Active reading exercise: Isotopes


In 1932 James Chadwick, a British scientist, discovered the neutron. His discovery explains how isotopes are formed. Isotopes have the same number of protons, but different numbers of neutrons.

We can also say that the atomic number is the same, but the mass numbers are different. Isotopes are the same element because their atomic number does not change.

We refer to an isotope by adding its mass number to the element’s name. The isotope in the diagram below is called carbon-12.


Carbon isotopes

Carbon can occur as three different isotopes. They are called carbon-12, carbon-13 and carbon-14.

Carbon dating is used to identify the age of very old objects, for example the remains of extinct animals such as mammoths. The amount of the carbon isotope carbon-14 in an object is examined to determine its age.

Things to do

  1. Fill in the missing words and numbers to describe the similarities and differences between isotopes of the same element.                                                                                 a) Isotopes are atoms with the same            number but different                   number. b) They have the same number of protons and electrons but different numbers of                      .
  2. Name the scientist who discovered the neutron.
  3. Why are isotopes the same element?
  4. How do we refer to isotopes?
  5. Name the three isotopes of carbon.
  6. Why is the isotope carbon-14 useful?
  7. Using your knowledge about isotopes, fill in the gaps in the table a, b and c.
Isotope name Atomic number Mass number
 Tin-116  50 a
b c 118

Remember that the atomic number is the same for isotopes of the same element.


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