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

J. J Thomsons model of atom

(i) An atom consists of a sphere of positive charge in which electrons are embedded just like seeds in watermelon.

(ii) Total positive charge on the sphere is equal to the total negative charge present on the electrons. So that atom as a whole isn't electrically neutral.

(iii) It could not explain the results of Rutherford scattering experiment.

Rutherford scattering experiment

(i) An atom consists of small positively charged nucleus in the centre and electrons revolving around it.

(ii) There is very large empty space between the nucleus and the electrons.

(iii) All the mass of the atom is concentrated in the nucleus.

(iv) It could not explain the stability of the atom because the revolving electron will be accelerated towards the nucleus. Hence it will lose energy. It's orbit will become smaller and smaller and ultimately the electron will fall into the nucleus.

Bhor's model of atom

(i) An atom consists of small heavily positively charge nucleus in the centre and the electrons revolve around it in the circular paths called orbits.

(ii) In a particular atom, the orbits in which the electrons revolve are the discrete orbits having fixed radii and energy. These discrete orbits are therefore called energy levels or shells. The term shell is used to indicate that the atom is three dimensional ie, it is not like a plate but is like a ball. As the energy of the orbits is fixed these are also called stationary states. These are numbered as 1,1,3,4 etc as we move towards from the nucleus are they are represented by the letters K, L, M, N, etc. The energy of these shells increases as we moves outward from the nucleus.

(iii) As long as an electron is revolving in a particular orbit,it can neither lose energy nor gain energy. Thus, the atom is stable and does not collapse. This state of atom with the lowest energy is called ground state of atom.

(iv) Energy is lost or gained by an electron only when it jumps from one orbit to another. The energy falls on an electron and it absorbs this energy, it will jump to some outer shell. The atom is then said to be in exicted state, the atom is not stable. It loses or emits energy and jumps back to some inner energy level.