Question

Write the postulates of Bohr's model of an atom ?

Answer 1

According to Bohr's theory , electrons revolve in definite circular orbits around the nucleus and these orbits are designated by the letters K, L, M, N or by the numbers 1, 2 ,3, 4 and so on.

As long as the electrons revolve in a particular orbit they can neither gain energy nor loose energy. Such orbits are called as stationary orbits or stationary states.

When an electron jumps from an higher orbit to a lower orbit it releases energy in the form of raldiations and when an electron jumps from lower orbit to a higher orbit it absorbs energy in the form of raldiations.

The angular momentum of aln electron is an integral multiple of h/ 2π mvr = nh/ 2π

It explains the stability of an atom and also the line spectrum of hydrogen atom

Answer 2

$\bold{ \underline{ \underline{BO HR ' S \: MODEL \: OF \: ATOM :-}}}$

=> TO , overcome the drawback of Rutherford's model of atom and to exaplain the line spectrum of Hydrogen Neil's Bohr a Danish physicist in 1913 proposed a new model of atom based upon Planck's Quantum Theory .This new model is called Bohr's Model of atom.

$\bold{ \underline{ \underline{POSTULATES :-}}}$

The main postulates of Bohr's Model of atom are as follows :-

● An atom consists of a small , heavy positively charged nucleus in the centre and the electrons revolve around it in circular orbit.

● In Bohr's Model of atom the word stationary does not mean that the Electrons are stationary but it means that the energy of the Electron revolving in a particular orbit is fixed and doesn't change with Time.

● The different energy levels are numbered as 1,2,3.....etc or designed as K,L,M,N,O,P .....etc starting from the shell closest to the nucleus

● Like energy ,the angular momentum of an electron in an atom can have certain definate values or discrete values and not any value of its own. The only permissible values of angular momentum are given by the following expression :-

$\bold{mvr = \frac{nh}{2\pi} }$
here m is the mass of the Electron, v is the tangential velocity of revolving electron ,r is the radius of the orbit ,h is the Planck's constant and n is the integer.

● When the electrons in an atom are in their Lowest (normal ) energy level or State ,they keep on revolving in Thier respective Orbits without losing energy beacause energy can neither be lost nor gained continuously . This state is called normal or ground state.

● Energy is emitted or absorbed Only when the electrons jump from one orbit to the . The amount of energy emitted or absorbed is given by the difference of energies of the two levels of energy levels concerned i.e

∆E= E2 - E1

Where E2 and E1 are the energies of the Electron in the higher and lower energy levels respectively and ∆E is the difference in engergies of the two levels.

● Energy is absorbed when electron jumps from a lower energy levels to some higher energy level.

● Energy is emitted when an electron jumps from a higher level to some lower energy level.