AQA GCSE Physics

Revision Notes

4.1.9 Discovery of the Proton & Neutron

The Discovery of the Proton

  • The nuclear model was improved when the particles within the nucleus were discovered
  • Ernest Rutherford discovered that the positively charged nucleus of all materials could be broken down into smaller positively charged particles
  • The smallest of these parts had the same charge as a Hydrogen nucleus
    • This amount of charge is called the elementary charge
  • He called the positively charged particles in the nucleus as protons
    • ‘Proto’ means original – protons are the original unit of positive charge

Worked Example

An alpha particle is the nucleus of a Helium atom.

Alpha particle, downloadable IGCSE & GCSE Physics revision notes

Determine the charge of an alpha particle.

Step 1: Determine the number of protons in the particle

    • The alpha particle has two protons

Step 2: Determine the number of electrons

    • There are no electrons in the nucleus of an atom
    • Since the alpha particle is a helium nucleus, it has no electrons

Step 3: Determine the charge of the alpha particle

    • The total charge is the sum of the positive protons and negative electrons
    • There are 2 protons and 0 electrons

2 – 0 = 2

    • The charge on the alpha particle is +2

The Discovery of the Neutron

  • After the discovery of the proton, many scientists saw evidence of another particle in the nucleus but could not prove it
  • It was about 20 years after models of the nucleus at the centre of the atom, that the final particle was confirmed
  • In 1932, James Chadwick proved the existence of neutrons in the nucleus, he discovered:
    • A neutron has a mass similar to the proton
    • A neutron has no charge – it is neutral
  • The discovery of the neutron gave another, better model of the atom
    • A positive, dense nucleus made from neutrons and protons
    • Negative electrons on different energy levels orbit the nucleus

Carbon atom structure, IGCSE & GCSE, AS & A Level Chemistry revision notes

The models of the atom changed a lot at the start of the 20th Century. The discovery of the neutron allowed this model to be created

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