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structure & bonding help!

I have researched everywhere to figure what this is, but wasn't succesful... so can you guys help me :biggrin:

Explain how you can predict the shapes of, and the bond angles in, simple molecules. In your answer you should choose examples of four different molecular shapes THERE ARE 10 MARKS FOR QUESTION!

If you can answer it all great, if not then I would still like to know what you know.

"every little helps" :hifro:

Reply 1

zmzm

http://www.chemguide.co.uk/atoms/bonding/shapes.html

Bond pairs and lone electrons always 'repel to give maximum separation'.
Lone pairs of electrons repel bond pairs more strongly than bond pairs repel bond pairs.
This is also useful to explain it:
http://en.wikipedia.org/wiki/VSEPR_theory

Apart from knowing why it forms these shapes, you would probably nees to memorise the examples you need.

Reply 2

fudgesundae

Original post by translucent

I have researched everywhere to figure what this is, but wasn't succesful... so can you guys help me :biggrin:

You essentially have to look at the number of lone pairs of electrons and bonded pairs of electrons there are in a molecule. From this, you can predict the shape of a molecule, then the bond angle.

For example: BF3 has 3 bonded pairs and no lone pairs. This means the shape is trigonal planar and the bond angle is 120 degrees.

You essentially have to memorise the shape and bond angle of a molecule with a certain number of bond pairs and lone pairs.

Off the top of my head I can remember:

Linear - 180 degrees, 0 lone pairs, 2 bonded pairs (eg. CO2)
Bent/Angular - 104.5 degrees, 2 lone pairs, 2 bonded pairs (e.g. H2O)
Trigonal Planar - 120 degrees, 0 lone pairs, 3 bonded pairs (e.g. BF3)
Tetrahedral - 109.5 degrees, 0 lone pairs, 4 bonded pairs (e.g. CH4)
Trigonal Pyramid - 107 degrees, 1 lone pair, 3 bonded pairs (e.g. NH3)
Octahedral - 90 degrees, 0 lone pairs, 6 bonded pairs (e.g. SF6)

Those are the examples you need for AS.

For the actual theory behind why bond angles are what they are, you need to look up VSEPR. It states that electron pairs repel each other. But some electron pairs repel more than others. The order is:

lone pair - lone pair > lone pair - bonded pair > bonded pair - bonded pair

For the purposes of A level chemistry you need to know that a lone pair will reduce the bond angle by approximately 2.5 degrees.