question_answer 20)

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

Answer:

1. J.J. Thomson's model of atom	(i)  An atom consists of a sphere of positive charge in which electrons are embedded just like seeds in the water melon (ii) Total positive charge on the sphere is equal to the total negative charge present on the electrons so that atom as a whole is electrically neutral,                (iii) It could not explain the results of Rutherford's scatter experiments.
2. Rutherford's model of atom	(i) An atom consists of a small positively charged nucleus in the centre and the electrons are revolving around it. (ii) There is very large empty space between the nucleus and the electrons. (iii) All the mass of the atom is mainly concentrated in the nucleus.                                       (iv) It could not explain the stability of the atom because the revolving electron will be accelerated towards nucleus. Hence it will lose energy. Its orbit will become smaller and smaller and ultimately the electron will fall into the nucleus.
3. Bohr's model of atom	The main points of this model of atom (called postulates of Bohr's model of atom) are as follows: (i) An atom consists of a small heavy positively charged nucleus in the centre and the electrons revolve around it in 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, also called energy levels or shells. The term shell is used to indicate that atom is three-dimensional i.e., it is not like a plate but is like a ball. As energy of the orbits is fixed these are also called stationary states. These are numbered as 1, 1, 3, 4, etc. as we move outwards from the nucleus or they are represented by the letters K, L, M, N etc. as shown in Fig. The energy of these shells increases as we move outward from the nucleus. Thus, representing the energies of 1st, 2nd, 3rd, 4th, shell etc. by  etc., we have :   However, the gap between the successive energy shells decreases as we move outwards from the nucleus. Circular orbits or energy levels or shells around 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 the atom with lowest energy is called ground state of the atom. (iv) Energy is lost or gained by an electron only when it jumps from one orbit to the other. 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 the excited state. In the excited state, the atom is not stable. It loses or emits energy and jumps back to some inner energy level. In other words, an electron jumps from inner shell to outer shell by absorbing energy whereas energy is emitted when an electron jumps from an outer shell to an inner.