- In 1803 John Dalton presented his atomic theory based on three key ideas:
- Matter is made of atoms which are tiny particles that cannot be created, destroyed or divided.
- Atoms of the same element are identical and atoms of different elements are different.
- Different atoms combine together to form new substances.
- At the time the theory was correct but as science developed some parts of Dalton’s theory were disproved.
- In 1897 physicist J.J. Thomson discovered the electron. Using a cathode-ray tube he conducted an experiment which identified the electron as a negatively charged subatomic particle, hence proving that atoms are divisible.
Diagram showing an electron beam deflecting towards the positive plate, proving electrons are negatively charged
- Based on his investigations Thomson proposed a model of the atom known as the plum pudding model which depicted negative electrons spread throughout soft globules of positively charged material.
Diagram showing the plum pudding model of the atom
- In 1909 Ernest Rutherford presented his model of the atom based on the famous gold foil experiment.
- Rutherford shot a beam of positively charged particles at a thin sheet of gold foil and, based on the plum pudding model, expected the particles to pass through the foil because the positive charge of the nucleus was thought to be evenly spread out.
- Some particles were scattered, however, and a few were deflected directly back, which led him to postulate that most of an atom’s mass is concentrated in a region of space at the centre of the atom called the nucleus.
- In Rutherford’s model the atom consists mainly of empty space with the nucleus at the centre and the electrons orbiting in paths around the nucleus.
- This model was known as the nuclear model of the atom.
Diagram showing Ernest Rutherford’s nuclear model of the atom
- In 1913 Niels Bohr further developed the nuclear model by proposing that electrons orbit the nucleus in fixed shells or orbitals located at set distances from the nucleus.
- Each orbital has a different energy associated with it, with the higher energy orbitals being located further away from the nucleus.
- This model solved the question of why the atom doesn’t collapse inwards due to the attraction between the positive nucleus and negative electrons circling the nucleus.
- Bohr’s theory and calculations were in agreement with experimental results.
- Further investigation and experimentation revealed that the nucleus could be divided into smaller particles, each one having the same mass and charge. This work led to the discovery of the proton.
Diagram showing Niels Bohr’s model of the atom