iv.3. Rutherford’s Model of an Atom: Nucleus and Electron Paths in Atomic Structure

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iv.3. Rutherford’s Model of an Atom: Nucleus and Electron Paths in Atomic Structure

Rutherford (1871-1937)

He was born at Spring Grove on 30 August 1871. He was known as the ‘Father’ of nuclear physics. He is famous for his work on radioactivity and the discovery of the nucleus of an atom with the gold foil experiment. He got the Nobel prize in chemistry in 1908.

Rutherford’s Electron Experiment: Atomic Arrangement	Ernest Rutherford designed an experiment to know the arrangement of electrons within the atom.
Use of α- Particle: Rutherford’s Gold Foil Experiment	In this experiment, fast moving alpha (α)-particles were made to fall on a thin gold foil. Selection of Gold Foil: He selected a gold foil because he wanted as thin a layer as possible and this gold foil was about 1000 atoms thick. About α-particles: These particles are doubly-charged helium ions. Since they have a mass of 4u, the fast-moving α-particles have a considerable amount of energy. Deflection: It was expected that α-particles would be deflected by the subatomic particles in the gold atoms.
Deviated Result in Rutherford’s Alpha-Particle scattering experiment	The α-particle scattering experiment gave totally unexpected results. Most of the fast moving α-particles passed straight through the gold foil. Some of the α-particles were deflected by the foil by small angles. One out of every 12000 particles appeared to rebound.
Observations: Atom’s Empty Spaces and Concentrated Positive Charge	Rutherford concluded from the α-particle scattering experiment that– Most of the space inside the atom is empty because most of the α-particles passed through the gold foil without getting deflected. Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space. A very small fraction of α-particles were deflected by 1800, indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom. Scattering of α-particles by a gold foil
Calculating Nucleus Radius	He also calculated that the radius of the nucleus is about 105 times less than the radius of the atom.
Nuclear Model of an Atom and the Central role of the nucleus	On the basis of his experiment, he put forward the Nuclear Model of an Atom. There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus. The electrons revolve around the nucleus in circular paths. The size of the nucleus is very small as compared to the size of the atom.
Drawbacks of Rutherford’s model: Stable Reality of Atoms	The revolution of the electron in a circular orbit is not expected to be stable. Any particle in a circular orbit would undergo acceleration and would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know. Atoms are quite stable.

Neils Bohr

He was born in Copenhagen on 7 October 1885. He was appointed professor of physics at Copenhagen University in 1916 and got the Nobel prize for his work on the structure of atom in 1922.

Among Professor Bohr’s numerous writings, three appearing as books are:

(i) The Theory of Spectra and Atomic Constitution,

(ii) Atomic Theory and,

(iii) The Description of Nature.