** BOHR’ S ATOMIC THEORY: **In 1913, NEIL BOHR presented another model of an atom which was based on MAX PLANK’S QUANTUM THEORY. According to BOHR ‘S model , revolving electron in an atom does not absorb or emit energy continuously. The energy of a revolving electron is Quantized as it revolves only in orbits of fixed energy, called “energy levels” by him.

** POSTULATES OF BOHR’S ATOMIC MODEL: **Bohr’s atomic mode was based upon the following main postulates.

- The hydrogen atom consists of a tiny nucleus and electrons are revolving in one of the circular orbits of radius “r” around the nucleus.
- Each orbit has a fixed energy that is quantized.
- As long as electron remains in a particular orbit, It does not radiate or absorb energy. The energy is emitted or absorbed only when an electron jumps from one orbit to another.
- When an electron jumps from lower orbit to higher orbit ,it absorbs energy and when it jumps from higher orbit to lower orbit it radiates energy. This change in energy (∆E)is given by plank’s equation.

∆E = E2 – E1 = hv

Where h= Plank’s constant=6.63 × 10-34Js and V= frequency of light E2= energy of higher orbit E1= energy of lower orbit and ∆E = change in energy.

- Electron can revolve only in orbits of a fixed angular momentum that is mvr = nh/2π Where n= quantum number or orbit number and n= 1,2,3, m=mass of electron r= radius of orbit v= velocity of electron.

** For your information**:

__Neil Bohr__was a Danish physicist who joined Rutherford in 1912 for his post doctoral research. In 1913, Bohr presented his atomic model based upon Quantum theory. He won the 1922 Noble Prize for Physics for his work on the structure of an atom.

** (B) ENERGY OF THE PAST MOVING ELECTRONS IS QUANTIZED OR ANGULAR MOMENTUM OF MOVING ELECTRONS IS QUANTIZED**:

According to Bohr postulate of atomic model, angular momentum of electrons is quantized. Quantum means fixed energy. It shows that amount of energy of each and every electron in a specific shell is fixed and remains constant as long as electrons remains in a particular orbit.

** Formula to calculate angular momentum of electrons in orbits (shells): **The formula used to find the angular momentum of moving electrons in orbits is as follows.

mvr = nh/2π.

In the formula “n” shows the number of shell in which electrons revolve. For first orbit it is 1, for second orbit it is 2, for third orbit it 3, and so on.

- For first orbit angular momentum may be given as:

mvr = nh/2π.

mvr = 1× 6.63 ×10^{ -34} JS/2×3.14

mvr = 3.315 ×10-^{34} / 3.14

mvr = 1.0 ×10-^{34 }JS (kg.m^{2}.S^{-1})

- For second orbit angular momentum may be given as:

mvr = nh/2π.

mvr = 2× 6.63 ×10^{ -34} JS/2×3.14

mvr = 2.12 ×10^{ -34} JS

- For Third orbit angular momentum may be given as:

mvr = nh/2π.

mvr = 3× 6.63 ×10^{ -34} JS/2×3.14

mvr = 3.0 × 10^{ -34} JS