Valence-shell electron-pair repulsion tetrahedral arrangement

Section 1 10 The shapes of molecules can often be predicted on the basis of valence shell electron pair repulsions A tetrahedral arrangement gives the max imum separation of four electron pairs (left) a trigonal planar arrange ment is best for three electron pairs (center) and a linear arrangement for two electron pairs (right)... 

Valence shell electron pair repulsion (VSEPR) model (Section 110) Method for predicting the shape of a molecule based on the notion that electron pairs surrounding a central atom repel one another Four electron pairs will arrange them selves in a tetrahedral geometry three will assume a trigo nal planar geometry and two electron pairs will adopt a linear arrangement... 

This simple method of deducing the structure of molecules is called Valence Shell Electron Pair Repulsion Theory (VSEPRT). It says that all electron pairs, both bonding and nonbonding, in the outer or valence shell of an atom repel each other. This simple approach predicts (more or less) the correct structures for methane, ammonia, and water with four electron pairs arranged tetrahedrally in each case. 

Valence-shell electron-pair repulsion (VSEPR) theory and the concept of hybridization suggest that the water molecule has two O—H bonds and two non-bonded pairs arranged tetrahedrally. More accurate calculations show that this does not provide a true picture of the total electron density in H20. 

The valence shell electron pair repulsion theory states that electron pairs aroimd the central atom of the molecule arrange themselves to minimize electronic repulsion the electrons orient themselves as far as possible from each other. Two electron pairs around the central atom lead to a linear arrangement of the attached atoms three indicate a trigonal planar arrangement, and four result in a tetrahedral geometry. Both lone pair and bonding pair electrons must be taken into accoimt when predicting structure. Molecules with fewer than four and as many as five or six electron pairs around the central atom also exist. They are exceptions to the octet rule. 

Lewis structures of all but the simplest molecules do not show the shape of the molecule. A collection of rules known as valence-shell electron repulsion theory (VSEPR theory), in which regions of electron density (attached atoms and lone pairs) are supposed to adopt positions that minimize their repulsions, is often a helpful guide to the local shape at an atom, such as the tetrahedral arrangement of single bonds around a carbon atom. This theory should also be familiar from introductory chemistry courses. 

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