[icemann]

Hello, I have a question about single vs. double bonding.  I am just beginning a chemistry course.  My text has an example of nitrate ion NO₃^-.  I am using dot diagrams to draw structural diagrams.  My text says that if the octet of the central atom is incomplete, move a lone pair of electrons from a peripheral atom between the central and peripheral atoms.  Therefore there is a double bond.  I understand this.  The next chapter asks me to explain why BF₃ is an exception to the octet rule, it is because the central atom only has 6 surrounding electrons, the single bonds.  My question is why, in this case, I wouldn't move an electron pair from F atom to create a double bond between one of the F atoms and the central B atom?  This would complete the central octet. Is it because the electronegativity difference between oxygen and nitrogen is much less than that of flourine and boron?

[rabolisk]

Kind of. In BF₃, if we moved a lone pair to create a double bond, you would get a full octet for all atoms. However, the formal charge on fluorine would now be +1 and on boron would be -1, which will not happen due to electronegativity. Basically, boron is fine with an incomplete octet, and fluorine will never form a double bond.

[icemann]

Thank you Rabolisk, I understand why now.  My textbook did not mention formal charge...I havn't taken Chemistry for a number of years so I suppose I forgot about it.