Editing Energy level (section)

== Energy level transitions ==
{{Further|atomic electron transition|molecular electron transition}}
[[File:Atomic Absorption (hv corrected).png|thumb|right|200px|An increase in energy level from {{math|''E''<sub>1</sub>}} to {{math|''E''<sub>2</sub>}} resulting from absorption of a photon represented by the red squiggly arrow, and whose energy is {{math|''[[Planck constant|h]] [[Frequency|&nu;]]''}}.]]
[[File:Schematic diagram of atomic line spontaneous emission (hv corrected).png|thumb|left|200px|A decrease in energy level from {{math|''E''<sub>2</sub>}} to {{math|''E''<sub>1</sub>}} resulting in emission of a photon represented by the red squiggly arrow, and whose energy is {{math|''h &nu;''}}.]]

Electrons in atoms and molecules can change (make ''[[Atomic electron transition|transitions]]'' in) energy levels by emitting or absorbing a [[photon]] (of [[electromagnetic radiation]]), whose energy must be exactly equal to the energy difference between the two levels.

Electrons can also be completely removed from a chemical species such as an atom, molecule, or [[ion]]. Complete removal of an electron from an atom can be a form of [[ionization]], which is effectively moving the electron out to an [[atomic orbital|orbital]] with an infinite [[principal quantum number]], in effect so far away so as to have practically no more effect on the remaining atom (ion). For various types of atoms, there are 1st, 2nd, 3rd, etc. [[ionization energy|ionization energies]] for removing the 1st, then the 2nd, then the 3rd, etc. of the highest energy electrons, respectively, from the atom originally in the [[ground state]]. Energy in corresponding opposite quantities can also be released, sometimes in the form of [[photon energy]], when electrons are added to positively charged ions or sometimes atoms. Molecules can also undergo transitions in their [[molecular vibration|vibrational]] or rotational energy levels. Energy level transitions can also be nonradiative, meaning emission or absorption of a photon is not involved.

If an atom, ion, or molecule is at the lowest possible energy level, it and its electrons are said to be in the ''ground state''. If it is at a higher energy level, it is said to be ''excited'', or any electrons that have higher energy than the ground state are ''excited''. Such a species can be excited to a higher energy level by [[light absorption|absorbing]] a photon whose energy is equal to the energy difference between the levels. Conversely, an excited species can go to a lower energy level by spontaneously emitting a photon equal to the energy difference. A photon's energy is equal to the Planck constant ({{math|''h''}}) times its [[frequency]] ({{mvar|f}}) and thus is proportional to its frequency, or inversely to its [[wavelength]] ({{mvar|λ}}).<ref name="chemguide" />
: {{math|1=Δ''E'' = ''hf'' = ''hc'' / ''λ''}},
since {{math|''c''}}, the speed of light, equals to {{math|''fλ''}}<ref name="chemguide" />

Correspondingly, many kinds of [[spectroscopy]] are based on detecting the frequency or wavelength of the emitted or [[Absorption spectroscopy|absorbed]] photons to provide information on the material analyzed, including information on the energy levels and electronic structure of materials obtained by analyzing the [[spectrum]].

An asterisk is commonly used to designate an excited state. An electron transition in a molecule's bond from a ground state to an excited state may have a designation such as σ&nbsp;→&nbsp;σ*, π&nbsp;→&nbsp;π*, or n&nbsp;→&nbsp;π* meaning excitation of an electron from a σ&nbsp;bonding to a σ&nbsp;[[antibonding]] orbital, from a π&nbsp;bonding to a π&nbsp;antibonding orbital, or from an n non-bonding to a π&nbsp;antibonding orbital.<ref name="chemguide" /><ref>[http://www.chem.ucla.edu/~bacher/UV-vis/uv_vis_tetracyclone.html.html Theory of Ultraviolet-Visible (UV-Vis) Spectroscopy]</ref> Reverse electron transitions for all these types of excited molecules are also possible to return to their ground states, which can be designated as σ*&nbsp;→&nbsp;σ, π*&nbsp;→&nbsp;π, or π*&nbsp;→&nbsp;n.

A transition in an energy level of an electron in a molecule may be combined with a [[vibrational transition]] and called a [[vibronic transition]]. A vibrational and [[rotational transition]] may be combined by [[rotational–vibrational coupling|rovibrational coupling]]. In [[rovibronic coupling]], electron transitions are simultaneously combined with both vibrational and rotational transitions. Photons involved in transitions may have energy of various ranges in the electromagnetic spectrum, such as [[X-ray]], [[ultraviolet]], [[visible light]], [[infrared]], or [[microwave]] radiation, depending on the type of transition. In a very general way, energy level differences between electronic states are larger, differences between vibrational levels are intermediate, and differences between rotational levels are smaller, although there can be overlap. [[Translation (physics)|Translational]] energy levels are practically continuous and can be calculated as kinetic energy using [[classical mechanics]].

Higher [[temperature]] causes fluid atoms and molecules to move faster increasing their translational energy, and thermally excites molecules to higher average amplitudes of vibrational and rotational modes (excites the molecules to higher internal energy levels). This means that as temperature rises, translational, vibrational, and rotational contributions to molecular [[heat capacity]] let molecules absorb heat and hold more [[internal energy]]. [[Conduction (heat)|Conduction of heat]] typically occurs as molecules or atoms collide [[Heat transfer|transferring the heat]] between each other. At even higher temperatures, electrons can be thermally excited to higher energy orbitals in atoms or molecules. A subsequent drop of an electron to a lower energy level can release a photon, causing a possibly coloured glow.

An electron further from the nucleus has higher potential energy than an electron closer to the nucleus, thus it becomes less bound to the nucleus, since its potential energy is negative and inversely dependent on its distance from the nucleus.<ref>{{cite web |url=http://chemed.chem.wisc.edu/chempaths/GenChem-Textbook/Electron-Density-and-Potential-Energy-899.html |title=Electron Density and Potential Energy |access-date=2010-10-07 |url-status=dead |archive-url=https://web.archive.org/web/20100718111313/http://chemed.chem.wisc.edu/chempaths/GenChem-Textbook/Electron-Density-and-Potential-Energy-899.html |archive-date=2010-07-18 }}</ref>