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revealing the structure of the atom East Anglia,

Strips of blue paraffin wax in a white case.
Paraffin wax thought to have been used by James Chadwick in his neutron experiments, c. 1932. Science Museum/ Science & Society Picture Library

In 1911 Ernest Rutherford had already shown that atoms were more complex than previously thought, drawing them as electrons orbiting a central nucleus. He later suggested that this nucleus was made up of particles called ‘protons’. 

However, measurements of atomic mass suggested that there was another particle present. From observations this mysterious particle seemed to weigh slightly more than a proton, and be uncharged, making it difficult to detect. A Cambridge contemporary of Rutherford’s, James Chadwick (1891-1974), spent more than a decade trying to account for this additional mass. 

Finally, following experiments with beryllium radiation and paraffin wax, Chadwick found conclusive proof that another particle, called the neutron, was actually present. In February 1932 he published his findings in the journal Nature, rewriting existing theories about the structure of atoms.

After Chadwick’s discovery, it was soon postulated that neutrons could mediate a nuclear chain reaction, which eventually led to the atomic bomb, and later to nuclear power production.

Science Museum

East Anglia
University of Cambridge, Cambridgeshire
Key Individuals
Ernest Rutherford, James Chadwick,