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Geometry of atomic and molecular orbitals


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Molecular orbital theory and hybridization, sigma and pi bonds


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Covalent bonds between two adjacent atoms form when their orbitals overlap. [Diagram goes here - download the original pdf to see it.] This shows the formation of a molecular orbital from two atomic orbitals. There are two types of molecular orbital (1) A s - bond("sigma" bond) arises from the head-on overlap of two atomic orbitals. [Diagram goes here - download the original pdf to see it.] A s -bond arises from the head-on overlap of atomic orbitals (2) A p - bond arises from the side-ways overlap of atomic orbitals. [Diagram goes here - download the original pdf to see it.] A p-bond arises from the sideways overlap of atomic p-orbitals. A single bond comprises just one s-bond; a double bond comprises one s-bond and one p-bond. In the formation of a molecular orbital, as one atom approaches another, the shapes of the orbitals of one or both atoms can change. This change in the shape of an atomic orbital as atoms approach each other is called "hybridization". The idea behind hybridization is that the s, p and d atomic orbitals can combine to form new orbitals. The manner in which they do this is described by the Valence Shell Electron Pair Repulsion Theory (abbreviated to VSEPR theory). This is described elsewhere; here we are concerned with the shapes of the hybrid orbitals. Let us explain why this theory of hybridization of atomic orbitals is necessary by means of an example. It is found, for example, from experiment that carbon will form four single bonds. In carbon tetrachloride carbon forms four identical bonds with chlorine atoms; each one of these bonds is equally spaced from the other in a tetrahedral arrangement. [Diagram goes here - download the original pdf to see it.] However, carbon has one s orbital and three p orbitals in its valence shell. These orbitals, as described above, are not equivalent, and they have different geometries - the s orbital has a spherical geometry, and the p orbitals have "dumbbell" geometries. Thus, in order to account for the symmetry of the carbon bonds in carbon tetrachloride we have to postulate that the approach of the chlorine molecules has caused the s and the p orbitals to amalgamate and hybridise (change) into new orbitals. In this case, since the new hybrid orbitals arise from the combination of one s orbital and three p orbitals, we say that they are sp3 hybrid orbitals. These orbitals give rise to a basic tetrahedral geometry. We can show the formation of hybrid orbitals using orbital diagrams as well. The atomic orbital diagram for carbon before hybridization is [Diagram goes here - download the original pdf to see it.] During hybridization the 2s and 2p orbitals combine to form 2sp3 orbitals. [Diagram goes here - download the original pdf to see it.]
Contents of
Geometry of atomic and molecular orbitals

1 The Geometry of atomic and molecular orbitals
2 The geometry of atomic orbitals
3 Molecular orbital theory and hybridization, sigma and pi bonds
4 Hybrid orbitals
5 Valence shell electron pair repulsion theory
6 How to tell what hybridisation takes place
7 Non-bonding pairs - lone pairs
8 Water
9 Ammonia
10 The ammonium ion
11 Boron trifluoride
12 Exercise in the Geometry of atomic and molecular orbitals

Related articles: (1) Ionic Boinding and Salts, (2) Geometry of atomic and molecular orbitals