The Student Room Group
Competition - post a photo you've taken of nature >>
Competition - post a photo you've taken of nature >>
Competition - post a photo you've taken of nature >>

Chem help: how do I determine the bond angle of a molecule?

I know the bond angles of molecules for a certain amount of electron pairs that don't involve lone pairs, however, when it comes to determine the bond angles INCLUDING the lone pairs I start to get confused.

So far I know that for each lone pair (in a tetrahedral arrangement) I have to subtract by 2.5 degrees. But what about for 3, 5 and 6 electron pairs?
Original post by funkybinoculars
I know the bond angles of molecules for a certain amount of electron pairs that don't involve lone pairs, however, when it comes to determine the bond angles INCLUDING the lone pairs I start to get confused.

So far I know that for each lone pair (in a tetrahedral arrangement) I have to subtract by 2.5 degrees. But what about for 3, 5 and 6 electron pairs?


http://www.chemguide.co.uk/atoms/bonding/shapes.html
Reply 2
Avatar for derpz
derpz
5
Original post by funkybinoculars
I know the bond angles of molecules for a certain amount of electron pairs that don't involve lone pairs, however, when it comes to determine the bond angles INCLUDING the lone pairs I start to get confused.

So far I know that for each lone pair (in a tetrahedral arrangement) I have to subtract by 2.5 degrees. But what about for 3, 5 and 6 electron pairs?


You should know how to work out the actual shape first:

1) Get the group number of the primary element
2) If its a negative charge, add that to the group number from 1) or subtract the positive charge
3) Add the bonded pairs
4) Divide by 2 to get the bonded pairs
5) Now subtract the bonded pairs by the number of bonds to find the loan pairs.



As for the bond angles, you just need to remember each bond angle along with the shape.

Also, remember that bonded pairss repel the most, then its bonded and lone pairs and lone pairs repel each other the least which is the cause of the specific bond angle (sometimes asked in exams)

If the first part doesnt make sense let me know and I'll give an example:smile:
Reply 3
Avatar for rosh2004
rosh2004
4
You will have to remember the angles. calculating it is not that easy.. just check the syllabus, take a few minutes to learn the angles for the molecles or arrangement of molecules mentioned in syllabus and solve some topical pastpapers for them. :smile: This will hardly take an hour
Original post by derpz
You should know how to work out the actual shape first:

1) Get the group number of the primary element
2) If its a negative charge, add that to the group number from 1) or subtract the positive charge
3) Add the bonded pairs
4) Divide by 2 to get the bonded pairs
5) Now subtract the bonded pairs by the number of bonds to find the loan pairs.




As for the bond angles, you just need to remember each bond angle along with the shape.

Also, remember that bonded pairss repel the most, then its bonded and lone pairs and lone pairs repel each other the least which is the cause of the specific bond angle (sometimes asked in exams)

If the first part doesnt make sense let me know and I'll give an example:smile:


Hey, thank you.

I'm just confused because in the CGP book it says SO2 has a bond angle of 119 degrees, instead of 120 degrees. I understand why it is less (because of the lone pair). however, will I need to know how to calculate this? Or is the bond angle always 119 degrees if there's just one lone pair within the 3 electron pairs around the central atom? Additionally, what would be the angle if there was 2 lone pairs instead? Am I making sense? :s-smilie:
Reply 5
Avatar for derpz
derpz
5
Original post by funkybinoculars
Hey, thank you.

I'm just confused because in the CGP book it says SO2 has a bond angle of 119 degrees, instead of 120 degrees. I understand why it is less (because of the lone pair). however, will I need to know how to calculate this? Or is the bond angle always 119 degrees if there's just one lone pair within the 3 electron pairs around the central atom? Additionally, what would be the angle if there was 2 lone pairs instead? Am I making sense? :s-smilie:


What exam board aare you doing, AQA (AS chem)?
Original post by derpz
What exam board aare you doing, AQA (AS chem)?


Edexcel AS :frown:
Reply 7
Avatar for derpz
derpz
5
Original post by funkybinoculars
Edexcel AS :frown:


Check the spec or just check with your teacher, for AQA you need to learn that method but Im unsure about Edexcel
Original post by derpz
Check the spec or just check with your teacher, for AQA you need to learn that method but Im unsure about Edexcel


ha ha yeP i looked at the spec and I think I was only meant to know it. Thank you for the help though :smile:

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you