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NH3 Molecular Geometry

Some points About Ammonia has been given below:
  • Ammonia has four groups of electrons and so it has the same electronic geometry as other molecules like CH4.
  • In case of NH3 however, one group has an unshared pair of electrons, and as per VSEPR the four electron groups around a central atom will be directed towards the corners of tetrahedron to give maximum separation.
  • N has tetrahedral electronic geometry, which refers to the geometric arrangement of electron groups around central atom.
  • The molecular geometry however excludes the lone pairs on the central atom and describes only the arrangement of atoms around the central atom.
  • Ammonia (NH3) is an example of molecule in which the central atom has both shared and unshared electron pairs. The shape of ammonia molecule (NH3) is trigonal pyramid.
  • There are three bonding pairs of electrons and one non bonding pair. The bond angle in a molecule of ammonia are 107o, a value very close to tetrahedral angle (109o.5').
  • We can write a general tetrahedral structure for the electron pairs of ammonia by placing the non bonding pair at one corner.
  • A tetrahedral arrangement of the electron pairs explains the trigonal pyramidal arrangement of the four atoms.
  • The bond angles are 107o (not 109o.5') because the non bonding pair occupies more space than the bonding pairs.

Ammonia Ball and Stick Model 
NH3 Molecular Geometry


Molecular geometry is explained by the arrangement of atoms. The nitrogen atom is at the apex of the trigonal pyramidal arrangement and the other three atoms are at the corners of the triangular base of the pyramid.

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Hybridization of NH3

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In order to arrive the molecular shapes, chemist combine the envisioned shapes of individual atoms to form the known shapes of molecules and this concept is called hybridization. It describes the need to modify the atomic orbitals of isolated atoms to conform to the known shapes of molecules.
In NH3 the lone pair of electrons occupies one of the sp3 hybrid orbitals. Each of the other three sp3 orbitals participates in bonding by sharing the electrons with another atom. They overlap with the half-filled H 1s orbital in ammonia. 

SP3 Hybridization in Ammonia

Molecular Geometry Chart

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VSEPR or valence shell electron-pair repulsion dictates that electron pairs will repel one another, thus creating a molecular geometry where each electron pair is as far as possible from every other electron pair. Un-bonded pairs are slightly more repellant than other pairs. 

VSEPR theory gives a good explanation of the geometry found in the chart.


Number of bonds
Number of unused electron pairs

Type of hybrid orbital
Angle between bonded atoms
Geometry
Example
3
1
sp3
90o- 109.5o
Pyramidal
NH3

The lone pair is a pair of valence electrons that is associated with only one nucleus in contrast to a bonded pair., which is associated with two nuclei. Lone pair of electrons occupy more space than the bond pair of electrons.

Due to the repulsion of bond pair and lone pair in ammonia the bond angles in ammonia are less than the expected 109.5o

Repulsion of Ammonia Bond Pair Lone Pair
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