Thomson’s model of an atom

Thomson proposed the model of an atom to be similar to that of the Christmas pudding. Thomson proposed that

  1. An atom consists of a positively charged sphere and electrons are embedded into it.
  2. The negative and positive charges are equal in magnitude so the atom as a whole is electrically neutral.

Drawbacks

Although Thomson’s model explained that atoms are electrically neutral but the result of the experiment carried out by other scientists like alpha particle scattering experiment could not be explained by Thomson’s model of atom and it also could not explain that there was empty space in atom so it failed.

Rutherford’s model of an atom

 Rutherford and his students in 1911 performed alpha particle scattering experiment in which they bombarded very thin gold foil with alpha particles.

Observations:-

  1. Most of the alpha particles passed straight without any deflection or change in path.
  2. Some of the alpha particles passed after deflecting at small angles.
  3. Out of 20,000 only one alpha particle suffered deflection from larges angle.

Conclusions :-

From above observations following conclusions were made:-

  1. Most of the space inside the atom is empty.
  2. Deflection of particles from small angles indicate the presence of positive charge in the nucleus (centre).
  3. Bouncing back of very less alpha particles from large angles indicates the presence of densely packed mass in the centre of atom.

On the basis of observations and conclusions, Rutherford proposed the nuclear model of atom according to this model,

  1. There is a positively charged spherical centre in an atom, called the nucleus. Nearly all the mass of an atom resides in the nucleus that is protons and neutrons are packed into it.
  2. The electrons revolve around the nucleus in very well-defined orbits. Most of the part of an atom is empty.
  3. The size of nucleus is very small as compare to the size of atom.

Drawbacks

According to the classical theory of electro dynamics, any charge or particle in a circular orbit would undergo acceleration and during centripetal acceleration the charged particle would lose the energy in the form of radiations and thus it will lose energy also and it will start moving in a spiral path which will keep on decreasing its radius and that electron will ultimately fall into the nucleus and the atom will collapse this shows that Atom is highly unstable, but we all know that atoms are quite stable and due to this reason the model was discarded.

Bohr’s Model of an atom

Neils Bohr in 1913 used the concept of quantisation to clear the objections on Rutherford‘s model of atom .

Here are the postulates of Bohr’s atomic model:-

1. The electrons continue revolving in their respective orbits without losing energy does each orbit is associated with the definite energy hence, it is also called energy level.

These orbits are represented by the letters K,L, M, N, . Or the numbers, n= 1,2,3,4,….

        2.  The electrons keep revolving in the same orbit as the centrifugal force of a revolving electron is counter balanced by the electrostatic force between the electron and the nucleus.

        3. The electrons are allowed to move in only those orbitals whose angular momentum (mvr) is a whole number multiple of h/2π. That is it is quantised. 

mvr = nh/2 π ; n= 1,2,3,4,…..

        4.The electron absorbs energy for jumping from lower energy orbital(shell) to Higher energy shell, and it loses the energy while coming from high energy orbital to the low energy orbital. ∆E = E2 – E1 = hc/λ

Drawbacks

        1. This model was unable to explain the bond formed by atoms to form molecules.

        2. It could not explain the splitting of spectral lines in magnetic and electric field.

        3.This model was able to explain the spectral lines of hydrogen atom only.

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