What happens when an electron returns to its ground state from its excited state

Energy is released by the electron in the form of light

Explanation:

Let's start by explaining that each atom in its natural state has a specific structure of its energy levels. Where the lowest energy level is called the ground state.  

So, when an atom is in its ground state, its electrons fill the lower energy orbitals completely before they begin to occupy higher energy orbitals.

Then, when an atom is excited, it means that it has left its ground state (in which each electron occupies its place in its orbit, around the nucleus), when this happens some electron jumps out of the orbit it occupied in its fundamental state to an outer orbit, further away from the nucleus and then return to the ground state, emitting in the form of light the energy received.  

To understand it better:  

An excited electron is an unstable electron, and when passing from one orbit to another, it produces light of a specific wavelength (color) that depends on the amount of energy the electron loses.

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