The Foundations of Chemistry

The Lewis formula for BF 3 is derived from the following: (a) each B atom has three
electrons in its valence shell and (b) each B atom is bonded to three F (or Cl, Br, I) atoms.
In Example 7-6 we wrote the Lewis formula for BCl 3. Both F and Cl are members of
Group VIIA, and so the Lewis formulas for BF 3 and BCl 3 should be similar.
We see that BF 3 and other similar molecules have a central element that does notsatisfy
the octet rule. Boron shares only six electrons.

B. VSEPR Theory

Boron, the central atom, has three regions of high electron density (three bonded atoms,
no lone pairs on B). The VSEPR theory predicts trigonal planarelectronic geometryfor
molecules such as BF 3 because this structure gives maximum separation among the three
regions of high electron density. There are no lone pairs of electrons associated with the
boron atom, so a fluorine atom is at each corner of the equilateral triangle, and the mole-
cular geometryis also trigonal planar. The maximum separation of any three items (electron
pairs) around a fourth item (B atom) is at 120° angles in a single plane. All four atoms are
in the same plane. The three F atoms are at the corners of an equilateral triangle, with
the B atom in the center. The structures of BCl 3 , BBr 3 , and BI 3 are similar.
Examination of the bond dipoles of BF 3 shows that the electronegativity difference (see
Table 6-3) is very large (2.0 units) and that the bonds are very polar.

However, the three bond dipoles are symmetrical, so they cancel to give a nonpolar mole-
cule.

C. Valence Bond Theory

To be consistent with experimental findings and the predictions of the VSEPR theory, the
VB theory must explain three equivalentBXF bonds. Again we use the idea of hybridiza-
tion. Now the 2sorbital and two of the 2porbitals of B hybridize to form a set of three
equivalent sp^2 hybrid orbitals.

Three sp^2 hybrid orbitals point toward the corners of an equilateral triangle:

2 p

sp^2

2 p

2 s

B[He] B[He]

hybridize

B F

2.0 4.0

2.0



(EN) 

EN

FF

F

B

Net molecular dipole  0

A model of BF 3 , a trigonal planar

AB 3 molecule.

8-6 Trigonal Planar Electronic Geometry: AB 3 Species (No Lone Pairs of Electrons on A) 317

B

2 s

++

2 p

2 p

or

three sp^2

hybrid

orbitals

B B

120 °

B

B

Trigonal planar geometry is sometimes

called plane triangularor, simply,

triangular.

The B^3 ion is so small (radius

0.20 Å) that boron does not form

simple ionic compounds.