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VSEPR-Valence shell electron pairrepulsion

Valence shell electron pair repulsion (VSEPR) rules are a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic

repulsion.[1]

 It is also named Gillespie – Nyholm theory after its two main developers. The

acronym "VSEPR" is sometimes pronounced "vesper" for ease of pronunciation; however, the

 phonetic pronunciation is technically more correct.

The premise of VSEPR is that the valence electron pairs surrounding an atom mutually repel

each other, and will therefore adopt an arrangement that minimizes this repulsion, thus

determining the molecular geometry. The number of atoms bonded to a central atom plus the

number of lone pairs of its nonbonding valence electrons is called its steric number. 

VSEPR theory is usually compared and contrasted with valence bond theory, which addresses

molecular shape through orbitals that are energetically accessible for bonding. Valence bondtheory concerns itself with the formation of sigma and pi bonds. Molecular orbital theory is

another model for understanding how atoms and electrons are assembled into molecules and

 polyatomic ions.

VSEPR theory has long been criticized for not being quantitative, and therefore limited to the

generation of "crude", even though structurally accurate, molecular geometries of covalent

molecules. However, molecular mechanics force fields based on VSEPR have also been

developed.

Valence Shell Electron Pair Repulsion Theory 

Valence shell electron pair repulsion theory, VSEPR, is a super-simple technique for predictingthe shape or geometry of atomic center in small molecules and molecular ions:

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Crucially, atomic center with VSEPR determined geometry can be joined together into molecular 

entities like cyclohexane and glucose:

This molecular building-block logic can be extended, enabling large bio molecular structures like

DNA to be modelled and understood:

 The VSEPR Technique 

Six or so steps are required to generate the VSEPR geometry of an atomic centre such as:

Carbon in methane, CH4

 Nitrogen in ammonia, NH3Xenon in xenon tetra fluoride, XeF4

Iodine in the iodide di fluoride ion, [IF2] – 

First, determine the number of electrons in the outer (valence) shell about the central

atom (C, N, Xe, I, etc.):

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Carbon, for example has four valence electrons, nitrogen 5, etc.

Second, find valency and number of electrons associated with the ligand X:

Third, construct a valid Lewis structure of the molecule in question showing all of 

the bonds and all of the lone pairs (nonbonded pairs) of electrons.

If the structure is a molecular ion, add one valence electron for each negative chargeand remove one valence electron for each positive charge.

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 Not all Lewis structures have eight electrons about the central atom A (as emphasized

 by very simple Lewis octet theory). For example,

Sulfuric acid, H2SO4, has two monovalent OH functions and two doubly bonded

oxygen that behave as single ligands:

Phosphorus pentachloride, PCl5, has 10 electron

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Valence- Shell Electron- Pair Repulsion

(VSEPR) Models 

The 3-dimensional structure of BF3 is different from PF3, and this is difficult tocomprehend by considering their formulas alone. However, the Lewis dot structure for them are

different, and the electron pair in :PF3 is the reason for its structure being different from BF3 (no

lone pair).

Three-dimensional arrangements of atoms or bonds in molecules are important properties as are bond lengths, bond angles and bond energies. The Lewis dot symbols led us to see the non- bonding electron pairs, whose role in determining the shape of a molecule was examined by N.V.

Sidgwick and H.E. Powell in 1940, and later by R.S. Nyholm and R.J. Gillespie. They have

developed an extensive rationale called valence-shell electron-pair repulsion (VSEPR) model of 

molecular geometry.

 The Valence-Shell Electron-PairRepulsion (VSEPR) models consider theunshared pairs (or lone electron pairs) and the

 bonding electrons. This considerations of lone

and bonding electron pairs give an excellentexplanation about the molecular shapes. The

VSEPR model counts both bonding and

nonbonding (lone) electron pairs, and call the

total number of pairs the steric number (SN ). If the element A has m atoms bonded to it

and n nonbonding pairs, then

SN = m + n 

SN is useful for predicting shapes of molecules.

If X is any atom bonded to A (in single, double,or triple bond), a molecule may be represented

 by AXmEn where E denote a lone electron pair.

This formula enable us to predict its geometry.The common SN , descriptor, and examples are

given in the table on the right.

 Note that the SN is also called the number of VSEPR pairs or number of electron pairs. The

VSEPR model has another general rule: lone pairs of electrons take up more space than bondedpairsmaking the bond angle, say H-O-H for water less than the tetrahedral angle of 109.5 °.

 Actually, the H-O-H angle in water is 105 °.

The geometry of the molecules with their SNs equal to 2 to 6 are given in the Table. The firstline for each is the shape including the lone electron pair(s). If the lone electron pairs are

ignored, the geometry of the molecule is given by another descriptor.

Molecular shapes and steric numbers (SN)

Example SN Descriptor

BeCl2, CO2 2 Linear 

BF3, SO3 SO2E, OO2E

3Trigonal planar  bent

CH4 

 NH3E

H2OE2 

4

Tetrahedral

 pyramidal

 bent

PF5 

SF4EClF3E2 

5

Trigonal bypyramidal

 butterflyT-shape

SF6, OIF5 BrF5E

XeF4E2 

6octahedral pyramidal

square planar 

E represents a lone electron pair.

SN is also called the number of VSEPR pairs 

or number of electron pairs.

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 The VSEPR theory assumes that each atom in a molecule will achieve a geometry that

minimizes the repulsion between electrons in the valence shell of that atom. The five compounds

shown in the figure below can be used to demonstrate how the VSEPR theory can be applied tosimple molecules.