Discovery of the Proton & Neutron (AQA GCSE Physics: Combined Science)

• 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

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

• 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

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