Every atom consists of a nucleus surrounded by electrons that exist in specific regions of space called shells or energy levels. These electrons are held near the nucleus by the attractive force between the negatively charged electrons and the positively charged protons within the nucleus. The energy of an electron is determined by which shell it occupies; shells closer to the nucleus have lower energy than those farther away.
Atoms constantly interact with their environment, often involving the transfer of energy. Energy exists in discrete, quantifiable packets called quanta, or photons when referring to light. This arrangement of electrons in distinct energy levels dictates how an atom can absorb and release energy.
Energy Absorption and Electron Excitation
When an atom encounters energy, such as a photon of light or heat, one of its electrons may absorb that energy and move to a higher energy level. This process is termed excitation, and the atom is then in an excited state. An electron in the lowest possible energy level is described as being in its ground state.
A fundamental principle governing this interaction is the quantization of energy, which means electrons can only exist at certain discrete energy values. Consequently, an atom can only absorb energy if the incoming photon possesses an amount of energy that precisely matches the difference between the electron’s current energy level and one of the higher, unoccupied levels.
If the energy of the incoming photon does not match one of these exact energy gaps, the photon will not be absorbed and will simply pass through the atom. When the correct energy is absorbed, the electron instantly jumps to the higher, less stable shell. The farther the electron jumps from the nucleus, the higher the energy it has absorbed.
The Release of Energy: Emission and Relaxation
The excited state achieved by the electron is temporary. An atom’s natural tendency is to return to its lower-energy ground state. This return to a lower energy level is called relaxation or de-excitation.
To move back down, the electron must release the excess energy it absorbed. This energy is most often released in the form of a photon, a particle of electromagnetic radiation. The energy of the emitted photon is exactly equal to the difference in energy between the higher and lower shells the electron transitioned between.
Because the energy levels within an atom are specific and unique to that element, the energy of the emitted photon is also specific. This energy determines the frequency and wavelength of the light, which in turn determines its color if the photon falls within the visible spectrum. Relaxation can also occur nonradiatively, such as through collisions with other atoms, which dissipates the energy as heat.
Practical Examples of Atomic Energy Transfer
A flame test uses heat to excite the electrons of metal ions, such as sodium or copper, in a sample. As these excited electrons relax, they emit light at specific wavelengths, producing the characteristic colors used to identify the metal, like the bright yellow-orange of sodium.
In a neon sign, electricity is passed through a low-pressure gas to excite the atoms. The electrons in the gas atoms are excited by collisions and then relax to emit light, creating the continuous glow. The distinct color of the light—red for neon, blue for argon—is a direct result of the unique energy level spacing in each element.
Fluorescent materials, such as those coating the inside of a fluorescent light bulb, also rely on this process. They absorb high-energy, invisible ultraviolet (UV) photons and then undergo a two-step relaxation process. The material releases the energy in the form of lower-energy visible light, effectively converting UV radiation into a visible glow.