Question

A)explain all the postulates of Bohr's atomic model.(short points).

B) explain draw backs of Rutherford's model of atom.(short points).

Answer 1

A) The important postulates in his theory are,

1. Electrons revolve round the nucleus with definite velocities in concentric circular orbits situated at definite distances from the nucleus. The energy of an electron in a certain orbit remains constant. As long as it remains in that orbit, it neither emits nor absorbs energy. These are termed stationary states or main energy states.
2. Bohr proposed that the angular momentum of an electron is quantized. Thus, the motion of an electron is restricted to those orbits where its angular momentum is an integral multiple of h2πh2π, where h is Planck’s constant.
3. Thus we have the relationship mvr = nh2πnh2π, where m is mass of electron, v is velocity of electron of said orbit, r is radius of that orbit, n is a simple integer.
4. The stationary states or allowed energy levels are only those where n = 1, 2, 3, …… This is called Bohr quantum condition.
5. The energy of an electron changes only when it moves from one orbit to another. An electronic transition from an inner orbit to outer orbit involves absorption of energy. Similarly, when an electron jumps from an outer orbit to inner orbit it releases energy, which is equal to the difference between the two energy levels.
6. The energy thus released in the form of a radiation of a certain frequency appears in the form a line in the atomic spectrum. If the energy of an electron in the outer orbit (n2) is E2 and energy of electron in the inner orbit (n1) is E1 then E2 - E1 = ΔE = hνν.
7. The value of n could be small integers 1, 2, 3 and these correspond to the first, second, third, and so on. Quantum states are shells for the electron; n is termed as principal quantum number.
8. Based on the Bohr theory Bohr calculated the radii of the various orbits and the energies associated with the electrons present in those shells.

B)
Rutherford atomic model failed to explain about the stability of electrons in a circular path.

As per Rutherford’s model, electrons revolve around the nucleus in a circular path. But particles that are in motion on a circular path would undergo acceleration, and acceleration causes radiation of energy by charged particles. Eventually, electrons should lose energy and fall into the nucleus. And this points to the instability of atom. But this is not possible because atoms are stable. Hence, Rutherford failed to give an explanation on account of this.