Atomic energy levels

The atomic energy levels describe the energy level of electrons within an atom, occupying a specific orbital.

An electron can absorb energy to transfer to an orbital further away from the nucleus, or emit (release) energy to transfer to an orbital closer to the nucleus.

Ground state
When all the electrons (e) in an atom are at their lowest energy level, they will occupy orbitals. No more energy can be emitted from the electrons, and the atom is said to be in its ground state.

Excited state
If we apply external energy to the electron (e), (for example, that of a photon which collides with the electron), the electron will increase its potential energy and "jump" to an orbital further away from the nucleus. If we keep on adding energy to the electron, it will eventually have enough energy to leave the atom completely, and this atom is now an ion.