Molecule shape (1 Viewer)

[highshill]

Hi guys, I understand how to draw Lewis structure but struggle to know what shape the form for example how can you judge that a shape may be a trigonal planar or trigonal pyramid as both require 3 bonds to the central atom,
please any links to where I can practice this skill.

Thanks

 

[BeerCanJim]

I am almost certain that you don't need to know the shapes of molecules for the HSC.

The shape of the molecule doesn't only depend on the number of atoms bonded with the central atom. But also lone pairs (two unpaired electrons).
-BF3 have three fluorine bonded with bromine (no unpaired electrons). So it is a trigonal planar.
-NH3 have three hydrogen bonded with nitrogen, with one lone pairs. We treat the lone pair as an extra bonding atom when determining the structure. A central atom bond with 4 atoms is tetrahedral, one of the 'atom' is a lone pair. But in reality, we only have three bonding pairs, so the shape of the molecule is trigonal pyramidal.

Trigonal pyramidal is just tetrahedral with a lone pair, the bond angles are slightly different tho. Similarly,
Bent is tetrahedral with two lone pairs (e.g. H2O)
Bent is also trigonal planar with one lone pair (e.g. SO2)

 

[jazz519]

It's not really required in the hsc (more like uni stuff) but here is all the shapes:http://imgur.com/a/VBM0k