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### Planar vs Pyramidal

Posted: Fri Nov 16, 2018 4:17 pm
Is there a difference between trigonal planar and trigonal pyramidal compounds? Is there a difference in their bond angles?

### Re: Planar vs Pyramidal

Posted: Fri Nov 16, 2018 4:34 pm
Yes, there is a difference. So trigonal pyramidal has a pyramid shape and if you can try to imagine it with a base in a triangular shape. (it may be helpful to look for a 3D image). Also, t's bond angles are slightly less than 109 degrees and this is because it has one lone pair of electrons that cause electron repulsion among the other bonds. Trigonal planar is when there are no lone pair electrons, it is pretty stable and has a bond angle of 120 degrees.

### Re: Planar vs Pyramidal

Posted: Fri Nov 16, 2018 4:50 pm
Or in a simpler manner of explaining, trigonal planar has 3 bonding regions while trigonal pyramidal has 4! The only difference is the lone pair of electrons on trigonal pyramidal, which forces the bonds to be repelled.

### Re: Planar vs Pyramidal

Posted: Fri Nov 16, 2018 6:24 pm
A trigonal planar has 3 bonding regions while trigonal pyramidal has 4. Also if there are 2 pairs of LP electrons they are not necessarily opposite to each other because the 3 surfaces you can put them on are all "neighboring" so when you visualize this, keep this in mind that although there is repulsion, they are still going to be near each other.

### Re: Planar vs Pyramidal

Posted: Fri Nov 16, 2018 9:15 pm
When there are three electron densities around the central atom, the electron arrangement is trigonal planar. Since there are no lone pairs, the molecular shape is also trigonal planar. When there are four electron densities around the central atom, the electron arrangement is tetrahedral. However, when one of the electron densities is a lone pair, it repulses the other three atoms causing the molecular shape to be trigonal pyramidal. The bond angles for trigonal planar is 120 degrees, while the bond angles for trigonal pyramidal are slightly less than 109.5 degrees.

### Re: Planar vs Pyramidal

Posted: Sat Nov 17, 2018 3:20 pm
With reference to trigonal planar, it has 3 bonds with no lone pairs. However, trigonal pyramidal has 3 bonds with one lone pair, thereby giving it a tetrahedral arrangement but a trigonal pyramidal shape. With regards to the bond angles, those in a trigonal planar would be 120, while those in a trigonal pyramidal would be less than 109.5 because of the increased repulsion.