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Molecular Geometry

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Stereochemistry means chemistry in space and describes chemistry as a function of molecular geometry. Historically it happened the other way round certain molecular phenomena were observed ad in order to explain them the stereochemical principles had to be developed. 

Properties of molecules depend not only on the bonding of atoms but also on the molecular geometry - the three dimensional arrangement of the molecules atoms in space. The combination of the polarity of the bonds and the geometry of the molecule determine the molecular polarity.

What is Molecular Geometry? 

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"Molecular geometry is a three dimensional arrangement of atoms in a molecule." With molecules containing three or more atoms, the geometry is not so obvious. The major features of molecular geometry can be predicted on the basis of a quite simple principle - electron pair repulsion.

The simple procedure that enables to predict with considerable success the overall geometry of a molecule or ion and the number of electrons surrounding a central atom in its Lewis structure. The chemical formula reveals little information about a molecules geometry.

VSEPR Theory

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According to VSEPR model the valence electron pairs surrounding an atom repel one another. The orbitals containing those electron pairs are oriented to be as far apart as possible. The geometry that the molecule ultimately assumes minimizes the repulsion. This approach to the study of molecular geometry is called the valence shell electron pair repulsion (VSEPR) model.

Two general rules govern the use of VSEPR model.
  1. If electron pair repulsion is concerned, double bonds and triple bonds can be treated like single bonds. This approximation is good for qualitative purposes. The reality of multiple bonds are "larger" than single bonds; that is because there are two or three bonds between two atoms the electron density occupies more space.
  2. If a molecule has two or more resonance structures, apply VSEPR model to any one of them.
The structural theory deals with the bond angles is called the VSEPR theory, whereas the one describes changes in the orbitals that contain the valence electrons is called the hybridization theory. VSEPR uses as its basis the fact like charges will orient themselves in such a way as to diminish the repulsion.

Determination

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Molecular geometry is determined by the relative positions of the atomic nuclei in the molecule. It is most conveniently described by the so-called internal coordinates that is bond lengths, bond angles and the angles of torsion. The two experimental methods used to determine the molecular geometries are
  1. Diffraction methods
  2. Spectroscopic methods
The steps involved in the determination of molecular geometry are listed below.
  • Draw the Lewis structure.
  • Determine the number of bonds and the number of lone pairs around each atom.
  • Choose appropriate electron pair geometry
  • Predict the bond angles keeping in mind the lone pair occupies more space around the central atom than the bonding pairs.

Chart

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The molecular geometry table for molecules with two to six electron pair bonds around a central atom (A) is shown below.

Species type Orientation of electron pairs
Predicted bond angles
Example
Ball and stick model
AX2 Linear
180o BeF2 Beryllium Fluoride
AX3 Trigonal planar
120o BF3 Boron Trifluoride
AX4 Tetrahedron
109.5o CH4 Methane
AX5 Trigonal bipyramid
90o
120o
180o
PF5 Phosphorus Pentafluoride
AX6 Octahedron
90o
180o
SF6 Sulfur Hexafluoride

Chemical Bonding and Molecular Geometry

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The Lewis structure and VSEPR method is used to predict the number of bond pairs and lone pairs, and the molecular shape, of any particular molecule. However the type of electron involved in the bonding is not specified. The valence bond description of chemical bonding which does specify the nature of the electrons involved in the bonding. This description of bonding retains the electron pair concept that is so useful in writing the Lewis electron dot structures and thus it uses a localised orbital approach.

Molecular geometry is a simple model that allows to predict molecular geometries. This valence shell electron pair model usually predicts the correct general shape of a molecule. However, valence bond theory provides further insight into why bonds form and the same time reveals that bonds have definite directions in space, resulting in particular molecular geometries.

Types

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The different types of molecular geometry is listed below.

Different types of molecular geometry Example
Linear
Carbon Dioxide
Trigonal planar
Sulfur Trioxide
Tetrahedral
Methane
Trigonal bipyramidal
Iodine Penta Fluoride
Octahedral
Sulfur Hexafluoride

Molecular Geometry of CO2

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The Lewis dot structure of CO2 indicates that there are only two electron groups in the form of double bonds that extend from the central carbon atom. Carbondioxide is a linear molecule.

Carbon dioxide

Molecular Geometry of H2O

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The Lewis dot structure model of water indicates that there are two bonding pairs and two lone pairs on the central oxygen atom. The VSEPR model dictates that the electron groups should be arranged in a tetrahedral shape.

Water

Molecular Geometry of SO2

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Sulfur dioxide provides an example of a molecule with three electrons with three electron groups about the S atom. The shape of SO2 is more angular rather than trigonal planar.

Sulfur Dioxide

Molecular Geometry of SF4

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In SF4 molecule the central S atom has five pairs of electrons whose arrangement should be trigonal bipyramidal. It has the following two structures in which the lone pair in equatorial position is more stable.

Sulfur Tetrafluoride

Molecular Geometry of CCl4

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CCl4 has four polar C-Cl bonds, the molecular geometry of Carbon tetrachloride is tetrahedral, the four bond dipoles point to the vertices of the tetrahedron and therefore cancel each other.

Carbon Tetrachloride

Molecular Geometry of CH4

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CH4 has a tetrahedral electronic geometry. Methane has four bond pairs so it has a tetrahedral molecular shape. Methane is a typical five atom molecule. The four hydrogen atoms are at the corners of a tetrahedron and the carbon at the center. The molecule is three dimensional.

Methane

Molecular Geometry of HCN

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In HCN the central carbon atoms two VSEPR electron groups are a single bond and a triple bond. The HCN molecule has linear molecular geometry and describes the arrangement of three or more atoms placed at an expected bond angle of 180o.

Molecular Geometry of HCN