Bond Angles

How do you work these out for the bond angles between the atoms of non linear molecules? Or do you just have to know them?

Reply 1

spoon1

you pretty much have to just know them really....

but you can use your common sense for some of them, for example, a molecule with two bonding pairs and two lone pairs will have a smaller bond angle than a molecules with two bonding pairs and one lone pair, because there is more repulsion between the bonding pairs and lone pairs....

Reply 2

spoon1

two bonding pairs and one lone pair.....

which, im not sure is even physically possible

Reply 3

Christophicus

spoon1

which, im not sure is even physically possible

SO2 has two double bonds and 1 lone pair. Does that count?

Reply 4

freddie88

you just have to learn them.

a non-bonding pair is less localised than a bonding pair.
a double or triple bond causes more repulsion than a single.
an electronegative ligand will attract the pair, making the bond more contracted.

and remember that mono-capped octahedrons don't exist.

Reply 5

freddie88

spoon1

which, im not sure is even physically possible

of course it is. why wouldn't it be?

Reply 6

perkyDani

in different txt bks they give different names to molecule shapes- like is h20 a tbent linear or a tetrahedral? do the lone electron pairs count as part of the shape?

Reply 7

Kiesha

perkyDani

in different txt bks they give different names to molecule shapes- like is h20 a tbent linear or a tetrahedral? do the lone electron pairs count as part of the shape?

Yep I think so but I think you have to call it "distorted tetrahedral" or whatever.

Reply 8

oxymoron

Kiesha

Yep I think so but I think you have to call it "distorted tetrahedral" or whatever.

Well, strictly H2O is based on a (slightly distorted) tetrahedral because of the 4 pairs of electrons. But it is not a tetrahedral shape because the lone pairs don't count in this. I would call it a "bent" or "non-linear" molecule

Similarly NH3 is based on a tetrahedral but should be called a "pyramidal" molecule.

CH4 on the other hand can be called tetrahedral. :smile:

Reply 9

charco

Study Forum Helper

One thing is the shape adopted by the regions of electron density and another is the actual shape of the molecule.

Questions invariably ask for the molecular shape (angles etc) in which case you must ignore the non bonding pairs ...

eg... water has its electron pairs arranged in a tetrahedral arrangement (bond angle in a perfect tetrahedron = 109.5º) but...

as two of the pairs of electrons are non-bonding they are 'invisible' and therefore don't count in the molecular geometry so...

the remaining H-O-H angle is still 109º (or actually about 104.5º due to the VSEPRT) and the molecular shape is bent, angular or non-linear (pick your preference)

For more about the shapes of molecules take a look at:
http://www.ibchem.com/IB/ibc/bonding/bon_htm/14.1.htm