Valence-shell electron-pair repulsion trigonal planar 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 1.10) 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 themselves in a tetrahedral geometry, three will assume a trigonal planar geometry, and two electron pairs will adopt a linear arrangement. 

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. 

---