### Bond Angles

Posted:

**Sun Nov 12, 2017 3:14 am**On future exams, will we have to find bond angles and is so, what steps would we show for credit?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=43&t=23379

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Posted: **Sun Nov 12, 2017 3:14 am**

On future exams, will we have to find bond angles and is so, what steps would we show for credit?

Posted: **Sun Nov 12, 2017 9:10 am**

I think it is pure memorization for the bond angles of basic structures (ex: tetrahedral or trigonal planar). For the more advanced structures with lone pairs, I think you just have to know that the lone pairs will push the bonded atoms closer together and make the bond angle smaller than it was originally. I don't know if there is a way to mathematically calculate the specific bond angles of certain molecules with certain structures (I think they would just have to be given to you).

Posted: **Wed Nov 15, 2017 10:15 am**

You should be expected to find bond angles for any future VSEPR problems. It's mainly memorization but honestly it's not very much since you just need to know the bond angles of the 'parent' structures.

For AX2, Linear, all derivations will start with 180, for AX3, Trigonal Planar, all derivations will have 120 or less than that, etc.

For AX2, Linear, all derivations will start with 180, for AX3, Trigonal Planar, all derivations will have 120 or less than that, etc.

Posted: **Wed Nov 15, 2017 10:22 am**

Usually with bond angles and molecular/electron shape, it is pure memorization and nothing more. However, most of the bond angles are relatively easy to figure out by using logic and working through the lewis structure.

Posted: **Wed Nov 15, 2017 12:52 pm**

Do you think it's fine for us to just put bond angles with greater than or less than signs? like the bond angle is <109.5

Posted: **Wed Nov 15, 2017 10:47 pm**

Adam Enomoto 1K wrote:Do you think it's fine for us to just put bond angles with greater than or less than signs? like the bond angle is <109.5

I believe the answer to that is yes because many bond angles with lone pairs as a region of electron density are determined experimentally as they are different for many molecules. I would say that you are simply expected to know, for example, that the bond angles for a shape like trigonal pyramidal would be <109.5 because we learned that they should be 109.5 degrees for tetrahedral, but with the extra repulsion of a lone pair, we know that the bond angles are going to be a little bit less than 109.5. Hope this helps!

Posted: **Thu Nov 16, 2017 2:21 pm**

Yes putting less than 109.5 degrees for bond angles will be fine because molecules with lone electron pairs have bond angles that are determined experimentally and differ from molecule to molecule even if both have the same VSEPR shape.

Posted: **Thu Nov 16, 2017 2:38 pm**

You just need to know that a lone electron pair causes the bond angle between the central atom and other atoms to decrease slightly. Ex. CH4 has bond angles of 109.5 degrees but ammonia (NH3) has bond angles slightly less than 109.5 degrees. You do not need to know the exact bond angle of ammonia.

Posted: **Thu Nov 16, 2017 5:44 pm**

You can just memorize/visualize the bond angles for each VSEPR shape and then if there are any lone pairs present (AX_{2}E, AX_{3}E_{2}, etc.), the bond angle would be less than the "original" bond angles (listed below). You can just state that the bond angles are <___° (the bond angles without lone pairs).

AX_{2} = linear = 180°

AX_{3} = trigonal planar = 120°

AX_{4} = tetrahedral = 109.5°

AX_{5} = trigonal bipyramidal = 90°/120°/180°

AX_{6} = octahedral = 90°/180°

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