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The Hybridisation Theory... as applied to simple inorganic molecules

posted Dec 16, 2016, 7:21 AM by Grace Ong   [ updated Dec 16, 2016, 7:22 AM ]
The following post was first posted on Blogger on Monday, 18 February 2013.

I shall use the Hybridisation Theory to explain the shapes of the ammonia and beryllium chloride molecules.

The Ammonia Molecule

Electronic configuration of nitrogen:

electron-in-box diagram for nitrogen

If the atomic orbitals of the central nitrogen atom are not hybridised, the N−H bonds will be oriented at angles of the px, py and pz orbitals.

hypothetical unhybridised orbitals of nitrogen in ammonia

This does not explain why the ammonia molecule is trigonal pyramidal.

If the 2s and 2p orbitals of nitrogen are mixed to form four sp3 hybrid orbitals,

hybridisation of nitrogen

three of the sp3 orbitals will each form an end-on overlap with the 1s orbital of a hydrogen atom (to form three N−H σ-bonds), while the fourth sp3 orbital will house the unbonded pair of electrons:

sp3 hybrid orbitals of nitrogen in ammonia and shape of ammonia molecule

Note: In a typical situation,

No. of hybrid orbitals
No. of atoms the central atom
is bonded to
No. of non-bonded electron pairs
on the central atom

The electron geometry with respect to the central nitrogen atom is tetrahedral, and the resulting molecular geometry of the molecule is trigonal pyramidal. However the observed H−N−H bond angle is 107.5 °, less than the 109 ° of a tetrahedral angle. This is because the unbonded electron pair is closer to the nitrogen atom and occupies a greater volume than a bond pair, resulting in a lone pair-bond pair repulsion being greater than a bond pair-bond pair repulsion.

The Beryllium Chloride Molecule

Electronic configuration of beryllium:

electron-in-box diagram of beryllium
Since there are only two chlorine atoms bonded to beryllium, and no unbonded electron pairs on beryllium, only one 2p orbital is used to combine with the 2s orbital of beryllium to form two sp hybrid orbitals:

hybridisation of beryllium

Each of the two sp hybrid orbitals then forms an end-on overlap with a 3p orbital of a chlorine atom (to form two Be−Cl σ-bonds):

sp hybrid orbitals of beryllium in beryllium chloride

The resulting molecular geometry of the molecule is linear, with a bond angle of 180 °.

shape of beryllium chloride