Angles less than 109.5 degrees

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Baoying Li 1B
Posts: 91
Joined: Sat Aug 17, 2019 12:18 am

Angles less than 109.5 degrees

Postby Baoying Li 1B » Sun Nov 17, 2019 12:03 pm

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?

Joowon Seo 3A
Posts: 80
Joined: Sat Aug 24, 2019 12:17 am

Re: Angles less than 109.5 degrees

Postby Joowon Seo 3A » Sun Nov 17, 2019 12:08 pm

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.

MeeraBhagat
Posts: 74
Joined: Sat Aug 24, 2019 12:15 am

Re: Angles less than 109.5 degrees

Postby MeeraBhagat » Sun Nov 17, 2019 12:31 pm

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.

Sofia Barker 2C
Posts: 81
Joined: Wed Sep 18, 2019 12:21 am

Re: Angles less than 109.5 degrees

Postby Sofia Barker 2C » Sun Nov 17, 2019 1:47 pm

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.

Siya Shah 1J
Posts: 50
Joined: Sat Aug 17, 2019 12:15 am

Re: Angles less than 109.5 degrees

Postby Siya Shah 1J » Sun Nov 17, 2019 2:21 pm

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.

Emily Vainberg 1D
Posts: 86
Joined: Sat Jul 20, 2019 12:15 am

Re: Angles less than 109.5 degrees

Postby Emily Vainberg 1D » Sun Nov 17, 2019 2:50 pm

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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