Editing Formaldehyde (section)

==Forms==
Formaldehyde is more complicated than many simple carbon compounds in that it adopts several diverse forms. These compounds can often be used interchangeably and can be interconverted.{{CN|date=January 2025}}
*Molecular formaldehyde. A colorless gas with a characteristic pungent, irritating odor. It is stable at about 150&nbsp;°C, but it polymerizes when condensed to a liquid.
*[[1,3,5-Trioxane]], with the formula (CH<sub>2</sub>O)<sub>3</sub>. It is a white solid that dissolves without degradation in organic solvents. It is a [[trimer (chemistry)|trimer]] of molecular formaldehyde.
*[[Paraformaldehyde]], with the formula HO(CH<sub>2</sub>O)<sub>n</sub>H. It is a white solid that is insoluble in most solvents.
*[[Methanediol]], with the formula CH<sub>2</sub>(OH)<sub>2</sub>. This compound also exists in equilibrium with various [[oligomer]]s (short polymers), depending on the concentration and temperature. A saturated water solution, of about 40% formaldehyde by volume or 37% by mass, is called "100% formalin".

A small amount of [[stabilizer (chemistry)|stabilizer]], such as [[methanol]], is usually added to suppress [[oxidation]] and [[polymerization]]. A typical commercial-grade formalin may contain 10–12% methanol in addition to various metallic impurities.

"Formaldehyde" was first used as a [[generic trademark]] in 1893 following a previous trade name, "formalin".<ref>{{Citation | title = Formalin | publisher = Merriam-Webster, Inc. | url = https://www.merriam-webster.com/dictionary/formalin | date = 15 January 2020 | access-date = 18 February 2020 | archive-date = 18 April 2020 | archive-url = https://web.archive.org/web/20200418141055/https://www.merriam-webster.com/dictionary/formalin | url-status = live}}</ref>
<gallery |centered="" caption="Main forms of formaldehyde">
Structural formula of formaldehyde.svg|Monomeric formaldehyde (subject of this article)
File:S-Trioxane.svg|Trioxane is a stable cyclic trimer of formaldehyde.
File:Paraformaldehyd.svg|Paraformaldehyde is a common form of formaldehyde for industrial applications.
Methanediol-2D.png|Methanediol, the predominant species in dilute aqueous solutions of formaldehyde
</gallery>

===Structure and bonding ===
Molecular formaldehyde contains a central carbon atom with a [[Carbonyl group|double bond to the oxygen atom]] and a [[Carbon–hydrogen bond|single bond to each hydrogen atom]]. This structure is summarised by the [[Structural formula#Condensed formulas|condensed formula]] H<sub>2</sub>C=O.<ref name="Wells">{{cite book | first1 = A. F. | last1 = Wells | title = Structural Inorganic Chemistry | edition = 5th | year = 1984 | publisher = Oxford University Press | isbn = 978-0-19-965763-6 | pages = 915–917, 926}}</ref> The molecule is planar, Y-shaped and its [[molecular symmetry]] belongs to the [[Molecular symmetry#Common point groups|''C''<sub>2v</sub> point group]].<ref>{{Greenwood&Earnshaw2nd|page=1291}}</ref> The precise [[molecular geometry]] of gaseous formaldehyde has been determined by [[gas electron diffraction]]<ref name="Wells" /><ref name="CKIM">{{cite journal | title = Electron Diffraction Studies of Formaldehyde, Acetaldehyde and Acetone | first1 = Kato | last1 = Chuichi | first2 = Konaka | last2 = Shigehiro | first3 = Iijima | last3 = Takao | first4 = Kimura | last4 = Masao | journal = [[Bulletin of the Chemical Society of Japan|Bull. Chem. Soc. Jpn.]] | year = 1969 | volume = 42 | issue = 8 | pages = 2148–2158 | doi = 10.1246/bcsj.42.2148 | doi-access = free}}</ref> and [[microwave spectroscopy]].<ref name="CRC">{{cite book | title = [[CRC Handbook of Chemistry and Physics]] | edition = 93rd | publisher = [[CRC Press]] | year = 2012 | editor = William M. Haynes | pages = 9–39 | isbn = 978-1439880500}}</ref><ref name="Duncan">{{cite journal | title = The ground-state average and equilibrium structures of formaldehyde and ethylene | first = J. L. | last = Duncan | journal = [[Molecular Physics (journal)|Mol. Phys.]] | year = 1974 | volume = 28 | issue = 5 | pages = 1177–1191 | doi = 10.1080/00268977400102501 | bibcode = 1974MolPh..28.1177D}}</ref> The [[bond length]]s are 1.21 [[Ångstrom|Å]] for the [[carbon–oxygen bond]]<ref name="Wells" /><ref name="CKIM" /><ref name="CRC" /><ref name="Duncan" /><ref name="March">{{cite book | first1 = Michael B. | last1 = Smith | first2 = Jerry | last2 = March | title = March's Advanced Organic Chemistry | publisher = John Wiley & Sons | year = 2007 | edition=6th | isbn = 978-0-471-72091-1 | pages = 24–25, 335}}</ref> and around 1.11 Å for the [[carbon–hydrogen bond]],<ref name="Wells" /><ref name="CKIM" /><ref name="CRC" /><ref name="Duncan" /> while the H–C–H [[bond angle]] is 117°,<ref name="CRC" /><ref name="Duncan" /> close to the 120° angle found in an ideal [[trigonal planar molecular geometry|trigonal planar molecule]].<ref name="Wells" /> Some [[Excited state|excited electronic states]] of formaldehyde are [[Trigonal pyramidal molecular geometry|pyramidal]] rather than planar as in the [[ground state]].<ref name="March" />