The Rutherford atomic model was proposed by Ernest Rutherford in 1911. It is also called the Planetary Model of the Atom. It introduced the concept of a dense, positively charged nucleus at the centre of an atom, with electrons orbiting it, forming the basis for modern atomic theory. According to Rutherford’s atomic model, the positively charged particles and the majority of the mass of an atom were said to be concentrated in a small volume. He referred to this area of the atoms as the nucleus.
Rutherford Gold Foil Experiment
To determine how electrons are arranged in an atom, the Alpha (α) Particle Scattering Experiment was organized by Rutherford. Rapidly moving α-particles were directed to bombard a thin sheet of gold.
- The gold foil was selected so as to obtain an extremely thin layer. The thickness of the gold foil was about 1000 atoms.
- Doubly charged helium ions are known as α-particles. Rapidly moving α-particles possess a great deal of energy, as they have a mass of about 4 amu.
- The hypothesis was that α-particles would be deflected by the subatomic particles in the gold atoms.
- Rutherford didn't expect to witness significant deflections, as the α-particles were considerably heavier than the protons.
- However, the experiment produced entirely unanticipated results.
Observations
- A large percentage of alpha particles travelled through the gold film without being deflected, indicating that the majority of space in an atom is empty. As a result, an atom's main portion must be empty.
- The positive charge in an atom is concentrated in a relatively small volume and is not dispersed evenly. When bombarded, the gold foil only deflected a small number of alpha particles. They experienced extremely slight angles of deflection. So he arrived at the stated conclusion.
- Most alpha particles passed straight through the foil. In addition, relatively few particles had 180° deflected. As a result, he came to the conclusion that the positively charged particles only occupied a small portion of an atom's overall volume.
Conclusion
- The majority of the space in an atom is vacant because a large proportion of the α-particles directed toward the gold sheet passed through it without any deflection.
- Only a few α-particles were diverted off their route, suggesting that the atom's positive charge takes up relatively little space.
- Since a very tiny percentage of α-particles completely rebounded, this implied that the atom's mass and positive charge are concentrated in a small volume and not uniformly distributed.
Postulates
- Most of the mass is concentrated in the nucleus: An atom contains a very small, dense, positively charged central region called the nucleus. Almost all the mass of the atom is concentrated in this nucleus
- Electrons revolve around the nucleus: Negatively charged electrons revolve around the nucleus in the space surrounding it.
- Atom is mostly empty space: Since the nucleus is extremely small compared to the size of the atom, most of the atom is empty space
- Electrostatic attraction holds the atom together: The negatively charged electrons are attracted to the positively charged nucleus by electrostatic force.
- Atom is electrically neutral: The total positive charge of the nucleus is equal to the total negative charge of electrons, so the atom as a whole is neutral.
Drawbacks
- Rutherford's model predicts that electrons will orbit around the positively charged nucleus, a prediction that is not considered stable. A charged particle in rapid motion along a circular route would lose energy repeatedly and eventually collapse into the nucleus. This condition causes an atom to be unstable, whereas we know that atoms are extremely stable.
- Because it merely postulated the existence of protons in the nucleus, the Rutherford model could not resolve the problem of atomic mass.
- Rutherford's atomic model doesn't explain the arrangement of electrons in the atom, which makes it incomplete in this regard.