SP3 Hybridization in Ammonia
 

Hello...

I have a small conceptual problem. As far as i know, hybridization occurs when electrons prefer to live in different orbitals with same spin rather than same orbitals with different spin.

As far as hybridization in Carbon atom is concerned, it takes place because there are 2 electrons in 2S subshell ........ and 2 in 2P subshell i.e. Px = 1 , Py = 1 , Pz = 0

That is why one of the two electrons in 2S shifts to Pz and SP3 hybridization takes place.


However , the problem is with ammonia(NH3). The electronic configuration is 1 S2 , 2 S2 , 2 P3. i.e. Px = 1, Py = 1 , Pz = 1. There is no reason why the 2 S2 shuould over lap with 2P3 and form SP3 orbitals. BUT STILL THEY DO ... and form one lone pair and 3 Half filled bond pairs. (Which afterwards form covalent bonds with Hydrogen atoms)

My question is that why do the 2S2 electrons have to form SP3 orbitals with the 2P3 electrons, when they still remain the same i.e. 2P electrons in different orbitals with same spin and 2S electrons as a lone pair.

Do Reply ... thanks

Hybridisation is the formation of molecular orbitals (ie orbitals used in bonding found in molecules) from atomic orbitals.

The arrangements of electron pairs in bonded atoms is different from that of non-boded atoms - this is an observed fact. Hybridisation is an attempt to explain this fact. It seems clear that ALL atoms perform these changes when bonding and the actual reasons why are unclear without recourse to higher quantum theory considerations.

There are two basic theories as to what happens when two atoms combine:
1.Molecular orbital theory
2. Linear combination of atomic orbitals

In MO theory the molecule forms hybridised molecular orbitals (with their own unique shapes and form) from the orbitals of the bonding atoms. There are an equal numer of molecular orbitals as atomic orbitals.
In LCAO the overlap of the already hybridised orbitals creates regions of electron density that hold the atoms together.

Both theories have their merits although MO is more sophisticated.

At pre-university level is is simply convenient to say that the electros move into degenerate hybridised orbitals before bonding and to use carbon as the example due to its ability to sp, sp2 and sp3 hybridise.

However, as you stated NH3 is also sp3, and so is H2O and probably HF is as well - these are molecules and the atomic orbitals have hybridised into the arrangements found in molecules ie sp3 for four distinct electron pairs.

see: http://www.ibchem.com/IB/ibc/bonding/bon_htm/14.2.htm

for a little more info...