In an atom or ion, the allowed transitions between energy levels are governed by the principles of quantum mechanics. These transitions correspond to the absorption or emission of photons (electromagnetic radiation) when an electron moves between different energy levels.
The energy levels of an atom are quantized, meaning that electrons can only occupy specific energy states. The energy levels are often represented by quantum numbers, such as the principal quantum number (n), azimuthal quantum number (l), magnetic quantum number (m), and spin quantum number (s).
The most common and well-known model for understanding the energy levels of atoms is the Bohr model, which describes electron transitions in hydrogen-like atoms. According to the Bohr model, electrons occupy discrete energy levels designated by the principal quantum number (n). The energy of these levels increases as n increases, and the spacing between energy levels decreases as the distance from the nucleus increases.
When an electron transitions between energy levels, it can absorb or emit energy in the form of a photon. The energy of the absorbed or emitted photon corresponds to the energy difference between the initial and final energy levels of the electron. The relationship between the energy of a photon (E) and its frequency (ν) is given by the equation E = hν, where h is Planck's constant.
The allowed transitions between energy levels depend on the specific electronic configuration of the atom or ion. The selection rules dictate which transitions are allowed or forbidden based on the conservation of various quantum numbers, such as angular momentum and parity.
For example, in hydrogen-like atoms, the selection rules state that the change in the principal quantum number (Δn) must be ±1 for an allowed transition. Additionally, the change in the azimuthal quantum number (Δl) must be ±1, and the change in the magnetic quantum number (Δm) must be 0 or ±1.
However, it is important to note that these selection rules are simplified representations and are not universally applicable to all atoms and ions. More complex atoms have more intricate energy level structures, and the selection rules become more involved.
In summary, the allowed transitions between energy levels in an atom or ion involve the absorption or emission of photons, and these transitions are determined by the specific electronic configuration and selection rules associated with the atom or ion.