Question

Question #e029f

Answer 1

Phosphorus trifluoride, `PF_3`, has a trigonal pyramidal molecular geometry because of the lone pair of electrons present on the central atom.

By comparison, boron trifluoride, `BF_3`, is trigonal planar because there are no lone pairs of electrons present on the central atom.

To get a better understanding of why this is happening you must take a look at each molecule's electron geometry, which takes into account any lone pairs present on the central atom.

The Lewis structure for `BF_3` looks like this

![http://www.tutor-homework.com/Chemistry_Help/Molecular_Geometry/052_Boron_Trifluoride_BF3.html]()

Notice that boron forms three bonds with the three fluorine atoms and has no lone pairs. This means that its electron geometry will be the same as its molecular geometry - trigonal planar.

Now look at the Lewis structure of `PF_3`.

![https://prhschemistry.wordpress.com/2012/12/18/vsepr-theory-gumdrop-shapes/]()

This time, a lone pair of electrons is present on the central atom. According to VSEPR Theory, the electron geometry will be tetrahedral

![http://www.chemistryland.com/CHM151W/08-Bonds/VSEPR/VSEPR.html]()

and the molecular geometry, the ones that does not include the lone pair of electrons, will be trigonal pyramidal.

![http://www.webelements.com/compounds/phosphorus/phosphorus_trifluoride.html]()