In science, energy level refers to an allowed state of a quantum system corresponding to a particular amount of stored energy. [1] The term “energy level”, in basic definition, typically refers to the position of an electron in one of the layered “levels” of shells in the orbital structure of the atom, where each step higher (distally away) from the nucleus corresponds to a higher energy level or less stable position for the electron.

Molecules
Supposedly, not just electrons, but all nuclei and molecules have well-defined energy levels, such that molecules and molecular systems can only pass from one energy level to another, with no in-between state. [1] This is often referred to as the quantum leap. This would seem to have huge implications to that of the energy levels of human molecules.

History
The energy level model seems to stem from the 1913 article “On the Constitution of Atoms and Molecules” by Niels Bohr. [2] This was expanded on more American physical chemist Gilbert Lewis’ 1916 article “The Atom and the Molecule”, in which he discussed how “electrons may pass with readiness from one position in the outer shell to another”, also discussing this in the context of thermodynamic stability and chemical bonding. [3]

American physicist William Sidis, in 1920, stated that “the idea of the energy-level, of unavailable energy (or entropy)” was brought in by the second law of thermodynamics. [4]

References
1. Gribbin, John. (1998). Q is for Quantum: an Encyclopedia of Particle Physics. Touchstone.
2. (a) Bohr, Niels. (1913). "On the Constitution of Atoms and Molecules, Part I". Philosophical Magazine 26: 1–24.
(b) Bohr, Niels. (1913). "On the Constitution of Atoms and Molecules, Part II Systems Containing Only a Single Nucleus". Philosophical Magazine 26: 476–502.
(c) Bohr, Niels. (1913). "On the Constitution of Atoms and Molecules, Part III Systems containing several nuclei". Philosophical Magazine 26: 857–875.
3. Lewis, Gilbert. (1916). “The Atom and the Molecule”, J. Am. Chem. Soc., 38, 4.
4. Sidis, William J. (1920). The Animate and the Inanimate, [PDF], (published in 1925, R.G. Badger).

External links
Energy level – Wikipedia.

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