Compare all the proposed models of an atom given in this chapter.

A comparison between the models of an atom proposed by J.J. Thomson, Rutherford and Bohr is given below:

Thomson's model of an atom

J,J Thomson proposed the model of an atom in which the atom consisted of a positively charged sphere. The electrons are embedded in the sphere. The negative and the positive charges are equal in magnitude. Hence, the atom is electrically neutral.

Limitation J.J. Thomson's model of the atom could not explain the results of alpha particle scattering experiment carried out by Rutherford. According to Thomson’s model atom was considered as a sphere which was positively charged and electrons were embedded in it. It could not explain why most of the positively charged alpha particles passed straight through the gold foil. It also failed to explain the deflections and rebounding of the alpha particles . J. J. Thomson's model of the atom was just based on imagination. It did not have any experimental evidence to support.

Rutherford's model of an atom

Rutherford put forward the model of an atom which had a positively charged center called nucleus. It also had the electrons which revolved around the nucleus in well defined orbits. He also proposed that the size of the nucleus is very small as compared to the size of the atom and nearly all the mass of an atom is centred in the nucleus. From the data he also calculated that the radius of the nucleus is about 105 times less than the radius of the atom.

Limitation. Although it has a few drawbacks as well. According to this model the atom would not be stable. This phenomenon can be explained as follows:

According to this model, the orbital revolution of the electron is not expected to be stable. Any particle which is in a circular orbit would undergo an acceleration. Charged particles would radiate energy during acceleration. Electrons are negatively charged. Thus, the revolving electron would lose energy. It will finally fall into the nucleus. If this was the situation, then the atom should be highly unstable. Hence, matter would not exist in the form that we know.

Bohr's model of an atom

According to Bohr’s model of an atom:

(i) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.

(ii) While revolving in discrete orbits the electrons do not radiate energy.

These orbits are called energy levels. Energy levels in an atom are shown by circles.

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

Further, the distribution of electrons into different orbits of an atom was suggested by Bohr and Bury. The following rules are the rules followed for writing the number of electrons in different energy levels or shells: (i) The maximum number of electrons present in a shell is given by the formula 2n², where ‘n’ is the orbit number or energy level index, 1,2,3,…. Hence the maximum number of electrons in different shells can be written as : first orbit or K-shell will be = 2 × 12 = 2, second orbit or L-shell will be = 2 × 22 = 8, third orbit or M-shell will be = 2 × 32 = 18, fourth orbit or N-shell will be = 2 × 42= 32, and so on. The maximum number of electrons that can be accommodated in the outermost orbit is 8. Electrons are not accommodated in a given shell, unless the inner shells are filled. That is, the shells are filled in a stepwise manner.

Advantage. Bohr's model of an atom explains the stability of atoms by saying that there is no loss in the energy of revolving electrons of the atom as long as they keep on revolving in the same energy level due to which the atoms remain stable