Question

why an electron have constant energy when it revolve around nucleus in an orbit

Answer 1

This postulate was introduced by Bohr to explain the physics of the atom. Thus, the problem was resolved electron fall into the nucleus. According to the laws of classical physics electron could have any energy or speed. In this case, moving on orbit of the nucleus he had to radiate energy. Eventually, he would have lost their energy in a fraction of a second and would have fallen into the nucleus. This suggested that the atom must be unstable. But experiments showed that the atoms are living much longer. Therefore, Bohr introduced the stationary orbits with constant energy for the electrons in the atom.