MODELS OF AN ATOM

Atomic models of atoms-

J.J. Thomson model of an atom-

  • J.J. Thomson was the first one to propose a model for the structure of an atom.
  • He was awarded the Nobel prize in Physics in 1906 for his work on the discovery of electrons.

 

Thomson proposed that:

  • An atom possesses a spherical shape in which positively is uniformly distributed and the electrons are embedded in it.
  • The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.
  • An important feature of this model is that the mass of the atom is assumed to be uniformly distributed over the atom.
  • Although this model was able to explain the overall neutrality of the atom, but was not consistent with the results of later experiments.

Rutherford’s Nuclear Model of Atom

  • Ernest Rutherford discovered the nucleus of the atom in 1911, discovered alpha and beta rays, and proposed the laws of radioactive decay.
  • Rutherford conducted a series of experiments using alpha-particles. A beam of alpha-particles was directed against a thin foil of gold, platinum, silver, or copper. The foil was surrounded by a circular fluorescent zinc screen. Whenever an alpha-particle struck the screen, it produced a flash of light.

 

 

OBSERVATION

CONCLUSION

(a) Most of the -particles went straight without suffering any deflection.

(a) Many of the particles went straight through the metal foil undeflected, indicating
large empty space within the atom.

 

(b) Some of them were deflected through small angles.

(b) Some of the -particles were deflected from their original paths through moderate angles, indicating that whole of the positive charge is concentrated in a space called nucleus. It is proposed to be present at the center of the atom.

(c) A very small number (about 1 in 20,000) did not pass through the foil at all but suffered large deflections or even rebound.

 

(c) A very small number of the -particles suffered strong deflections or even rebound on their path indicating that the nucleus is rigid and -particles recoil due to direct collision with the positively charged heavy mass.

 

 

 

 

 

According to this model:

  • The positive charge and most of the mass of the atom was densely concentrated in extremely small region. This very small portion of the atom was called nucleus by Rutherford.
  • The nucleus is surrounded by electrons that move around the nucleus with a very high speed in circular paths called orbits. Thus, Rutherford's model of atom resembles the solar system in which the nucleus plays the role of sun and the electrons that of revolving planets.
  • Electrons and the nucleus are held together by electrostatic forces of attraction.

 Limitations of Rutherford's Atomic Model:

  • An electron in the nuclear model describing planet like orbits is under acceleration. According to the electromagnetic theory of Maxwell, charged particles when accelerated should emit electromagnetic radiation. Therefore, an electron in an orbit will emit radiation, the energy carried by radiation comes from electronic motion. The orbit will thus continue to shrink. Thus, the Rutherford model cannot explain the stability of an atom.

 

  • This model of an atom fails to explain the distribution of electrons in different orbit around the nucleus.
  • According to Rutherford's model of an atom, the atomic spectrum should be continuous. But atomic spectrum is found to be discontinuous. Rutherford model fails to explain the discontinuity of the atomic spectrum.
  • This model also fails to explain the line spectra of atoms, which show discrete lines, each line corresponds to a fixed frequency.

What are nucleons? –  Protons and Neutrons are collectively called as Nucleons.

 

Bohr's Model of an Atom

 

Bohr suggested that –

  • Electrons spin around the nucleus in an individualized separate path or unattached orbit.
  • The electrons do not emit any energy while moving Indies special orbits.
  • These orbits are also called as Energy Levels.
  • They are represented using letters or numbers as shown in the figure below –