Energy is the fundamental currency of the universe, and it governs the behavior of all matter, including the atoms that make up every substance. Within an atom, the electrons orbiting the nucleus are not free to possess any amount of energy but are instead confined to specific, discrete energy levels. These fixed energy states determine how an atom interacts with external energy sources, such as light or heat.
What Defines Ground and Excited States
The concept of energy states is rooted in the quantum nature of matter, where energy is “quantized,” meaning it comes in fixed packets. For an atom, the ground state represents the most stable configuration—the lowest possible energy level for all of its electrons. This is the default arrangement an atom assumes when no external energy is being supplied.
Electrons are confined to specific orbits or shells around the nucleus, corresponding to discrete energy levels. An analogy for this is a staircase, where an electron can stand only on a step. The ground state is the lowest step, closest to the nucleus where the electron is most tightly bound.
The excited state refers to any higher energy level an electron occupies. If an electron moves away from the nucleus to a shell farther out, the atom has absorbed energy and is now in an excited state. Since multiple higher energy levels exist, an atom can have a first, second, or third excited state, depending on the energy gained. The excited state is inherently unstable because it possesses excess energy compared to its most natural arrangement.
How Atoms Change Energy Levels
The transition between the ground and excited states depends on the exchange of energy. To move an electron from the ground state to an excited state, a process called excitation must occur, requiring the atom to absorb a precise amount of energy. This energy is often supplied by a photon (a particle of light) or by collisions caused by heat or an electrical current.
The energy of the incoming photon must exactly match the energy difference between the initial and target energy levels. If the photon’s energy is slightly too much or too little, the electron cannot make the jump and the energy is not absorbed. Once an electron is in a higher, excited state, the atom is unstable and will instantly seek to return to its ground state.
The return to a lower energy level is called de-excitation or relaxation, which involves the release of the excess energy that was previously absorbed. This energy is typically released as a new photon, which is the mechanism that creates light. The energy of the emitted photon is precisely equal to the difference in energy between the two levels the electron jumps between. This de-excitation process occurs extremely rapidly.
Observing State Transitions in Everyday Life
The process of de-excitation is directly responsible for many phenomena involving light and color. For instance, the vivid colors produced by a neon sign or a firework display result from excited electrons returning to their ground state. Different elements have unique sets of energy levels, meaning their electrons release photons of specific energies that correspond to distinct colors of visible light.
Another application is fluorescence, where a material absorbs a high-energy photon, typically from the ultraviolet range, and then immediately re-emits the energy as a lower-energy photon in the visible light spectrum. The electron jumps to a high excited state upon absorption, loses a small amount of energy as heat, and then drops back down. This results in the emission of a longer-wavelength, visible light photon, which is why certain objects glow under a black light.
Lasers rely on the precise control of these energy states to produce their coherent, intense beam of light. A laser medium is engineered to maintain a condition called population inversion, where a majority of the atoms are forcefully kept in an excited state rather than the normally populated ground state. When a single photon stimulates an excited atom to de-excite, it releases a second, identical photon that is perfectly in-phase with the first, creating a cascade of identical light particles that form the laser beam.