What is the VESPER treatment of ammonia gas?

1 Answer
Apr 23, 2017

Answer:

The ammonia molecule has FOUR regions of electron density around the central nitrogen atom..............and this leads to #sp^3#, #"trigonal pyramidal geometry."#

Explanation:

Nitrogen has 5 valence electrons; hydrogen has 1 valence electron. And thus we have #5+3xx1=8# electrons to distribute around the central nitrogen atom.

Six of these electrons constitute the individual #N-H# bonds; and the remaining 2 electrons are conceived to be the nitrogen-centred lone pair.

And thus there are 4 orbitals, each containing 2 electrons, around the central nitrogen atom. The most stable geometric arrangement for these 4 orbitals, which MINIMIZES electrostatic interaction between LIKE electronic charges, is as a TETRAHEDRON, i.e. #sp^3# hybridization, with #109.5^@# between electron pairs to a first approximation. This is a simple manifestation of #"VESPER"#, which will be described in more detail in your text.

But we describe molecular geometry on the basis of the disposition of ATOMS not lone pairs of electrons. And thus we describe the geometry of ammonia as #"trigonal pyramidal"# with #/_H-N-H# compressed to #104-105^@# because of the disproportionate influence of the nitrogen lone pair (this lone pair is closer to the nitrogen centre than the #N-H# bonding pair, and tends to contract #/_H-N-H#).

On the other hand we know that ammonia is a BASIC molecule, and it undergoes the base association reaction:

#NH_3(aq) + H_2O(l) rightleftharpoonsNH_4^(+) + HO^-#.

In ammonium ion, ALL the hydrogens are equivalent, and the molecule assumes a highly symmetrical tetrahedral shape. Thus #/_H-N-H=109.5^@#, the #"so-called ideal tetrahdedral angle"#.

And if you can explain the geometry of ammonia, you can also easily account for the geometry of phosphine, methane, and water.......Try it out............