The boron trifluoride molecule
The boron trifluoride molecule is depicted as havingthree single bonds and an electron-deficient central boron atom.Use the concept of formal charge to suggest why a structureinvolving a double bond to one fluorine, which would provide anoctet to the boron, is not favored.
Answer:
If we add one double bond between boron and one of the fluorineswe get the following Lewis Structure
The fluorine that shares a doublebond with boron has six electrons around it (four from its two lonepairs of electrons and one each from its two bonds with boron).This is one less electron than the number of valence electrons itwould have naturally (Group seven elements have seven valenceelectrons), so it has a formal charge of +1. The two flourines thatshare single bonds with boron have seven electrons around them (sixfrom their three lone pairs and one from their single bonds withboron). This is the same amount as the number of valence electronsthey would have on their own, so they both have a formal charge ofzero. Finally, boron has four electrons around it (one from each ofits four bonds shared with fluorine). This is one more electronthan the number of valence electrons that boron would have on itsown, and as such boron has a formal charge of -1.
This structure is supported by thefact that the experimentally determined bond length of the boron tofluorine bonds in BF3 is less than what would be typicalfor a single bond. However, this structure contradicts one of themajor rules of formal charges: Negative formal charges are supposedto be found on the more electronegative atom(s) in a bond, but inthe structure depicted in Figure, a positive formal chargeis found on fluorine, which not only is the most electronegativeelement in the structure, but the most electronegative element inthe entire periodic table (χ=4.0). Boron on the otherhand, with the much lower electronegativity of 2.0, has thenegative formal charge in this structure. This formalcharge-electronegativity disagreement makes this double-bondedstructure impossible.
However the large electronegativitydifference here, as opposed to in BH3, signifiessignificant polar bonds between boron and fluorine, which meansthere is a high ionic character to this molecule. This suggests thepossibility of a semi-ionic structure such as seen in Figure
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