**HEISENBERG’S UNCERTAINTY PRINCIPLE**

The position and the velocity of the bodies which we come across in our day-to-day life can be determined precisely at a particular moment of time. Hence, the paths or trajectories of such bodies can be predicted. However, in the case of small particles such as electrons it is impossible to determine simultaneously its position and velocity at a given instant with absolute certainty. Hence, it is not possible to talk of trajectory of an electron. Werner Heisenberg (1927) put forward this fact in the form of a well-known principle, called Heisenberg’s uncertainty principle. This principle, which is a direct consequence of the dual nature of matter and radiation, states that It is impossible to measure simultaneously both the position and velocity (or momentum) of a microscopic particle with absolute accuracy or certainty Heisenberg’s principle rules out the concept of definite paths or trajectories of electrons in an atom as proposed by Bohr. In order to define the trajectory of an object one must know the location and velocity of the object at various moments. Since for a subatomic particle such as an electron, it is not possible to simultaneously determine the position and velocity with absolute accuracy, it is not possible to talk of the trajectory of an electron.

Instead it is only possible to predict the probable region in a given space where we can find electron. Thus, Heisenberg replaced the concept of definite orbits by the concept of probability.