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Valence shell electron pair repulsion (VSEPR) theory (/ ˈ v ɛ s p ər, v ə ˈ s ɛ p ər / VESP-ər, [1]: 410 və-SEP-ər [2]) is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. [3]
They have central angles from 104° to 109.5°, where the latter is consistent with a simplistic theory which predicts the tetrahedral symmetry of four sp 3 hybridised orbitals. The most common actual angles are 105°, 107°, and 109°: they vary because of the different properties of the peripheral atoms (X).
In accordance with the VSEPR (valence-shell electron pair repulsion theory), the bond angles between the electron bonds are arccos(− 1 / 3 ) = 109.47°. For example, methane (CH 4) is a tetrahedral molecule. Octahedral: Octa-signifies eight, and -hedral relates to a face of a solid, so "octahedral" means "having eight faces". The bond ...
In VSEPR theory the electron pairs on the oxygen atom in water form the vertices of a tetrahedron with the lone pairs on two of the four vertices. The H–O–H bond angle is 104.5°, less than the 109° predicted for a tetrahedral angle, and this can be explained by a repulsive interaction between the lone pairs. [2] [3] [4]
Against the expectations of VSEPR theory but consistent with Bent's rule, the bond angles of ammonia (NH 3) and nitrogen trifluoride (NF 3) are 107° and 102°, respectively. Unlike VSEPR theory, whose theoretical foundations now appear shaky, Bent's rule is still considered to be an important principle in modern treatments of bonding.
HSAB theory is also useful in predicting the products of metathesis reactions. In 2005 it was shown that even the sensitivity and performance of explosive materials can be explained on basis of HSAB theory. [2] Ralph Pearson introduced the HSAB principle in the early 1960s [3] [4] [5] as an attempt to unify inorganic and organic reaction ...
The VSEPR theory places each pair of valence electrons in a bond or a lone pair found in a local region of the molecule based on the Pauli exclusion principle. While this is frequently taught in chemistry textbooks in conjunction with orbital models such as orbital hybridisation and molecular orbital theory, the approach is completely different.
Molecular orbital theory was seen as a competitor to valence bond theory in the 1930s, before it was realized that the two methods are closely related and that when extended they become equivalent. Molecular orbital theory is used to interpret ultraviolet–visible spectroscopy (UV–VIS). Changes to the electronic structure of molecules can be ...