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Seesaw and Triganol Bipyramid

Posted: Mon Oct 19, 2015 6:39 pm
by Macy Matsukawa 3J
Hello. In lecture today, I did not clearly understand the difference between seesaw shape and triganol bipyramid shape. For example SF4, is it seesaw shape because of the one lone pair on sulfur? Could I just tell the difference by saying that a triagonal bipyramid shape has 5 regions of electron density with no pairs of lone electrons and seesaw shape also has 5 but one of them is a pair of lone electrons?

Re: Seesaw and Triganol Bipyramid

Posted: Mon Oct 19, 2015 9:41 pm
by Pauline Tran 2G
Yes, so the main difference between the seesaw shape and the trigonal bipyramidal shape is the fact tat the seesaw shape has a lone pair in the equatorial position whilst the trigonal bipyramidal shape does not have a lone pair.

Re: Seesaw and Triganol Bipyramid

Posted: Mon Oct 19, 2015 9:47 pm
by Ronald Yang 2F
Trigonal bipyramidal and seesaw shapes have the same number of regions of electron density, as you said: 5. If you were to look at PCl5 and SF4, both have an electron shape of trigonal bipyramidal, as they both have 5 regions of electron density. However, PCl5 has no lone pairs, whereas SF4 has one lone pair. Therefore, when we look at molecular shape, that one lone pair makes SF4 differ in molecular shape. PCl5 would still have a molecular shape of trigonal bipyramidal, but SF4, due to its lone pair, has one of its equatorial atoms replaced with a lone pair, yielding a molecular shape of a seesaw.

Re: Seesaw and Triganol Bipyramid

Posted: Mon Oct 19, 2015 9:48 pm
by Jennifer Cheng 2L
The trigonal pyramidal structure actually has four areas of electron density. Take SO32- for example. The sulfur atom is connected to three oxygen atoms (3 areas of electron density) and then it has a lone pair (1 area of electron density). So that gives it a total of four areas of electron density. This gives it the shape of trigonal pyramidal. There are only four atoms involved in this structure: sulfur and three oxygen atoms.

**Edit: Sorry I read trigonal bipyramidal as trigonal pyramidal. Trigonal bipyramidal also has five areas of electron density but all five are bonded pairs.

In contrast, SF4 has five atoms involved in its structure: a sulfur atom and four oxygens. There are five areas of electron density (compared to four areas of electron density in SO32-). The sulfur atom is bonded to four fluorine atoms (4 areas of electron density) and has one lone pair (1 area of electron density). This gives it the shape of seesaw.

It's helpful to draw out what the structures would look like if there was a bonded atom in place of the lone pair for visualization purposes. Hope this helped.

Re: Seesaw and Triganol Bipyramid

Posted: Mon Oct 19, 2015 9:59 pm
by Ronald Yang 2F
Jennifer Cheng 4H wrote:The trigonal pyramidal structure actually has four areas of electron density.


You are totally correct, but don't mix up trigonal pyramidal with trigonal bipyramidal.