Editing Ammonia (section)

=== Structure ===
[[File:NH3-Dipole-Moment.png|thumb|Molecular structure of ammonia and its three-dimensional shape. It has a net dipole moment of 1.484&nbsp;[[Debye|D]].]]
[[File:Ammonia-2D-dot-cross.svg|class=skin-invert-image|thumb|[[Dot and cross diagram|Dot and cross]] structure of ammonia]]

The ammonia molecule has a [[trigonal pyramidal]] shape, as predicted by the [[valence shell electron pair repulsion theory]] (VSEPR theory) with an experimentally determined bond angle of 106.7°.<ref name="CRC-2013">{{cite book | editor= Haynes, William M. | year = 2013 | title = CRC Handbook of Chemistry and Physics | edition = 94th | publisher = [[CRC Press]] | isbn = 9781466571143|pages=9–26| title-link = CRC Handbook of Chemistry and Physics }}</ref> The central nitrogen atom has five outer electrons with an additional electron from each hydrogen atom. This gives a total of eight electrons, or four electron pairs that are arranged [[tetrahedron|tetrahedrally]]. Three of these [[electron pair]]s are used as bond pairs, which leaves one [[lone pair]] of electrons. The lone pair repels more strongly than bond pairs; therefore, the bond angle is not 109.5°, as expected for a regular tetrahedral arrangement, but 106.7°.<ref name="CRC-2013"/> This shape gives the molecule a [[dipole]] moment and makes it [[Polar molecule|polar]]. The molecule's polarity, and especially its ability to form [[hydrogen bond]]s, makes ammonia highly miscible with water. The lone pair makes ammonia a [[Base (chemistry)|base]], a proton acceptor. Ammonia is moderately basic; a 1.0&nbsp;[[Molar concentration|M]] [[aqueous solution]] has a [[pH]] of 11.6, and if a strong acid is added to such a solution until the solution is neutral ({{nowrap|1=pH = 7}}), 99.4% of the ammonia molecules are [[protonated]]. Temperature and [[salinity]] also affect the proportion of [[ammonium]] {{chem2|[NH4]+}}. The latter has the shape of a regular [[tetrahedron]] and is [[isoelectronic]] with [[methane]].

The ammonia molecule readily undergoes [[nitrogen inversion]] at room temperature; a useful analogy is an [[umbrella]] turning itself inside out in a strong wind. The [[energy barrier]] to this inversion is 24.7&nbsp;kJ/mol, and the [[resonance frequency]] is 23.79&nbsp;[[GHz]], corresponding to [[microwave]] radiation of a [[wavelength]] of 1.260&nbsp;cm. The absorption at this frequency was the first [[Microwave spectroscopy|microwave spectrum]] to be observed<ref name="Cleeton-1934">{{cite journal|doi=10.1103/PhysRev.45.234 |title=Electromagnetic Waves of {{convert|1.1|cm|0|abbr=on}}. Wave-Length and the Absorption Spectrum of Ammonia|year=1934|author=Cleeton, C. E.|journal=Physical Review|volume=45|issue=4|pages=234 |first2=N. H.|last2=Williams|bibcode = 1934PhRv...45..234C }}</ref> and was used in the first [[maser]].