Bond angle and shape

[Irene Kang 3F]

Can anyone explain what exactly affects the bond angle and how we are supposed to know the new shape and angles?

[VeronicaShepherd3B]

Bond angles are determined but the repulsion between atoms or groups that are connected to a central atom. We can use VSEPR shape to determine the angles and the lewis dot structure.

[Kaethe Zappacosta 2L]

Hi! Bond angles are memorized values, unless an atom is replaced with a lone pair. For tetrahedral, for example, if one of the atoms is removed and a lone pair takes its place, the bond angle would be slightly less than 109.5, as lone pairs have more repulsion than bonded atoms. So, bond angles are determined by shape, and these values are memorized for each shape (or you can just visualize the structure), and if lone pairs exist, you know the angle values will be slightly off.
Hope this helps!

[emily3L]

On top of the information provided by these responses, I think that it is also important to note the order of repulsion strength:

lone - lone pair > lone - bonding pair > bonding - bonding pair

[Shreya Arcot 3K]

Bond angles are determined by the number of lone pairs on the central atom. Lone pairs cause repulsion, which also impacts bond angles.

For example, the tetrahedral shape (with 0 lone pairs) has a bond angle of 109.5 degrees. When one of these bonding regions (out of 4) is a lone pair, the bond angle is slightly less than 109.5 degrees. When 2 of these bonding regions (out of 4) is a lone pair, the bond angle is even lower.

[005493723]

Bond angles are determined by the shape of a molecule, and I think you have to memorize the few main ones but you can also kind of reason it out based on what you know about angles, shapes, and electrons (ex: a straight line is 180 so anything slightly bent will be just under that). A tetrahedral with no lone pairs has angles of 109.5 degrees, but when there is a lone pair it is slightly less than 109.5 as electrons repel.

[Jessica Arcos 2H]

Lone pairs can change bond angles because of repulsion, causing some bond angles in the molecule to get smaller. For example, in a linear molecule, the bond angles are 180 but when there's a lone pair added to the central atom, the bond angle decreases. Making the bond angle go from 180 to <120. To determine the new shape of the molecule you would have to know the VSEPR shapes.

[Arad_Badiee_2B]

For instance, when looking at a tetrahedral vs trigonal pyramid structure, the whole reason that the tetrahedral is 109.5 and the trigonal pyramid is <109.5 is that the lone pairs are more repulsive than the bonding pairs.

[Connor_Olsen_2K]

Bond angle is determined by the amount of bonds and lone pairs on a central atom. It has to be memorized, but can be inferred when thinking in the 3d plane. The bonds will repel as far away from each other as possible, meaning the bond angles are easily predictable.

[Anna Guan]

Bond angles are something you memorize based on which VSEPR structure it is. You determine the VSEPR structure by looking at the Lewis Structure and how many atoms or lone pairs surround the central atom to see what 3D shape it might make.