Isotopes are atoms of the same element with different numbers of neutrons. The proton number stays the same, so the element stays the same, but the mass of the atom changes.
This lesson is part of SPM Science elements and materials. It builds on reading atomic structure information.
What stays the same and what changes?
The proton number defines the element, so all isotopes of an element share it. The neutron number differs, so the nucleon number differs too.
Electrons match the proton number in a neutral atom, which is why isotopes share their chemical behaviour.
Worked example: two isotopes of chlorine
Chlorine has two common isotopes, written ³⁵₁₇Cl and ³⁷₁₇Cl. Compare them.
| Particle | ³⁵₁₇Cl | ³⁷₁₇Cl |
|---|---|---|
| Protons | 17 | 17 |
| Neutrons | 35 − 17 = 18 | 37 − 17 = 20 |
| Electrons | 17 | 17 |
| Electron arrangement | 2.8.7 | 2.8.7 |
Why do they react alike? Both have the same 7 outer electrons, so each gains one electron to form Cl⁻ in the same way.
Why does one weigh more? It has 2 extra neutrons, each with a mass close to that of a proton.
What uses do syllabus questions expect?
Questions ask you to match a use to the property that supports it. The property is usually that a radioactive isotope gives out radiation that instruments can detect.
| Use | Property that makes it work |
|---|---|
| Tracing a flow in an industrial pipe | Radiation can be detected outside the pipe |
| Dating an ancient wooden object | A radioactive isotope changes at a steady rate |
| Sterilising medical equipment | Radiation can damage microorganisms |
Write each use as an explanation. The exam does not ask how to handle the material, and students should never try to.
The mistake that costs marks
The slip is to say isotopes are “different elements” because their masses differ. The proton number decides the element, and it is the same for isotopes.
| Statement | Mark? | Reason |
|---|---|---|
| Isotopes have different numbers of protons | No | Proton number defines the element |
| Isotopes have different numbers of neutrons | Yes | The correct difference |
| Isotopes react differently | No | Same electrons, same chemistry |
Check yourself
Carbon has isotopes ¹²₆C and ¹⁴₆C. State the number of neutrons in each and explain why both are carbon.
Answer
¹²₆C has 12 − 6 = 6 neutrons. ¹⁴₆C has 14 − 6 = 8 neutrons.
Both are carbon because they have the same proton number, 6. The proton number defines the element, and only the neutron number differs.
What to study next
Continue with classifying materials from properties, then test the whole topic in the elements and materials practice set.
If you want a teacher to go over the isotope wording with you, see online one-to-one Science tuition.