CHEMISTRY COMMUNITY

This post doesn't have a question, just a chart from my TA that might make memorization easier!

# of regions of e- density. arrangement of e- densities (bond angles).
2 linear (180)
3 trigonal planar (120)
4 tetrahedral (109.5)
5 trigonal bipyramidal (120, 90)
6 octahedral (90)

This helps so much, thank you!!! I was super worried about memorizing them.

Is there a way to calculate the bond angles or is it just a memorization thing?

Hey Rebekah. Based on what I have seen so far, it seems like you have to memorize them.

Rebekah Kaufman 1L wrote:Is there a way to calculate the bond angles or is it just a memorization thing?

Unfortunately, Its the one thing we MUST memorize for the course. It should not be too hard though with practice we should be fine while also strengthing our lewis structure skills!

We made a similar chart in discussion too. I want to point out that I believe our TA mentioned something about the angles being less than 109.5 in some cases, I'm not exactly sure why or in what instances, but I think he mentioned lone pairs. I also think Lavelle said that we don't have to know the exact number, just know that it's less than 109.5.

Thank you for sharing it with us. It is helpful. (:

Jennifer 1G wrote:We made a similar chart in discussion too. I want to point out that I believe our TA mentioned something about the angles being less than 109.5 in some cases, I'm not exactly sure why or in what instances, but I think he mentioned lone pairs. I also think Lavelle said that we don't have to know the exact number, just know that it's less than 109.5.

I remember this too but I don't quite remember for what instances the angle would be less than 109.5 degrees, can anybody explain why?

Thank you so much!

This chart is indeed helpful, thanks for sharing!

melodyso1K wrote:

Jennifer 1G wrote:We made a similar chart in discussion too. I want to point out that I believe our TA mentioned something about the angles being less than 109.5 in some cases, I'm not exactly sure why or in what instances, but I think he mentioned lone pairs. I also think Lavelle said that we don't have to know the exact number, just know that it's less than 109.5.

I remember this too but I don't quite remember for what instances the angle would be less than 109.5 degrees, can anybody explain why?

Hi! This would mostly have to do with the molecules in the formula. Here’s what I have in my notes:

Molecules with the same formula have the same general shape but their bond angles may differ slightly.
For example, comparing the difference between CH4 and CH3Cl, both have the VSEPR formula AX4 and thus have a tetrahedral shape. However, in CH3Cl, chlorine is larger than hydrogen, which pushes the other atoms further away than would just the 4 H atoms in CH4. This thus changes the angles.

So, even though the molecules are both AX4, their bond angles differ slightly.

Hope this helps!

How do we memorize angles if sometimes the angle depends on that specific molecule's lone pair composition?

AnnaYan_1B wrote:

melodyso1K wrote:

Jennifer 1G wrote:We made a similar chart in discussion too. I want to point out that I believe our TA mentioned something about the angles being less than 109.5 in some cases, I'm not exactly sure why or in what instances, but I think he mentioned lone pairs. I also think Lavelle said that we don't have to know the exact number, just know that it's less than 109.5.

I remember this too but I don't quite remember for what instances the angle would be less than 109.5 degrees, can anybody explain why?

Hi! This would mostly have to do with the molecules in the formula. Here’s what I have in my notes:

Molecules with the same formula have the same general shape but their bond angles may differ slightly.
For example, comparing the difference between CH4 and CH3Cl, both have the VSEPR formula AX4 and thus have a tetrahedral shape. However, in CH3Cl, chlorine is larger than hydrogen, which pushes the other atoms further away than would just the 4 H atoms in CH4. This thus changes the angles.

So, even though the molecules are both AX4, their bond angles differ slightly.

Hope this helps!

Everything said so far is absolutely correct (to the best of my knowledge). I just wanted to present an additional situation with changed bond angles, which Jennifer touched upon in her original question. Lone pair electrons have a high repulsion strength, so if a molecule has a lone pair (or multiple lone pairs), this too could decrease bond angles.

Jennifer 1G wrote:We made a similar chart in discussion too. I want to point out that I believe our TA mentioned something about the angles being less than 109.5 in some cases, I'm not exactly sure why or in what instances, but I think he mentioned lone pairs. I also think Lavelle said that we don't have to know the exact number, just know that it's less than 109.5.

the angles would be >109.5 when there is a lone pair involved in the # of e- densities around a molecule

the angles would be >109.5 when there is a lone pair involved in the # of e- densities around a molecule

The angles would be less than, not greater than.

Just to be clear, they are less than because the loan electron pair pushes the other atoms farther away from it (and thus closer to other atoms). This then results in increasingly smaller angles between atoms as you move left to right along the chart.

These were all very great and very helpful, thanks everyone!

when one of those regions is lone pair or pairs, is it basically the same thing but just with > or < ?

when one of those regions is lone pair or pairs, is it basically the same thing but just with > or < ?