Valence-shell electron-pair repulsion linear 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... 

Having introduced methane and the tetrahedron, we now begin a systematic coverage of the VSEPR model and molecular shapes. The valence shell electron pair repulsion model assumes that electron-electron repulsion determines the arrangement of valence electrons around each inner atom. This is accomplished by positioning electron pairs as far apart as possible. Figure 9-12 shows the optimal arrangements for two electron pairs (linear),... 

Valence shell electron pair repulsion theory places the two electron pairs on Be 180° apart, that is, with linear electronic geometry. Both electron pairs are bonding pairs, so VSEPR also predicts a linear atomic arrangement, or linear molecular geometry, for BeCl2. 

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. 

For example, if there are only two electron pairs in the valence shell of an atom, these pairs tend to be at opposite sides of the nucleus, so that repulsion is minimized. This gives a linear arrangement of electron pairs that is, the electron pairs mainly occupy regions of space at an angle of 180° to one another (Figure 10.2). 

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