Bond Angles

[Nimisha_Seshadri_3C]

How do we determine the bond angles of a molecule when given a lewis structure?

[Jayden Arevalo 2B]

I think there isn't a particular formula or way to determine bond angles; it is a bit of memorization. But, it's helpful to understand why the bond angles in trigonal planar are 120 degrees since there are 3 even angles in the same plane that must add up to 360 degrees. Just think about the planes that each atom resides in and the 360 degree rule.

[Michael Vigman 2D]

I'd agree with Jayden that it may be simpler to memorize the bond angles for various VSEPR shapes / electron density geometries. However, in finding a single bond angle within a shape (if there are two bond angles present), look at the geometry of the molecule and identify electron densities that fall in the same plane.

I tend to look at the molecule and see which electron density regions I could intersect using an imaginary piece of paper (on a single plane). Then, since electron repulsions try to maximize the distance between each other we can divide the molecules in that plane from 360 degrees. Additionally, remember that lone pairs can 'push" bond angles to be lower than expected.

[Crystal Ros 2L]

The key to this is knowing that electron dense areas repel one another, so they will be attempting to get as far away from each other as possible. Knowing this, the Lewis structure can tell us the VSEPR model. By looking at the VSEPR model we can see which bond angles will be present within a molecule.

[Reza Hemmati 3L]

Hi!

What I do to find the bond angles is find the total regions of electron density(number of bonded atoms plus the number of lone pairs on the central atom), then compare that to a VSEPR shape with no lone pairs. So, if the total was 3, I'd compare it to a trigonal planar; if it was 4, I'd compare to tetrahedral; if it was 5, I'd compare to trigonal bipyramidal; if it was 6, I'd compare it to octahedral. For those with a sum of 3 or 4, the bond angles are the same as trigonal planar or tetrahedral, respectively.* With those with a sum of 5 or 6, the only bond angles that the structures can have are those of trigonal bipyramidal or octahedral respectively, but they may be missing a bond angle or two, so you'd have to visualize/draw out the structure and remove bonding pairs, similar to what Dr. Lavelle has done in lecture with the yellow molecule models.

*There's also one more tiny thing to account for when comparing, which is that lone pairs influence bond angles by making them slightly lower, if the lone pairs don't cancel their repulsion out by being on direct opposite sides of the molecule. This is signified by putting a < sign in front of the bond angle.

Hope this helps! ^^

[505739628]

I know most people have just been saying to memorize the bond angles from VSEPR structures but I actually recommend trying to understand why the bonds form these angles instead of just memorizing because then you will retain the information better! For me, I just think of the structure and how the bonds would arrange themselves to reduce repulsions as much as possible (so the bonds are as far apart as they can be). Obviously the more practice you do the better you will get and the bond angles will become easier as you do problems over and over again. Hope this helps!

[Vanessa Wiratmo 3k]

Its better to use the VSEPR model to determine bond angles because you can look at electron density and lone pairs that way. This way, its easier to visualize the atoms in their respective positions and conclude bond angles. It also great to memorize them and how lone pairs affect angles.

[Raizel Ferrer 1H]

I usually have to reference to the VSEPR model to determine the bond angles of a molecule when given a lewis structure to the find the molecule's electron arrangement and to find the shape of the molecule to visualize how spaced apart the lone pairs are.

[Kavya Anand 2B]

Hi, while there isn’t a set way to determine bond angles, it helps to visualize the VSEPR molecular structures and understand that the electron domains will have repulsion and therefore want to be as far away from each other as possible.

[505784197]

I think for some it can be quite arbitrary. For example, a trigonal pyramid is just less than 109.5 because the lone pairs push the rest of the bonding pairs a bit lower because of electron-electron repulsion.