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Trigonal Pyramidal Bond Angle

A molecule is composed of certain number of atoms which are bonded with chemical bonds. We know that atoms have valance electrons which are placed in the outermost shell of atoms. These valence electrons take part in bonding to complete the octet configuration of atoms. Such chemical bonds which are formed by equal sharing of valance electrons of atoms are called as covalent bonds. The compounds or molecules which are formed by covalent bonds are known as covalent compounds or covalent molecules.

In covalent molecules, atoms are arranged in certain geometry in three dimensional space which is called as molecular geometry. There are different kinds of molecular geometry such as linear, trigonal planer, tetrahedral, trigonal pyramidal and octahedral. Various geometries can be identified with the position of atoms in the molecule. If there are only bonding electrons and no lone pair or non-bonding electron, then molecule will follow regular geometry with no distortion. According to VSEPR theory, the presence of lone pair in any molecule can change the molecular geometry because lone pairs tend to repel the bonding electrons and molecule tends to adopt such geometry which reduces the repulsion to provide stability to the molecule.

 Number of  Bonding Electrons 
 Number of lone pairs of electrons   Molecular Geometry   Example 
 2  0  Linear  $BeF_{2}$, $CO_{2}$
 1  1  Linear  CO, $N_{2}$
 3  0  Trigonal planar
 $BF_{3}$, $CO_{3}^{2-}$
 2  1  Bent or V-shape
 $O_{2}$, $SO2$
 1  2  Linear  $O_{2}$
 4  0  Tetrahedral
 $CH_{4}$, $SO_{4}^{2-}$ 
 3  1  Trigonal pyramidal  $NH_{3}$, $H_{3}O^{+}$
 2  2  Bent  $H_{2}O$, $ICl_{2}+$
 1  3  Linear  $HF$, $OH^{-}$
 5  0  Trigonal bipyramidal
 4  1  Seesaw  $SF_{4}$, $TeCl_{4}$, $IF_{4}^{+}$ 
 3  2  T-shaped  $ClF_{3}$
 2  3  Linear  $I_{3}^{-}$, $XeF_{2}$
 6  0  Octahedral  $SF_{6}$, $PF_{6}^{-}$, $SiF_{6}^{2-}$ 
 5  1
 square pyramidal  $BrF_{5}$, $SbCl_{5}^{2-}$
 4  2  square planar
 $XeF_{4}$, $ICl_{4}^{-}$

Let’s take an example of molecular geometry. In methane molecule, one center carbon atom is bonded with four hydrogen atoms in tetrahedral manner.  The bond angle in this molecule is 109.5°. All four C-H bonds are equal in energy and length because they are formed by same kind of hybrid orbital. Since there are four pairs of bonding electrons therefore they arrange in regular tetrahedral manner. If we replace one of the pair of bonding electrons with lone pair, it will not remain tetrahedral in shape. For example if we replace one of the hydrogen atom of methane molecule with lone pair of electrons like in ammonia molecule, there will be repulsion between lone pair and bonding electrons which push the bonding electrons close to each other and reduce the bond angle. It alters the molecular geometry and changes it from tetrahedral to trigonal pyramidal. 

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Trigonal Pyramidal Bond Angle Definition

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The molecular geometry: trigonal pyramidal can be defined as the shape in which three chemical bonds are arranged in trigonal manner with one lone pair on the central atom of the molecule. 

This molecular geometry is the distorted form of tetrahedral molecule in which the center atom is $sp^{3}$ hybridizes with one lone pair of electrons. Ammonia is the most common example of this geometry in which the center atom that is nitrogen atom is $sp^{3}$ hybridized and contains one lone pair and 3 bond pairs. Since the molecule is $sp^{3}$ hybridized therefore the molecular geometry should be tetrahedral but because of presence of one lone pair on nitrogen atom, the geometry is distorted to trigonal pyramidal with bond angle 107°. Here trigonal pyramidal bond angle is little less than the tetrahedral bond angle which is due to repulsion between lone pair and bond pair which  push the bond pairs close to each other and reduces the bond angle up to 107°. 

Trigonal Pyramidal of Ammonia

Trigonal Pyramidal Bond Angle Examples

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There are many examples of trigonal pyramidal bond angle such as ammonia molecule, sulphate ion and hydronium ion. The trigonal pyramidal geometry of ammonia molecule is formed from tetrahedral electron pairs with lone pair. We know there are 5 valance electrons in center nitrogen atoms therefore it requires three more electrons to complete its octet configuration which can be done by the formation of 3 covalent bonds with other three hydrogen atoms. Molecule acquires such geometry which keeps the lone pairs and bond pairs far away from each other. It reduces the bond angle from 109.5° to 107° and change the geometry from tetrahedral to trigonal pyramidal. 

As per the molecular formula, the shape of ammonia molecule should be trigonal planer like boron trifluoride. But It is not so due to presence of one lone pair in the molecule. Another example is hydronium ion $(H_{3}O_{+})$ in which center oxygen atom as one lone pair and also bonded with three hydrogen atoms. There is +1 charge over oxygen atom as oxygen atom must have 2 lone pairs.  In this example, the Lewis diagram shows O at the center with one lone electron pair and three hydrogen atoms attached.  The geometry is very similar to ammonia molecule but not like water molecule in which center oxygen atom has 2 lone pairs with 2 hydrogen atoms. Due to presence two lone pairs, repulsion is very high in water molecule therefore bond reduces from 109.5 degrees to 104 degrees and geometry turns to V-shape or bent geometry.

Smallest Bond Angle in Trigonal Pyramidal

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Smallest bond angle in trigonal pyramidal geometry is 107.4° in ammonia molecule. Other examples of trigonal pyramidal geometry are Xenon trioxide $(XeO_{3})$, chlorate ion $(ClO_{3}^{−})$ sulfite ion $(SO_{3}^{2−})$ etc. Let’s discuss one example of same geometry. Sulphite ion has sulphur atom as center atom which has one lone pair with three oxygen atoms and 2- charge over molecule. Out of three oxygen atom, one oxygen atom is bonded with double bond whereas remaining 2  oxygen atoms are bonded with single covalent bonds and carry negative charge. 

Trigonal Pyramidal Geometry of Sulphite Ion

Trigonal Pyramidal Bond Angle Hybridization

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Molecules with trigonal pyramidal geometry show $sp^{3}$ hybridisation with one lone pair on center atom of molecule.
Since $sp^{3}$ hybridization is related to tetrahedral geometry, but due to presence of one lone pair over center atom in the molecule, the molecular geometry changes to trigonal pyramidal.
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