Rutherford nucleus of an atom

Ernest Rutherford and his students describe the gold foil experiment to provide an atomic model in two parts, the nucleus of an atom and extranuclear electrons.

A beam of alpha particles obtained from spontaneously radioactive isotopes of polonium directed to very thin platinum or gold foil. The direction of motion of α-particles after emerging from the metal sheets traced on a fluorescent screen.

Rutherford gold foil experiment model of an atom

Rutherford gold foil experiment observations

  1. Rutherford observed that the vast majority of the alpha particles passed the straight line through the gold foil. Thus this suggested that the matter consists largely of emptiness.
  2. But a very limited few particles (about 1 in 8000) changed their direction quite sharply but in some cases the deflection more than 900.
  3. Even in a few cases, the alpha particle after the encounter moved backward or reflected back. But this number increases with the metal foil of the heavier nucleus.

Conclusions of the gold foil experiment

Earnest Rutherford concluded these point from his gold foil experiment

  1. All the positive charge and the entire mass of the atom concentrated in a very small part of the atom. These central core called the atomic nucleus.
  2. The dimension of the nucleus is negligible compared to the atom. The Radius of the atomic nucleus ∼ 10-13 being the same as that of an electron but the radius of an atom ∼ 10-8. Thus an atom must have a very empty structure.
  3. The large deflection of an alpha particle from its original path was due to Coulombic repulsion between the alpha particle and the positive nucleus of an atom. Hence entire positive charge also resides in the nucleus.
  4. Some alpha particle suffers little deflection while passing by an electron. This suggested, outside the nucleus, there are electrons that make the atom ultimately neutral.

From the above conclusions, Rutherford proposed an atomic model.

Rutherford model of an atom

According to the Rutherford model, the entire mass of the atom concentrated in a tiny positively charged nucleus. These electrons move around the nucleus in different orbits constantly otherwise the electrons fall on the nucleus. Thus an atom divided by Rutherford into two-parts

  1. The nucleus of an atom
  2. Extranuclear electrons

The nucleus of an atom

Almost the entire mass of the atom concentrated in a very small central core called the nucleus of an atom. Since the extranuclear electrons contribute negligibly to the total mass of the atom.

Thus the nucleus must carry particles that account both for the mass and positive charge of the atom.

Extranuclear electrons of an atom

But an atom is electrically neutral. Thus a positive nucleus must be surrounded by the negatively charge of an electron. Such a system cannot be stable if the election were in rest.

Therefore the electron moving in circular orbits around the nucleus and Coulombic attraction between the nucleus and the electron was equal to the centrifugal force of attraction.

Limitations of the Rutherford model

  1. The Rutherford model has limitations for conformity with the classical atomic theory of electromagnetic radiation. A moving charged particle always emits radiation. Thus the loss of kinetic energy and eventually hit the nucleus of an atom.
  2. If the energy of an electron loses continuously, the observed atomic spectra should be continuous, which consisting of broad bands merging one into the other.