2. Contents of this resource
• What are isotopes?
• What causes isotopes?
• Instability of isotopes.
• Stable and unstable isotopes.
3. What are isotopes?
• Atoms of the same element have the same atomic number.
• Atoms of different elements cannot have the same atomic number; but they
can have same mass numbers.
• Atoms of the same element can have different mass numbers although they
have the same atomic numbers.
• Atoms of the same element having different mass numbers are isotopes.
4. What causes isotopes?
• Atoms of the same element have the same atomic number.
• Mass of an atom comes from the protons and neutrons.
• Isotopes have different mass numbers showing that they have different
numbers of protons and neutrons.
• Since the number of protons is the same for a particular element, then
isotopes have different numbers of neutrons.
5. Instability of isotopes
• Isotopes have different number of nucleons.
• Nucleons are protons and neutrons.
• Difference in the number of nucleons, means difference in the mass of the
nucleus.
• The difference in the masses of the nuclei cause them to be unstable.
6. Stable and unstable isotopes
Stable Nuclei Unstable Nuclei Found in
Carbon-12, Carbon-13 Carbon-14 Plants and animals
Potassium-39 Potassium-40 Rocks and sea water
Uranium-235, Uranium-238 Rocks
