Editing Energy level (section)

== Molecules ==
[[Chemical bond]]s between atoms in a molecule form because they make the situation more stable for the involved atoms, which generally means the sum energy level for the involved atoms in the molecule is lower than if the atoms were not so bonded. As separate atoms approach each other to [[covalent bond|covalently bond]], their orbitals affect each other's energy levels to form bonding and antibonding [[molecular orbital]]s. The energy level of the [[bonding molecular orbital|bonding orbitals]] is lower, and the energy level of the [[antibonding molecular orbital|antibonding orbitals]] is higher. For the bond in the molecule to be stable, the covalent bonding electrons occupy the lower energy bonding orbital, which may be signified by such symbols as σ or π depending on the situation. Corresponding anti-bonding orbitals can be signified by adding an asterisk to get σ* or π* orbitals. A [[non-bonding orbital]] in a molecule is an orbital with electrons in outer [[electron shell|shell]]s which do not participate in bonding and its energy level is the same as that of the constituent atom. Such orbitals can be designated as '''n''' orbitals. The electrons in an n orbital are typically [[lone pair]]s.
<ref name="chemguide">[http://www.chemguide.co.uk/analysis/uvvisible/theory.html#top UV-Visible Absorption Spectra]</ref> In polyatomic molecules, different vibrational and rotational energy levels are also involved.

Roughly speaking, a molecular energy state (i.e., an [[eigenstate]] of the [[molecular Hamiltonian]]) is the sum of the electronic, vibrational, rotational, nuclear, and translational components, such that:
<math display="block">E = E_\text{electronic} + E_\text{vibrational} + E_\text{rotational} + E_\text{nuclear} + E_\text{translational}</math>
where {{math|''E''<sub>electronic</sub>}} is an [[eigenvalue]] of the [[electronic molecular Hamiltonian]] (the value of the [[potential energy surface]]) at the [[molecular geometry|equilibrium geometry of the molecule]].

The molecular energy levels are labelled by the [[molecular term symbol]]s. The specific energies of these components vary with the specific energy state and the substance.

=== Energy level diagrams ===
There are various types of energy level diagrams for bonds between atoms in a molecule.
; Examples : ''[[Molecular orbital diagram]]s'', ''[[Jablonski diagram]]s'', and ''[[Franck–Condon principle|Franck–Condon]]'' diagrams.