Question

Why did Heisenberg replace the concept of definite orbits by the concept of probability?

Answer 1

Because an electron in an atom has both velocity and some specific coordinates in the 3 dimensional space in the atom. It is experimentally verified that one cannot calculate both the velocity and the the position coordinate of the electron . Hence the values obtained are very inaccurate and uncertain using Bohr's model of the atom. Hence he replaced the concept of orbits