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Mon January 09, 2012 By:

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Mon January 09, 2012

The shape of a molecule is a description of the way the atoms in the molecule occupy space. Molecular geometry is the distances and angles between each of the different atoms in the molecule. It is the geometry used to describe the shape of a molecule. It is the shape of the molecule. So, lone pairs affect the shape of the molecules by undergoing repulsion with either bond pairs or other lone pairs.

Molecular geometry and molecular structure are two different things. Molecular geometry is the distances and angles between the each of the different atoms in the molecule. It is essentially the shape of the molecule. Whereas molecular structure is the way the molecule is assembled from its constituent atoms and not only includes the shape of the molecule, but also the nature of the bonding in the molecule (such as where there are single, double or triple bonds), the polarity of the molecule (if the electrons are spread out evenly throughout the molecule etc.

 A lone pair of electrons is a non-bonding pair of electrons. An 'electron cloud' may be a single, double or triple bond, or a lone pair of electrons. 'Electron clouds' being negatively charged repel one another and try to get as far away from each other as possible.  Even though the lone pairs aren't involved in bonding, they do influence the angles of the bonds because they repel the electrons involved in bonding. Lone pairs of electrons exert a great repelling effect than the bonding pairs. Lone pair-bonding pair repulsion is greater than bonding pair-bonding pair repulsion. For example in case of ammonia,  because of the presence of a lone pair of electrons, the lone pair-lone pair repulsion is greater than bonding pair-bonding pair repulsion, so the lone pair pushes the bonding pairs closer together than in a tetrahedral arrangement of the 'electron clouds' and gives it a distorted tetrahedral arrangement or shape called trigonal pyramidal.
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