**Heisenberg’s uncertainty principle:** It is impossible to find out simultaneously the exact position and exact velocity of a subatomic particle like electron since electron is very light and tiny particle. To observe an electron, a radiation of very short wavelength (i.e., of high energy) must be allowed to fall on it. The moment such a high energy radiation falls on an electron, its position is changed.

**Uncertainty principle changed Bohr’s concept of stationary orbits**: As the position and velocity of an electron cannot be measured simultaneously, it is not possible to assign a path or an orbit to an electron.

**Orbital:** is defined as three dimensional space around the nucleus of an atom where there is a maximum probability of finding an electron.