Covalent structures and bonding
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Why is boron trifluoride (BF3) stable with only three covalent bonds and hence only 6 electrons in its outer shell? Why doesn't it form BF4?
Igloo writes ...
There are only three outermost electrons in the boron atom.
Let’s consider the two possible types of bond which boron might form with fluorine.
First the possibility of ionic bonding. In order to form the boron ion, B3+, enormous amounts of energy would be needed since there are as many as three electrons to remove, and they are all relatively close to the nucleus, since boron is such a small atom. You will find from a data book that the three ionisation energies add up to almost 7000 kJ mol-1 (compare this with the 500 kJ mol-1 needed for a sodium atom). Even though fluorine is a highly electronegative non metal, three moles of fluorine atoms are simply incapable of withdrawing as much as 7000 kJ from one mole of boron atoms to form boron ions. Therefore ionic bonding is impossible for BF3.
Next, consider covalent bonding. Boron only has three outer electrons so the maximum number of covalent bonds it can form must be three. As you say, the covalent structure for BF3 only has six electrons associated with the boron atom, rather than the usual number of eight, but remember this: whenever bonds are created, energy is released, and in this case, although energy is needed to break the covalent bonds in solid boron and that F2 molecules need to be atomised into F atoms before BF3 is formed, the energy released when three B-F bonds are formed is much greater. Any book of data will tell you that the enthalpy of formation of BF3 is about -1100 kJ mol-1, so this shows that BF3 is about 1100 kJ more stable than the elements from which it is formed.
The majority of covalent compounds do have a “noble gas” structure, but BF3 is an exception.
By the way, the fact that BF3 is electron deficient is the reason why it is classified as an electrophile, and it is a very reactive one too.
To sum up, we say that BF3 is a stable molecule (as compared with its elements), but that it is a highly reactive compound because of its electron deficiency.
The molecule BF4 cannot be formed because boron has only three outer electrons. However, if BF3 meets up with fluoride ions, F-, then the BF4- ion will be formed, and this has a perfectly stable structure. Try to draw a dot-and-cross diagram for this ion.
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updated: 31 January 2006
