes.
Atoms of the same element that have the same number of protons but a different number of neutrons — and therefore a different mass number.
Because isotopes have the same number of protons, they also have the same number of electrons. This means they have identical chemical properties — they react in exactly the same way. However, they differ in their physical properties, particularly their mass.
Think of isotopes like different models of the same car brand. A Toyota Corolla and a Toyota Corolla with a heavier engine are fundamentally the same car — same design, same behaviour on the road — but one is heavier. The "brand" (element) hasn't changed, just the mass.
A key relationship to keep in mind: In MYP Sciences, one of the big ideas is Relationships — how structure relates to properties and behaviour. Isotopes are a perfect example: the number of electrons (determined by proton count) governs chemical behaviour, while the number of neutrons in the nucleus influences physical properties and stability. Same structure where it counts chemically, different structure where it affects mass and nuclear stability.
Protons, Neutrons, and Mass Number
To understand isotopes, you need to be confident with three key atomic numbers:
- Atomic number (Z): The number of protons in the nucleus. This is fixed for every atom of an element — it never changes.
- Mass number (A): The total number of protons plus neutrons in the nucleus.
- Number of neutrons: Found by subtracting the atomic number from the mass number.
So for isotopes of the same element, the atomic number stays the same, but the mass number changes because the neutron count changes.
Electrons are so tiny that they contribute almost nothing to the mass of an atom. When we talk about atomic mass, we are really just counting protons and neutrons.
What about ions? It is worth noting that atoms can also lose or gain electrons to form ions. If an isotope becomes an ion, its electron count changes — but it is still the same isotope, because the proton and neutron counts are unchanged. For example, has 17 protons, 18 neutrons, and 18 electrons (one extra, giving it a negative charge). Don't confuse isotopes (different neutron counts) with ions (different electron counts).
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