[G Campbell]

1. Boron triflouride, and phosphorous triflouride have the same general formula AX₃ but boron triflouride is trigonal planer in shape with bond angles of 120^o and phosphorous triflouride is pryramidal in shape with a bond angle of 96^o.

Use the VSEPR to explain these differences.

[charco]

check this out

http://www.ibchem.com/IB/ibc/bonding/bon_htm/14.1.htm

then try to answer the problem

[Grumples]

As a general rule, if you take the sum of the valence electrons on the central atom and the total number of e- the other atoms need to make a full octet, then divide by two, then you get the number of e- pairs around the central atom.  Here's the formula in a clearer form:

(# valence e- on central atom) + (total # of e- needed by other atoms for a full octet)
                                                        2

for instance, in ammonia, NH₃, the N has 5 valence electrons, and each of the hydrogens needs one more e- for a full octet.  Therefore, (5+3)/2= 4,  there are four pairs of electrons around the N.  since there are only three things bonded, that means that one  of those pairs is a non-bonding pair.  Use this formula for the two molecules you suggested, and you will find your answer.  Remember, a non-bonding pair of electrons effects the shape just as much as a bonding pair would.  

[Donaldson Tan]

in BF3, there are 3 bonding pairs attached to the central boron atom. By VSEPR, the boron atom must be sp2 hybridised and its electron pair geometry is trigonal planar. Given all bonding groups are identical, then the inter-group repulsion is the same magnitude, then the bond angle for BF3 will be the ideal trigonal planar angle of 120degrees.

for PF3, there are 3 bonding pairs and one lone pair on the central P atom. By VSEPR. the P atom must be sp3 hybridised and its electron pair geometry is tetrahedral. Since the lone pair is "invisible", the molecular shape would be trigonal pyramidal. The ideal tetrahedral angle is 109.5 degree. However, F being very electronegative, withdraws significant electron density within the P-F bond towards itself. This reduces the repulsion between each fluoro-group. However, the lone pair continue to exert electronic repulsion on the three fluoro-group significantly, in virtue of the short P-F bond length. As a result, PF3 is less 'squashed' compared to the ideal case of a perfectly tetrahedral molecule.The bond angle will be less than 109.5 degree.

[G Campbell]

Thanks for your help.