Editing Boric acid (section)

==Molecular and crystal structure==
The three oxygen atoms form a [[Trigonal planar molecular geometry|trigonal planar geometry]] around the boron. The B-O bond length is 136 pm, and the O-H is 97 pm. The [[molecular symmetry|molecular point group]] is ''C''<sub>3h</sub>.<ref>{{Greenwood&Earnshaw2nd|page=1291}}</ref>

Two crystalline forms of orthoboric acid are known: [[triclinic crystal system|triclinic]] with [[space group]] P{{overline|1}}, and [[hexagonal crystal system|trigonal]] with space group P3<sub>2</sub>. The former is the most common; the second, which is a bit more stable thermodynamically, can be obtained with a special preparation method.<ref name=rota2017/>

The triclinic form of boric acid consists of layers of {{chem2|B(OH)3}} molecules held together by hydrogen bonds with an O...O separation of 272 pm. The distance between two adjacent layers is 318 pm.<ref name=GandE/> While the layers of the triclinic phase are nearly trigonal with {{nowrap|1=γ = 119.76°}}, {{nowrap|1=''a'' = 701.87 pm}}, and {{nowrap|1=''b'' = 703.5 pm}} (compared to {{nowrap|1=''a'' = 704.53(4) pm}} for the trigonal form), the stacking of the layers is somewhat offset in the triclinic phase, with {{nowrap|1=α = 92.49°}} and {{nowrap|1=β = 101.46°}}. The triclinic phase has {{nowrap|1=''c'' = 634.72 pm}} and the trigonal one has {{nowrap|1=''a'' = 956.08(7) pm}}.<ref>{{cite journal |last1=Gajhede |first1=M. |last2=Larsen |first2=S. |last3=Rettrup |first3=S. |title=Electron density of orthoboric acid determined by X-ray diffraction at 105 K and ab initio calculations |journal=Acta Crystallographica Section B Structural Science |date=1 December 1986 |volume=42 |issue=6 |pages=545–552 |doi=10.1107/S0108768186097720 |bibcode=1986AcCrB..42..545G |url=https://www.crystallography.net/cod/9007675.html}}</ref><ref>{{cite journal |last1=Shuvalov |first1=Robert R. |last2=Burns |first2=Peter C. |title=A new polytype of orthoboric acid, H 3 BO 3 -3 T |journal=Acta Crystallographica Section C Crystal Structure Communications |date=15 June 2003 |volume=59 |issue=6 |pages=i47–i49 |doi=10.1107/S0108270103009685|pmid=12794317 |bibcode=2003AcCrC..59I..47S }}</ref>

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|[[File:Boric-acid-unit-cell-3D-balls.png|center|200px]]||[[File:Boric-acid-layer-3D-balls.png|center|250px]]
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|{{center|The [[unit cell]] of boric acid}}||{{center|[[hydrogen bond]]ing (dashed lines)<br />allows boric acid molecules to form<br />parallel layers in the solid state}}
|}