(Polar molecules, Non-polar molecules, etc.)
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Hi guys. I have a question that why is the angle of trigonal pyramidal less than 109.5 degrees but not less than 120 degrees since the angle of trigonal planar is 120 degrees. Also, same as the angular shape, why is the angle not less than 180 degrees but less than 120 degrees?
It is because of the arrangement of electron densities. Since in a trigonal planar arrangement there are three regions of electron density therefore the angle has to be <120, but in a tetrahedral arrangement there are four regions of electron density therefore the angle has to be <109.5.
The lone pair creates a stronger repulsion in the trigonal pyramidal shape, which is why the angle is less than 109.5. In a trigonal planar shape, there is no lone pair, so there is equal repulsion between the bonding regions creating equal 120 degree bonding angles.
There are four regions of electron density in molecules with a trigonal pyramidal shape. Normally, molecules with four regions of electron density have a tetrahedral shape with 109.5-degree bond angles between the 4 bonding electron pairs. However, trigonal pyramidal shapes are formed when one of the four regions of electron density is a lone pair, which has a stronger negative charge that pushes the bonding pairs closer together to form angles slightly less than 109.5 degrees. When thinking through the bond angles, remember that although the molecular shape is named after the positioning of the atoms in the molecule, these atomic positions are influenced by the presence of lone pairs.
Trigonal planar and trigonal pyramidal are not the same thing, even though each have three bonding pairs. Trigonal pyramidal has three bonding pairs and an additional lone pair, making a total of four regions of electron density. The lone pair causes repulsion, pushing the bonded pairs away from one another.
You should look up the big VSEPR chart and use that for reference. The 'less than' signs refer to when you go across a row. So Trigonal pyramid (which has 1 lone pair and three bond pairs) is less than 109 degrees because the shape to the left is tetrahedral (4 bond pairs, 0 lone pairs). The bond angles for tetrahedral is 109 degrees so you would be comparing trigonal pyramid to that shape not to trigonal planar.
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