Editing Urea (section)

===Molecular and crystal structure===
The structure of the molecule of urea is {{chem2|O\dC(\sNH2)2}}. The urea molecule is planar when in a solid crystal because of [[Orbital hybridisation#sp2|sp<sup>2</sup> hybridization]] of the N orbitals.<ref name="Godfrey 1997">{{cite journal | last1=Godfrey | first1=Peter D. | last2=Brown | first2=Ronald D. | last3=Hunter | first3=Andrew N. | title=The shape of urea | journal=Journal of Molecular Structure | volume=413–414 | date=1997 | doi=10.1016/S0022-2860(97)00176-2 | pages=405–414| bibcode=1997JMoSt.413..405G }}</ref><ref name="Ishida 2004">{{cite journal | last1=Ishida | first1=Tateki | last2=Rossky | first2=Peter J. | last3=Castner | first3=Edward W. | title=A Theoretical Investigation of the Shape and Hydration Properties of Aqueous Urea: Evidence for Nonplanar Urea Geometry | journal=The Journal of Physical Chemistry B | volume=108 | issue=45 | date=2004 | issn=1520-6106 | doi=10.1021/jp0473218 | pages=17583–17590}}</ref> It is non-planar with C<sub>2</sub> symmetry when in the gas phase<ref name="West 2015">{{cite journal | last1=West | first1=Aaron C. | last2=Schmidt | first2=Michael W. | last3=Gordon | first3=Mark S. | last4=Ruedenberg | first4=Klaus | title=A Comprehensive Analysis in Terms of Molecule-Intrinsic, Quasi-Atomic Orbitals. III. The Covalent Bonding Structure of Urea | journal=The Journal of Physical Chemistry A | volume=119 | issue=41 | date=2015-10-15 | issn=1089-5639 | doi=10.1021/acs.jpca.5b03400 | pages=10368–10375| pmid=26371867 | bibcode=2015JPCA..11910368W | url=https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1624&context=chem_pubs }}</ref> or in aqueous solution,<ref name="Ishida 2004" /> with C–N–H and H–N–H bond angles that are intermediate between the [[Trigonal planar molecular geometry|trigonal planar]] angle of 120° and the [[Tetrahedral molecular geometry|tetrahedral]] angle of 109.5°. In solid urea, the oxygen center is engaged in two N–H–O [[hydrogen bond]]s. The resulting hydrogen-bond network is probably established at the cost of efficient molecular packing: The structure is quite open, the ribbons forming tunnels with square cross-section. The carbon in urea is described as sp<sup>2</sup> hybridized, the C-N bonds have significant double bond character, and the carbonyl oxygen is relatively basic. Urea's high aqueous solubility reflects its ability to engage in extensive hydrogen bonding with water.

By virtue of its tendency to form porous frameworks, urea has the ability to trap many organic compounds. In these so-called [[clathrate]]s, the organic "guest" molecules are held in channels formed by interpenetrating helices composed of [[hydrogen bond|hydrogen-bonded]] urea molecules.  In this way, urea-clathrates have been well investigated for separations.<ref name="Worsch 2002">{{cite book | date=2002 | last1=Worsch | first1=Detlev | last2=Vögtle | first2=Fritz | title=Topics in Current Chemistry | chapter=Separation of enantiomers by clathrate formation | publisher=Springer-Verlag | isbn=3-540-17307-2 | doi=10.1007/bfb0003835 | pages=21–41}}</ref>