Heisenberg uncertainty principle
According to Heisenberg uncertainty principle, it is impossible to determine momentum and position of an electron in an atom simultaneously.
If position is to determine its momentum will change and vice versa because light(energy) is to be dropped on an electron to track it, which will change the position of the electron.
Momentum (p) = mass(m) x Velocity(v)
Hence if its not possible to determine the position of an electron , how can we say that electron revolve around the nucleus in discrete orbits. We can only find the probability of electron in some space, called orbital.