Atomic models - Thomson's and Rutherford's atom model

 Atom Model

Man had always been curious to know the details of the constituent of matter. Structure of atom, which is the smallest particle of matter which takes part in a chemical reaction has always been one of the main targets of scientists. 

     An explanation regarding the structure of atom is called an atom model. 

     Faraday, during this experiments on electrolysis showed that each atom, irrespective of the nature of the element gave up or received a fixed quantity of charge equal in magnitude to 1.59 × 10-19 C. This led to the electrical nature of matter. Every atom is electrically neutral and is found to be stable. Every atom model must be based upon these experimentally observed facts. 

Thomson's Atom Model

J.J. Thomson gave the first idea regarding structure of atom. The model is known after him as Thomson's atom model. According to this atom model whole of the positive charge is distributed uniformly in the form of a sphere. Negativity charged electrons are arranged within this sphere here and there (Fig. 1). The model is popularly known as plum-pudding model. Every electron is attached towards the center of uniformly charged sphere while they exert a force of repulsion upon each other. The electrons get themselves arranged in such a way that the forces of repulsion are exactly balanced. When disturbed, electrons vibrate to and fro within the atom and cause emission of visible, infrared and ultraviolet light. 

Fig. 1. Thomson's atom model. 

     Thomson's atom model satisfied the requirements of the atom and the demands of electromagnetic theory. According to this model, hydrogen can give rise to a single spectral line. Experimentally, hydrogen is found to give several series, each series consisting of several lines. This indicated that, "Thomson's atom model needed magnifications". 

Rutherford's Experiment

Rutherford performed experiments on the scattering of alpha particles by extremely thin metal foils. 

     A radioactive source (radon) of ∝-particles was placed in a lead box having a narrow opening (Fig. 2). This source emits ∝-particles in all possible directions. However, only a narrow beam of alpha particles emerged from the lead box, the rest being absorbed by the lead box. This beam of ∝-particles is made incident on a gold foil whose thickness is only one micron, i.e., 10-6 m. When passing through the metal foil, the ∝-particles get scattered through different angles. These particles fall on a fluorescent screen producing a tiny flash of light on the screen. This can be easily viewed by a low power microscope in a dark room. 

Fig. 2. Rutherford's ∝-scattering.

     In 1913, Geiger and Marsden performed a more sensitive experiment on the scattering of ∝-particles on the guidelines suggested by Rutherford. This experiment is described below :

     Apparatus consists of an air tight chamber 'C' which can be evacuated by a tune 'T' (Fig. 3). The chamber is capable of rotation inside a jacket 'R' about a vertical axis. The source 'S' of ∝-particles, radon, is placed inside a lead cavity 'L'. ∝-particles, after coming out of the narrow opening in the lead cavity strike a thin foil 'F'. This foil is made of some metal of high atomic weight like gold, silver or platinum. The foil is placed at the centre of chamber 'C'. Scattered ∝-particles are viewed through low power microscope 'M', which is provided with a fluorescent screen. As the chamber rotates about a central axis, the microscope M rotates along with it. But the cavity 'L' and foil 'F' remain fixed with the tube. 

Fig. 3. Experimental set up for study of ∝-scattering.

     The ∝-particles on striking the atom of the toilet scattered in different directions. By rotating the chamber, the number of particles scattered along different directions can be recorded by observing the scintillations on the fluorescent screen. 

     The above experiment gave the following results :

     (i) Most of the ∝-particles either passed straight through the metal foil or suffered only small directions. This could be explained by Thomson's atom model. 

    (ii) A few particles were deflected through angles which were less than or equal to 90°.

   (iii) Very few particles were deflected through angles greater than 90°. It was observed that only 1 in 8000 particles was found to be deflected greater than 90°. Sometimes a particle was found to be deflected through 180°. In other words, it was sent back in the same direction from where it came. The large angle of scattering came as a greater surprise. It could not be explained by Thomson's atom model. It was one of the main reasons for rejecting Thomson's atom model. 

    (iv) If 'Φ' is the angle made by a scattered particle with its original direction of motion and 'N' is the number of particles available in that direction, it was found that, 

    (v) If 't' is the thickness of the foil and 'N' is the number of ∝-particles scattered in a particular direction (Φ = constant) it was observed that

                    N/t = constant. 

Conclusions

     (i) The fact that most of the ∝-particles passed undeviated led to the conclusion that an atom has a lot of empty space in it. 

    (ii) ∝-particles are heavy particles having high initial speeds. These could be deflected through large angles only by a strong electrical force. This led Rutherford to the conclusion that whole of positive charge and nearly the entire mass of the atom were concentrated in a tiny central core. Rutherford named this core as nucleus. 

   (iii) The difference in deflection of various particles can be explained as follows :

     ∝-particles which pass at greater distances away from the nucleus shown as A and A' in (Fig. 4)  suffer a small deflection due to smaller repulsion exerted by the nucleus upon them. The particles like B and B' which pass close to the nucleus experiences a comparatively greater force and hence get deflected through greater angles. A particle 'C' which travels directly towards the nucleus is first slowed down by the repulsive force. Such a particle finally stops and then is repelled along the direction of its approach. Thus, it gets repelled back after suffering a deviation of 180°.

Fig. 4. Different deviations for different ∝-particles.

Rutherford's atom model

On the basis of conclusions drawn from Rutherford's experiment, a new atom model was proposed. This atom model known as Rutherford's atom model has following characteristics : 

     (i) An atom consists of equal amounts of positive and negative charge so that atom as a whole is electrically neutral. 

    (ii) The whole of positive charge of the atom and practically whole of its mass is concentrated in a small region which forms the core of the atom called nucleus. 

   (iii) The negative charge, which is contained in the atom of electrons distributed all around the nucleus but separated from it. 

   (iv) In order to explain the stability of electron at a certain distance from the nucleus, it was proposed by 'Rutherford' that the electron resolve round the nucleus in circular orbits. The electrostatic force of attraction between the nucleus and the electron provides the centripetal force. 

    (v) The nuclear diameter is of the order of 10-14 metres. This can be shown as follows :

     Let an ∝-particles having velocity 'v' approaches a nucleus (head-on) having a charge '+Ze' upon it (Fig. 5). 

Fig. 5. An ∝-particle approaching the nucleus. 

     The velocity of ∝-particle decreases till it comes to rest at a distance 'ro' from the nucleus. It is then repelled back along the direction of approach 'ro' gives the radius of nucleus. 

         Initial K.E. of ∝-particle = 1/2 (mv²) 

         Initial P.E. of ∝-particle = 0

              [∝-particle is supposed to start from infinity]

        Final K.E. of ∝-particle = 0

        Final P.E. of ∝-particle = 1/4πε₀ × q₁q₂/r₀

     According to law of conservation of energy, 

          1/2 (mv²) = 1/4πε₀ × q₁q₂/r₀

     ∴                r₀ = 2/4πε₀ × q₁q₂/mv²

     For ∝-particle,  q₁ = 2e, q₂ = Ze

     ∴               r₀ = 2/4πε₀ × 2Ze²/mv²

                          = 4/4πε₀ × Ze²/mv²

                          = 4 × 9 × 109 Ze²/mv²

     In one of the experiments, ∝-particles of velocity 2 × 107 ms-1 were bombarded upon gold foil (Z = 79). 

     Here 'Z' = 79, e = 1.59 × 10-19 C 

         m = 4 × 1.67 × 10-27 kg, v = 2 × 107 ms-1

r0 = 4 × 9 × 109 × [79 × (1.59 × 10-19)²/4 × 1.67 × 10-27 × ( 2 × 107)²]

  

 = 2.69 × 10-14 m

     This gives the radius of nucleus. 

Failure of Rutherford's atom model

     (i) According to electromagnetic theory, a charged particle in accelerated motion must radiate energy in the form of electromagnetic radiation. As the electron revolves in a circular orbit, it is constantly subjected to centripetal acceleration v²/r. So it must radiate energy continuously. As a result of this, there should be a gradual decrease in the energy of electron. The electron should follow a spiral path and ultimately fall into the nucleus (Fig. 6). Thus, the whole atomic structure should collapse. This is contrary to the actual fact that atom is very stable. 

Fig. 6. Electron spiralling inwards. 

    (ii) According to Rutherford's model, electrons can revolve in any orbit. If so, it must emit continuous radiations of all frequencies. But atoms emits spectral lines of only definite frequencies. 

Key Words

1. ∝-particles. Helium nucleus

2. Atom model. An explanation to the structure of atom. 

3. Atom. Smallest particle of matter which can take part in a chemical reaction. 

4. Fluorescent screen. A screen coated with such a material which causes glow as charged particle strike against it.