Covalent structures and bonding
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Why does phosphorus fluoride have 10 outer electrons? Is it due to the s,p,d configuration?
Phosphorus(V) Fluoride, PF5
In the case of PF5, there are indeed 10 outermost electrons, and this appears to contradict all that you have been taught about the need for atoms to attain a “noble gas” structure. The fact is that most molecules of elements in the first two periods do contain atoms with “noble gas” structures, but by no means all. A more advanced idea is that atoms attempt to achieve a more stable electronic structure, of which the noble gas structure, s2p6, is the commonest.
Many elements in the third period, e.g. phosphorus and sulphur, can form compounds in which the “octet” is increased to 10 or 12, for example PF5, PCl5, SF6, and since this is not shown by the elements in the period above (nitrogen and oxygen) you can see that this is possibly to do with the fact that d orbitals are available in shell 3, but not in shell 2.
You will find a really excellent article written and illustrated by Jim Clark. Read through all that he has to say, and, towards the end of the article you will find a full explanation of why phosphorus is able to expand the octet and form PCl5 (the arguments for the existence of PF5 are exactly the same)
Knowledge of all of this chemistry is possibly outside your A-level specification, but it could help to improve an answer in an A-level script, as well as satisfy your curiosity.
Risk assessment
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For further guidance see our tutorial on Risk Assessment.
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updated: 30 January 2006
