Why energy level are also know as stationary state ?

Explanation

### Step-by-Step Solution:

1. **Understanding Bohr's Theory**: According to Bohr's theory of the atom, electrons move in fixed circular paths around the nucleus, which are referred to as orbits or energy levels.

2. **Fixed Energy Levels**: In these orbits, the electrons have specific energy values. These energy levels are quantized, meaning that electrons can only occupy certain allowed energy states.

3. **No Energy Loss or Gain**: While an electron is in a particular orbit, it does not lose or gain energy. This is because the electron is in a stable state, where the forces acting on it (the attraction to the nucleus and the centrifugal force due to its motion) are balanced.

4. **Constant Energy**: Since the energy of the electron remains constant as long as it stays in its orbit, these energy levels are referred to as stationary states. The term "stationary" implies that there is no change in energy for the electron in that state.

5. **Terminology**: Therefore, these fixed energy levels can also be called stationary orbits, stationary shells, or stationary energy levels, all of which emphasize the stability and constancy of the energy of the electrons in those states.

### Summary:
Energy levels are known as stationary states because, according to Bohr's theory, electrons in these levels do not lose or gain energy while they revolve around the nucleus, resulting in a constant energy state.