Electrons in an atom occupy different energy levels or orbitals. The lowest energy level that an electron can occupy is known as the ground state. However, electrons can exist in higher energy levels, referred to as excited states. There are several reasons why electrons may not be in their ground state:
Absorption of energy: Electrons can absorb energy from various sources, such as photons or thermal energy. When an electron absorbs energy, it moves to a higher energy level or an excited state. This process is commonly observed in spectroscopy when atoms or molecules absorb specific wavelengths of light, causing their electrons to transition to higher energy levels.
Excitation through collisions: Electrons can gain energy through collisions with other particles. For example, in high-temperature environments or during high-energy events, collisions can impart energy to electrons, promoting them to higher energy levels.
Electron promotion during chemical reactions: In some chemical reactions, electrons are transferred between atoms or molecules. During these processes, electrons can move to higher energy levels in the products compared to the reactants.
Thermal fluctuations: Even in the absence of external energy sources, electrons can exhibit temporary excursions to higher energy levels due to thermal fluctuations. These fluctuations arise from the probabilistic nature of electron behavior described by quantum mechanics.
However, it's important to note that excited states are generally unstable, and electrons tend to return to lower energy levels by emitting energy in the form of photons. This emission can result in the characteristic spectral lines observed in atomic and molecular spectra. The stable configuration for electrons in an atom is the ground state, where they occupy the lowest available energy levels consistent with the Pauli exclusion principle and Hund's rule.