Question

What do you understand about stationary states?

Answer 1

Hint: A negative electron orbits an extremely tiny positive nucleus of an atom. Under the influence of the electric force of attraction between the electron and the positively charged nucleus, the electron travels in circular orbits around the nucleus.

Complete answer:
Let us learn about Bohr's Model of Atom:
Only certain orbits are stable, hence electrons can only adopt specific orbits. When an electron is in this orbit, it is considered to be stationary, and it does not produce energy in the form of radiation. (In Rutherford's model, electrons accelerate, generate electromagnetic radiation, lose energy, and spiral down to the nucleus, generating electromagnetic radiation.)
Only stationary states (energy levels/shells) are possible for atoms. A certain collection of electron orbits around the nucleus corresponds to each stationary condition. These states can be numbered as n = 1,2,3,4,..., where n is the quantum number.
Each stationary state has an energy $E_n$ that is distinct and well-defined. The energy of atoms are quantized. An atom's stationary states are ranked from lowest to highest energy:
$E_1 < E_2 < E_3 < E_4$
The ground state of an atom is the lowest energy state (most stable) of an atom with energy$E_1$. Excited states of the atom are other stationary states with energy $E_1,E_2,E_3$ and so on.
Before it can be absorbed, an atom can "jump" from one stationary state to another by producing or absorbing a photon with energy equal to the energy difference between two levels. It will not be absorbed if this is not done.

Note:
An atom can also go from a lower to a higher energy state by absorbing energy from an electron or another atom in an inelastic collision. Collisional excitation is the term for this phenomenon. The energy imparted by colliding particles is equal to the difference in energy states, plus any extra energy. Colliding particles can provide the most energy to assist the orbital electron in reaching the highest energy state feasible. Atoms will seek the state with the least amount of energy. If left alone, an excited atom will leap to lower and lower energy levels until it reaches the ground state.