Valence shell electron pair repulsion (VSEPR) theory (1957) is a model in chemistry, which is used for predicting the shapes of individual molecules, based upon their extent of electron-pair electrostatic repulsion, determined using steric numbers[1]. Scientific modelling is the process of generating abstract, conceptual, Graphical and or mathematical models. Chemistry (from Egyptian kēme (chem meaning "earth") is the Science concerned with the composition structure and properties In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by The steric number of a molecule is the number of atoms bonded to the central atom of a molecule plus the number of lone pairs on the central atom The theory is also called the Gillespie-Nyholm theory after the two main developers, and VSEPR is sometimes pronounced as "vesper" which is easier to say. Ronald J Gillespie, CM, ( August 21, 1924) a Chemistry professor at McMaster University, specializes in the field of Molecular Sir Ronald Sydney Nyholm (Born 29 January 1917 Broken Hill, New South Wales, Australia; Died 4 December 1971 Cambridge, England The premise of VSEPR is that a constructed Lewis structure is expanded to show all lone pairs of electrons alongside protruding and projecting bonds, for predicting the geometric shape and lone-pair behavior of a molecule through consideration of the total coordination number. Lewis structures, also called Lewis-dot diagrams are diagrams that show the bonding between Atoms of a lone pair is a (valence electron pair without bonding or sharing with other Atoms They are found in the outermost Electron shell of an atom so lone pairs Molecular geometry or molecular structure is the three- Dimensional arrangement of the Atoms that constitute a Molecule. The coordination number of an atom in a molecule or a crystal is the number of its nearest neighbours
VSEPR theory is based on the idea that the geometry of a molecule or polyatomic ion is determined primarily by repulsion among the pairs of electrons associated with a central atom. The pairs of electrons may be bonding or nonbonding (also called lone pairs). Only valence electrons of the central atom influence the molecular shape in a meaningful way. In Chemistry, valence, also known as valency or valency number, is a measure of the number of Chemical bonds formed by the Atoms
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Basic assumptions
- Pairs of electrons in the valence shell of a central atom repel each other.
- These pairs of electrons tend to occupy positions in space that minimize repulsions and maximize the distance of separation between them.
- The valence shell is taken as a sphere with electron pairs localizing on the spherical surface at maximum distance from one another.
- A multiple bond is treated as if it is a single electron pair and the two or three electron pairs of a multiple bond are treated as a single super pair.
- Where two or more resonance structures can depict a molecule the VSEPR model is applicable to any such structure.
Three types of repulsion take place between the electrons of a molecule:
- The lone pair-lone pair repulsion
- The lone pair-bonding pair repulsion
- The bonding pair-bonding pair repulsion.
A molecule must avoid these repulsions to remain stable. When repulsion cannot be avoided, the weaker repulsion (i. e. the one that causes the smallest deviation from the ideal shape) is preferred.
The lone pair-lone pair (lp-lp) repulsion is considered to be stronger than the lone pair-bonding pair (lp-bp) repulsion, which in turn is stronger than the bonding pair-bonding pair (bp-bp) repulsion. Hence, the weaker bp-bp repulsion is preferred over the lp-lp or lp-bp repulsion.
VSEPR theory is usually compared (but not part of) and contrasted with valence bond theory, which addresses molecular shape through orbitals that are energetically accessible for bonding. In Chemistry, valence bond theory explains the nature of a Chemical bond in a Molecule in terms of atomic valencies. Valence bond theory 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. In Chemistry, molecular orbital theory ( MO theory) is a method for determining molecular structure in which Electrons are not assigned to individual
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. In the context of Molecular mechanics, a force field (also called a forcefield) refers to the functional form and Parameter sets used [2]
AXE Method
The "AXE method" of electron counting is commonly used when applying the VSEPR theory. The A represents the central atom and always has an implied subscript one. The X represents how many sigma bonds are formed between the central atoms and outside atoms. In Chemistry, sigma bonds ( σ bonds) are the strongest type of covalent Chemical bond. Multiple covalent bonds (double, triple, etc) count as one X. The E represents the number of lone electron pairs present outside of the central atom. The sum of X and E, sometimes known as the steric number, is also associated with the total number of hybridised orbitals used by valence bond theory. The steric number of a molecule is the number of atoms bonded to the central atom of a molecule plus the number of lone pairs on the central atom
| Steric No. |
Basic Geometry 0 lone pair |
1 lone pair | 2 lone pairs | 3 lone pairs |
|---|---|---|---|---|
| 2 |  |
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| 3 |  |
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| 4 |  |
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| 5 |  |
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| 6 |  |
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| 7 |  |
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| Molecule Type | Shape | Electron arrangement† | Geometry‡ | Examples |
|---|---|---|---|---|
| AX1En | Diatomic |  |
|
HF, O2 |
| AX2E0 | Linear |  |
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BeCl2, HgCl2, CO2 |
| AX2E1 | Bent |  |
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NO2−, SO2, O3 |
| AX2E2 | Bent |  |
|
H2O, OF2 |
| AX2E3 | Linear |  |
|
XeF2, I3− |
| AX3E0 | Trigonal planar |  |
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BF3, CO32−, NO3−, SO3 |
| AX3E1 | Trigonal pyramidal |  |
 |
NH3, PCl3 |
| AX3E2 | T-shaped |  |
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ClF3, BrF3 |
| AX4E0 | Tetrahedral |  |
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CH4, PO43−, SO42−, ClO4− |
| AX4E1 | Seesaw |  |
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SF4 |
| AX4E2 | Square Planar |  |
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XeF4 |
| AX5E0 | Trigonal Bipyramidal |  |
|
PCl5 |
| AX5E1 | Square Pyramidal |  |
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ClF5, BrF5 |
| AX6E0 | Octahedral |  |
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SF6 |
| AX6E1 | Pentagonal pyramidal |  |
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XeF6 |
| AX7E0 | Pentagonal bipyramidal |  |
 |
IF7 |
When the substituent (X) atoms are not all the same, the geometry is still approxmiately valid, but the bond angles may be slightly different than the ones where all the outside atoms are the same. In Chemistry, the Linear Molecular geometry describes the arrangement of three or more atoms placed at an expected bond angle of 180º In Chemistry, trigonal planar is a Molecular geometry with one atom at the center and three atoms at the corners of a triangle all in one plane In Chemistry, the term " bent " can be applied to certain Molecules to describe their Molecular geometry. A tetrahedron (plural tetrahedra) is a Polyhedron composed of four triangular faces three of which meet at each vertex. In Chemistry, a trigonal pyramid is a Molecular geometry with one atom at the Apex and three atoms at the corners of a Trigonal base In Chemistry, the term " bent " can be applied to certain Molecules to describe their Molecular geometry. In Chemistry a trigonal bipyramid formation is a Molecular geometry with one atom at the center and 5 more atoms at the corners of a Triangular dipyramid Seesaw is a type of Molecular geometry where the central atom has one lone pair of electrons and there are four bonding groups also connected to that central atom In chemistry T-shaped molecular geometry describes a somewhat unusual geometry adopted by compounds where a central atom has three Ligands Ordinarily three-coordinated compounds minimize In Chemistry, the Linear Molecular geometry describes the arrangement of three or more atoms placed at an expected bond angle of 180º An octahedron (plural octahedra is a Polyhedron with eight faces In Geometry, a square pyramid is a pyramid having a square base The square planar molecular geometry in Chemistry describes the stereochemistry (spatial arrangement of atoms that is adopted by certain Chemical compounds As In Geometry, the pentagonal dipyramid (or Bipyramid) is third of the infinite set of Face-transitive Dipyramids The set In Geometry, a pentagonal pyramid is a pyramid with a Pentagonal base upon which are erected five triangular faces that meet at a point (the Diatomic molecules are molecules made only of two Atoms of either the same or different Chemical elements The prefix di- means two in Greek Structure HF forms orthorhombic crystals consisting of zig-zag chains of HF molecules In Chemistry, the Linear Molecular geometry describes the arrangement of three or more atoms placed at an expected bond angle of 180º Beryllium chloride is the Chemical compound with the formula BeCl2 Mercury(II chloride or mercuric chloride (formerly corrosive sublimate) is the Chemical compound with the formula HgCl sub>2 Carbon dioxide ( Chemical formula:) is a Chemical compound composed of two Oxygen Atoms covalently bonded to a single In Chemistry, the term " bent " can be applied to certain Molecules to describe their Molecular geometry. The nitrite Ion is NO2− The anion is bent being Isoelectronic with O3. OZONE is an object oriented Operating system written in the C programming language. In Chemistry, the term " bent " can be applied to certain Molecules to describe their Molecular geometry. Water ( H2[[oxygen O]] H OH) is the most abundant Molecule on Earth 's surface composing of about 70% of the Earth's surface as Oxygen difluoride is the Chemical compound with the formula OF2 In Chemistry, the Linear Molecular geometry describes the arrangement of three or more atoms placed at an expected bond angle of 180º Xenon difluoride is a powerful Fluorinating agent, with the chemical formula, is one of the most stable Xenon compounds. In chemistry triiodide (sometimes written tri-iodide can have several meanings In Chemistry, trigonal planar is a Molecular geometry with one atom at the center and three atoms at the corners of a triangle all in one plane Boron trifluoride is the Chemical compound with the formula BF3 In Chemistry, a carbonate is a salt or Ester of Carbonic acid. In Inorganic chemistry, a nitrate is a salt of Nitric acid with an Ion composed of one Nitrogen and three Oxygen atoms Sulfur trioxide (also spelled sulphur trioxide) is the chemical compound with the formula SO3 In Chemistry, a trigonal pyramid is a Molecular geometry with one atom at the Apex and three atoms at the corners of a Trigonal base Ammonia is a compound with the formula N[[hydrogen H3]] It is normally encountered as a Gas with a characteristic pungent Odor Phosphorus trichloride (formula P[[Chlorine Cl]]3 is the most important of the three Phosphorus chlorides. In chemistry T-shaped molecular geometry describes a somewhat unusual geometry adopted by compounds where a central atom has three Ligands Ordinarily three-coordinated compounds minimize Chlorine trifluoride is the Chemical compound with the formula ClF3 Bromine trifluoride is the Inorganic compound with the formula BrF3 In a Tetrahedral molecular geometry a central Atom is located at the center with four Substituents that are located at the corners of a Tetrahedron. Methane is a Chemical compound with the molecular formula. It is the simplest Alkane, and the principal component of Natural gas. A phosphate, an Inorganic chemical, is a salt of Phosphoric acid. Perchlorates are the salts derived from Perchloric acid ( H[[chlorine Cl]] O4) Seesaw is a type of Molecular geometry where the central atom has one lone pair of electrons and there are four bonding groups also connected to that central atom Sulfur tetrafluoride is the Chemical compound with the formula SF4 The square planar molecular geometry in Chemistry describes the stereochemistry (spatial arrangement of atoms that is adopted by certain Chemical compounds As Xenon tetrafluoride, Xe[[Fluorine F4]] is one of the chemical compounds derived from the Noble gas xenon In Chemistry a trigonal bipyramid formation is a Molecular geometry with one atom at the center and 5 more atoms at the corners of a Triangular dipyramid Phosphorus pentachloride is the Chemical compound with the formula PCl5 In Molecular geometry, square based pyramidal geometry describes the shape of certain compounds with the formula ML5 where L is a Ligand. Chlorine pentafluoride has formula ClF5 It was first synthesized in 1963 Bromine pentafluoride, Br[[fluorine F5]] is an Interhalogen compound and a Fluoride of Bromine. In Chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or Ligands are symmetrically arranged around Sulfur hexafluoride is an Inorganic compound with the formula. A pentagonal pyramidal structure is most commonly recognized as a pyramid with a base shaped like a pentagon Xenon hexafluoride is the Chemical compound with the formula XeF6 In Chemistry a pentagonal bipyramid (or dipyramid is a Molecular geometry with one atom at the centre with seven Ligands at the corners of a Pentagonal Iodine heptafluoride, also known as iodine(VII fluoride or even iodine fluoride is the compound I[[fluoride F]]7 In Organic chemistry, a substituent is an atom or group of atoms substituted in place of a Hydrogen atom on the Parent chain of a Hydrocarbon For example, the double-bond carbons in alkenes like C2H4 are AX3E0, but the bond angles are not all exactly 120 °. Structure This Hydrocarbon has four Hydrogen Atoms bound to a pair of Carbon atoms that are connected by a Double bond. Similarly, SOCl2 is AX3E1, but because the X substituents are not identical, the XAX angles are not all equal. Thionyl chloride (or thionyl dichloride) is an Inorganic compound with the formula S[[Oxygen O]] Cl 2
Examples
The methane molecule (CH4) is tetrahedral because there are four pairs of electrons. Methane is a Chemical compound with the molecular formula. It is the simplest Alkane, and the principal component of Natural gas. The four hydrogen atoms are positioned at the vertices of a tetrahedron, and the bond angle is cos-1(-1/3) ≈ 109°28'. A tetrahedron (plural tetrahedra) is a Polyhedron composed of four triangular faces three of which meet at each vertex. This is referred to as an AX4 type of molecule. As mentioned above, A represents the central atom and X represents all of the outer atoms.
The ammonia molecule (NH3) has three pairs of electrons involved in bonding, but there is a lone pair of electrons on the nitrogen atom. Ammonia is a compound with the formula N[[hydrogen H3]] It is normally encountered as a Gas with a characteristic pungent Odor It is not bonded with another atom; however, it influences the overall shape through repulsions. As in methane above, there are four regions of electron density. Therefore, the overall orientation of the regions of electron density is tetrahedral. On the other hand, there are only three outer atoms. This is referred to as an AX3E type molecule because the lone pair is represented by an E. The observed shape of the molecule is a trigonal pyramid, because the lone pair is not "visible" in experimental methods used to determine molecular geometry. The shape of a molecule is found from the relationship of the atoms even though it can be influenced by lone pairs of electrons.
A steric number of seven is possible, but it occurs in uncommon compounds such as iodine heptafluoride. Iodine heptafluoride, also known as iodine(VII fluoride or even iodine fluoride is the compound I[[fluoride F]]7 The base geometry for this is pentagonal bipyramidal.
See also
References
- ^ Modern Inorganic Chemistry W. Molecular geometry or molecular structure is the three- Dimensional arrangement of the Atoms that constitute a Molecule. A linear combination of atomic orbitals or LCAO is a Quantum superposition of Atomic orbitals and a technique for calculating Molecular orbitals Molecular modelling is a collective term that refers to theoretical methods and computational techniques to model or mimic the behaviour of Molecules The techniques L. Jolly ISBN 0-07-032760-2
- ^ VGS Box. Journal of Molecular Modeling, 1997, 3, 124-141.
External Links
- 3D Chem - Chemistry, Structures, and 3D Molecules
- IUMSC - Indiana University Molecular Structure Center
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