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Molecular Geometry

The group of similar or different atoms is called as a molecule. A molecule represents smallest and fundamental unit of a chemical compound which can take part in the chemical reactions. In any molecule, the atoms are bonded with each other in a certain manner with the help of their valence electrons. Each molecule can be represented by using a molecular formula which denotes the number of constituent atoms in that molecule.

Similarly, structural formula denotes the structure and geometry of a molecule. The molecular geometry of a molecule describes the proper position of constituent atoms in the molecule with their bond angles and bond length. All atoms are arranged in such a way there will be minimum repulsion and maximum distance between their electron pairs. The presence of the lone pairs on central atom and electro negativities of constituent atoms also affects the geometry of molecules.

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Central Atom Molecular Geometry

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The electronic geometry for a given number of electron pairs surrounding a central atom is always the same. That number of electron pairs will distribute themselves in the same way to maximize their separation. The same thing cannot be said for molecular geometry.

The molecular shape depends not only on the electronic geometry but also on the number of the electron pairs that are shared.
When the central metal atom is surrounded by five pairs of electrons the electronic geometry is trigonal bi pyramidal.
If all the electron pairs are shared, the molecular geometry will also be trigonal bi pyramidal. An example of such a molecule is PF5.

If one of the pairs of electrons is not shared, then the molecular shape is called distorted tetrahedron which sometimes is also called as seesaw.

An example of a molecule with trigonal bi pyramidal electronic geometry and the molecular shape as distorted tetrahedron is SF4

In this molecule SF4, there are five electron pairs which should lead to trigonal bi pyramidal structure.

However, out of the five electron pairs, one is a lone pair. This lone pair could occupy an axial position or an equatorial position.
When the lone pair is in axial position, then there will be three Lone pair - Bond pair repulsion but when lone pair is in equatorial position, then there will be two Lone pair - Bond pair repulsion.
Sf4 Molecular Geometry
Imagine rotating the arrangement so that the line joining the two axial position is the board on which the two seesaw riders sit, and the two bonded equatorial positions are the pivot of the seesaw.

SFMolecular Geometry

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Hybridization of an atom depends on the steric number, that is, the number of atoms attached with central atom and lone pairs of electrons present on it.

There are two ways in which the chemical bond of SF4 can be explained. 
  1. One is the new hybridization using one 3d orbital of the sulfur atom with the normal 3 sp3, resulting in the formation of the five coordinate bonds sp3d required for the decet.
  2. The other is the resonance among the conceivable ionic forms, that is in case of SF4 it should be a resonance hybrid of S+F3.F-, SF4, and so on.
Hybridization of Sf4
Therefore, S atom in SF4 is sp3d hybridized. The fifth sp3d orbital accommodates a lone pair of electrons. Hence SF4 is not square planar. The most stable arrangement of SF4 is when a lone pair is in axial position it would be 90o from the three closest other pairs and 180o from the other axial pair.

If it were an equatorial position, only the two axial pairs would be at 90o from the lone pair, and the two equatorial pairs would be farther away at 120o. The lone pair would, therefore, be less crowded in an equatorial position. The four F atoms then occupy the remaining four positions. The resulting arrangement is the seesaw arrangement.

For instance the lone pair - bond pair repulsion in the seesaw molecules SF4 causes distortion of the axial; S-F bonds away from the lone pair to an angle of 177o, the two equatorial S-F bonds, ideally at 120o, move much closer to an angle of 101.6o. The arrangement is shown below.
Angles in Sf4

Molecular Geometry Chart

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The Geometrical patterns and examples corresponding to those predicted molecule is listed in the following table -

Molecule
Steric Number Predicted
Geometry
                 Example
A$X_2$ 2 Linear  Linear Geometry          C$o_2$     
A$X_3$ 3 Trigonal planar Trigonal Planar               B$F_3$
A$X_4$ 4 Tetrahedral Tetrahedral              C$F_4$
A$X_5$
5 Trigonal
bipyramidal
Trigonal Bipyramidal              P$F_5$
A$X_6$ 6 Octahedral Octahedral     S$F_6$

Tetrahedral Molecular Geometry

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Tetrahedral molecular geometry is seen in those molecules which have AX4 arrangement with one central atom and 4 atoms bonding to it. Example of tetrahedral molecules are: Ammonium ion and methane.

Methane:

Tetrahedral Hybridization
Lewis structure of methane shows that the central C atom has four bonding electron pairs. These electron pairs repel each other and are thus directed to the four corners of a regular tetrahedron. The four new orbitals formed in a Tetrahedron by mixing one 's' and three 'p' orbitals. Thus, the hybridization in the molecule is sp3 hybridization. The sp3 hybridization is specifically called as Tetrahedral hybridization.

Bent Molecular Geometry

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Bent or angular shape is seen in molecules like water or sulfur dioxide, having AX2 molecules, with lone pairs on the central atom.

Example: Sulfur dioxide
The Lewis structure of SO2 as shown

Lewis Structure of Sulfur Dioxide

  1. The S atom is bonded to one O by a double bond and to the other O by a single bond.
  2. It has an unshared electron pair.
  3. Thus, S is surrounded by three electron pairs, two bonding pairs and one unshared pair.
  4. For maximum separation, the three electron pairs are directed to the corners of an equilateral triangle.
  5. The predicted bond angle is 120o.
  6. But, with the unshared electron pair repelling the bonding electron pairs, the bond angle is actually reduced to 119.5o.
  7. Thus, SO2 has a planar bent molecule with 119.5o.

Molecular Geometry Table

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Hybridization Number of Hybrid Orbitals
Shape of the molecule
Examples
sp 2 Linear BeCl3
sp2 3 Trigonal planar
BCl3
sp3 4 Tetrahedral CH4
dsp2 4 Square planar
XeF4
dsp3 5 Trigonal bipyramidal PCl5
d2sp3 6 Octahedral SF6

Molecular Geometry of CO2


Carbon dioxide has one Carbon bonded to two Oxygen atoms with a double bond. There are no lone pairs on carbon, as all the four valence electrons are used up in making the two double bonds. 

Thus, the molecule, as expected by AX2 type, exhibits a linear shape. They exhibit sp hybridization.

O = C = O

Molecular Geometry of H2O


In the Structural formula of water, the O atom is bonded to two H atoms by covalent bonds and two lone pairs. Thus, O is surrounded by two bonding pairs and two unshared electron pairs. The predicted bond angle is 109.5o.

But the two lone pairs on oxygen, the bond angle is reduced to 105o. Thus, water does not have a linear shape. It has a bent shape.

Structural Formula of Water

Molecular Geometry of NH3


The Lewis structure of NH3 shows that the central N atom has three bonding electrons and a lone electron pair. The three bonding pairs and the nitrogen have one lone pair. If there were four bond pairs, we would predict the same perfectly tetrahedral shape as Methane. But, since there is a lone pair in nitrogen, ammonia molecule is a slightly distorted tetrahedron with the H-N-H angle equal to 107o.

Lewis Structure Aammonia

H2S Molecular Geometry


Hydrogen sulfide, like the water molecule, has a bent shape due to the presence of a lone pair on the central sulfur atom. It has a bond angle of 92.1o. There are two lone pairs of electrons on the central Xe atom in XeF2. This molecule has a bond angle of 180o. So, it has a linear shape.

Xenon Flouride Shape

BF3 Molecular Geometry

  1. Here, there are three bonding pairs around the Boron atom.
  2. The three equal bond angles of 120o mean that the fluorine atoms are all equally far apart.
  3. The only lone pairs are on the fluorine atom.
  4. BF3 has a trigonal planar shape with a bond angle of 120o.
More topics in Molecular Geometry
Molecular Symmetry
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