Bond Angles

[ngarcia]

How do you know when a bond angle is "slightly less" than the expected bond angle measure?

[ahuang]

Typically shapes like bent or trigonal pyramidal have slightly less than a typical tetrahedral bond angle due to the number of electron pairs on the central atom.

[WesleyWu_1C]

Usually when there is at least one lone pair on the central atom, then the bond angle is slightly less than the expected bond angle.

[Alexa Mugol 3I]

If there are any lone pairs on the molecule, the bond angle will be slightly less because the lone pair has very strong repulsion, so it will push the surrounding atoms even more than a normal atom would. Therefore, more lone pairs --> slightly smaller bond angle.

[Julianna Laurentano 4G]

When there is a lone pair on the central atom, the bond angle is slightly less. This is because the lone pair of electrons on the central atom forces the bonding electrons closer together, therefore distorting the angle.

[madijohnson_4A]

As a side note, if there are multiple bond angles within the shape (like 120 and 90 degree angles within a trigonal bipyramidal molecule), we don't have to indicate which angles go where, we can just say the angles will be less than 120 and less than 90 for a shape like seesaw with 5 regions of electron density and a lone pair.

[Melvin Reputana 1L]

I would also like to add on that the presence of lone pairs creates repulsion forces between electrons. There are repulsion forces already present in the molecule, between the bonding pairs of electrons of the atoms. However, lone pairs express stronger repulsion forces. This greater force is what pushes the atoms away from the lone, causing neighboring atoms to become closer to each other which decreases the bond angle.