Momentum is a vector quantity which is a measure of the amount of motion when something moves. Consider a body of mass m, moving in a straight line with velocity v. Its momentum P is then

P=mv

Its unit is kg.m/s

 

Like linear momentum a body in a circular motion has angular momentum as well given by the cross product of the radius of the circular orbit and linear momentum.

L=r x P

Electron circling the nucleus has definite angular momentum as long as it revolves in the same orbit. For the quantization of energy levels, its angular momentum must be quantized.

Bohr says its

L=nh/2pi

In the next article we will see waht happens to angular momentum when electron jumps to different orbit.