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# Tetrahedral Bond Angles

The structure of organic molecules can be explained on the basis of valence bond theory or VSEPR (Valence shell electron pair repulsion theory). The valence bond theory is based on the concept of hybridisation. Hybridisation involves the mixing of atomic orbitals to form same number of hybrid orbitals. These hybrid orbitals are degenerate in nature or we can say that they have equal energy and the same shape. The overlapping of hybrid orbitals forms strong covalent bonds compare to un-hybrid orbitals. The overlapping of hybrid orbitals forms sigma bonds whereas the overlapping of un-hybrid orbital forms pi-bond in the molecule.

The bonded atoms in organic molecules are arranged in a certain manner which provides molecular geometry of the molecule. The angle between three bonded atoms is called as bond angle. The length of a bond also depends on the bond order which can be defined as the number of bonds between two bonded atoms.

The picture of tetrahedral bond angles are shown below.

VSEPR helps to understand the geometry of those molecules which do not follow valence bond theory and show deviation from a regular bond angle which also alters the molecular geometry. With the help of hybridisation, you can easily predict the regular molecular geometry with bond angle in the molecule such as sp3 hybridisation forms tetrahedral geometry with  $109^{\circ}{28}'$ bond angle. Let’s discuss few more points related to hybridisation and molecular geometry.

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## Sp3 Hybridization

1. Carbon (excited state ) IS2 2S1 2P­1 2PY1 2PZ1 does not contain any lone pair electron in forming CH4 molecule as all the 4 Sp3 orbitals which are formed due to mixing of 1s and 3p orbitals are singly occupied and therefore the bond angle is typical that of tetrahedral geometry.
2. Whenever carbon atoms react with 4 other atoms or group, it undergoes sp3 hybridization, the four sp3 hybrid orbitals containing a single electron tend to move as far as possible and finally it tends to be directed toward the corner of a regular tetrahedron.
3. The bond angle between any two sp3 hybrid orbital is $109^{\circ}{28}'$ approximately leading to the molecule a regular tetrahedral geometry as shown in the figure.

Example: molecules of CH4, CCl4, CHCl3 etc..,

Image showing four hybrid orbitals of carbon and tetrahedral bond angle

## Tetrahedral Bond Angle

1. The bond angle is also affected if the central atom bears a unbounded lone pair of electrons, as there is more effective repulsion due to the hybrid orbitals come closer and the bond angle is reduced.
2. So in electron geometry the bond angle in NH3 molecule is not typical that of tetrahedral bond angle but the value is reduced to 107o.
3. In water molecule, this tetrahedral bond angle is reduced further as the central atom oxygen contains 2 lone pairs of electrons.

Image showing the change in tetrahedral bond angle is due to the presence of lone pairs

See the hybridization state of the central atom in each of the molecule, CH4, NH3, H2O is sp3 but the bond angle is different in case of ammonia or oxygen.

## Definition of Bond Length

Bond length is the mean distance between the nucleus of two atoms in a compound. It is denoted by nm(nano meter) or Armstrong units. One Armstrong unit= 10 -10 meter. The following factors affect the bond length.
• Atomic radii: If atomic radii is more the bond length will be more
• Bond nature: If the number of bonds between atoms increases, the bond length will decrease. Hence the bond length between carbon- carbon in ethane is more than in ethene where there is a double bond between carbon carbon atom.
• Bond strength: Bond strength or bond dissociation energy is the energy required to break a bond into its constituent atoms. Bond strength is inversely related to bond length. Hence double bond will be stronger than single bond because in bond length is shorter in double bond.
• Bond order: It is the half of difference between number of electrons in bonding orbital and anti-bonding orbital. Electrons in the bonding orbital increase the stability while electrons in the anti-bonding orbital decrease the stability. Bond order is inversely related to bond length. Hence the bond length of nitrogen will be shorter with bond order 3 than that of oxygen with bond order 2.
• Bond length is used to determine the covalent radii. Bond length is determined by X-ray diffraction studies.
• Bond length may be different within the same molecule. This depends on the geometry. For example PCl5 is in trigonal bipyramidal geometry and two axial bonds are more longer than three equatorial bonds.

## Bond Angles

Bond angle is the angle between the bonded pair of electrons in a covalent compound. As there is direct overlap of atomic orbitals in the covalent compound the bonding electrons are projected in specific directions and there is a specific angle between the bonded pair of electrons.

## VSEPR Bond Angle

In a covalent compound the bonded pair of electrons experience electrostatic repulsive forces. Due to this they try to project as par as possible. A compound may contain different type of electrons like bonded pair electrons,unpaired electrons and lone paired electrons.

The repulsive force between different type of electrons are in the order of

Lone pair-lone pair>lone pair- bond pair>bond pair-bond pair

Hence there may be change in bond angles accordingly.

Similarly depending on the nature of geometry a compound may contain two or more than two type of bond angles. For example in trigonal bipyramidal geometry there are two type bond angles, 90o and 120o.

### Factors affecting bond angles

1. Size of the atom

When the atomic size increases there is more space available for expansion of bond pair electron over the repulsion from lone pair electron. Hence the bond angle increases. For example the bond angle in PH3 is more than in NH3.

2. Presence of lone pair of electrons

Lone pair electrons exert more repulsive force than bond pair electrons and hence there may be some reduction in normal bond angle.

## Bond Angle Chart

 S.No Hybridization No of Lone pairelectron Structure Bond angle Example 1 sp - Linear 180o BeCl2 2 sp2 - Trigonal 120o BF3 3 sp3 - Tetrahedral 109o CH4 4 sp3d - TrigonalBi pyramidal 90o,120o PCl5 5 sp3d2 - Octahedral 90o, 180o SF6 6 sp3d3 - Pentagonal Bi pyramidal 72o,180o IF7

## Some Special Cases in Bond Angles

• Ammonia: In this molecule there is one pair of lone pair electrons. As they create more repulsive force, the tetrahedral bond angles are reduced to 107o.
• BrF5: In this molecule sp3d2 hybridization takes place and there is one lone pair of electrons.
Hence the octahedral bond angles are changed to 84.8o and 180o. The shape of the molecule is square pyramidal.

## Bond Length and Bond Angle

• A chemical bond is characterized by certain parameters like bond length, bond angle, dipole moment and bond order.
• A chemical bond is formed due to transfer or sharing of valence electrons.
• Hence a chemical bond may be ionic, covalent or co-ordinate in nature.
• In this covalent bond is formed due to sharing of valence electrons.
• Physical overlap of atomic orbitals takes place and the bond is directional in nature.
• The bond parameters like bond length, bond angle are mostly associated with covalent bond only.