As previously discussed, atoms have a nucleus made of protons and neutrons surrounded by an electron cloud. An electron cloud is a way to describe where the electrons might be at any one time. Electrons aren’t stuck in certain spot of space relative to the nucleus. Instead, they tend to fill the shape of orbitals, which depends on the atom itself. Within these orbitals, an electron follows an orbit, or a path, around the atom, that is contained within the orbital shape. An atom can have different orbitals occupied by electrons. To get to each orbital requires a different amount of energy applied to the electron. That means that an electron at a high-energy-access orbital has received a lot of energy to get there.
When an electron is in its ground state, it’s in its orbital of lowest energy. Particles like atoms and electron tend to exist in low energy states. To exist in a high-energy state 1) requires energy that may not be available, and 2) tends to be unstable as that energy is likely to move to something else. If enough energy is applied to an electron, it may be repelled away from the other electrons in the lower energy orbitals and move to an orbital that requires the newly-gained energy for access. This concept can be a little tricky. Orbitals aren’t their own entity waiting around for an electron to get enough energy to access them. Instead, when an electron gains energy, it gains enough energy to push against the negatively charged electron’s pull from the positively charged nucleus. To come back “down” to a lower energy state, the electron can release, or emit, the energy.