Chemistry AS-Shapes of molecules!

How do you name them? Eg. how are you meant to know that PF4^+ is tetrahedral?? I really don't understand this topic. Could somebody please explain it to me? Someone said something about drawing it first???

Reply 1

Christophicus

18

*girlie*

How do you name them? Eg. how are you meant to know that PF4^+ is tetrahedral?? I really don't understand this topic. Could somebody please explain it to me? Someone said something about drawing it first???

- Count the number of lone electrons on each of the atoms in the molecule.
So, P has 5, F has 1 available space for an electron in its outer shell. There are 4 fluorine atoms, so 4*1 = 4.
- Allow for any ion charge. For example, if the ion has a 1- charge, add one more electron. For a 1+ charge, deduct an electron.
- So total number = 5 + 4 - 1 = 8
- Divide by 2 to get number of lone pairs of electrons = 8/2 = 4
- 4 bonding pairs of electron in the structure. We know this because there are 4 bonds. P-F, P-F, P-F, P-F
- Therefore there are no lone pairs of electrons since 4-4 = 0
- Therefore the shape is tetrahedral.
- When you draw PF4+, don't forget to include the + charge.

You will have to learn the shapes for the molecules in your specification. If it's edexcel, this page covers it well.
http://www.wbateman.demon.co.uk/asa2sums/sum1.3/sum1.3.htm
See parts, h and i.