An atom is in a ground state when all of the electrons in an atom are at their lowest energy levels. In an excited state, electrons spread out to higher energy levels, and not all are in their lowest levels.
A ground state atom possesses electrons in its lowest energy orbitals. This state has the lowest potential energy and is more stable than an atom in an excited state. An example of an atom that has electrons in its ground state is hydrogen. Hydrogen has two electrons filling its first potential energy level.
In an excited state, electrons do not fill their lowest energy orbitals. Molecules and atoms can obtain outside energy, resulting in a shift of an electron to a highest energy orbital. Excited state electrons are less stable than those in ground state and have more than minimum potential energy. When atoms are not in their ground state, they may revert to it, giving off energy as they return to the lower energy state.
An example of electrons going from a ground state to an excited state is a photochemical reaction. These reactions occur when energy in the form of light is absorbed by molecules. Electrons in the molecules jump orbitals, creating an excited state that allows molecules to change structures or combine with others.