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Why is boron trifluoride a trigonar planar and phosphorus triflouride a trigonar pyramidal?
Phosphorus trifluoride, ##PF_3##, has a trigonal pyramidal 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 for ##BF_3## looks like this
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 - trigonal planar.
Now look at the of ##PF_3##.
This time, a lone pair of electrons is present on the central atom. According to , the electron geometry will be tetrahedral
and the molecular geometry, the ones that does not include the lone pair of electrons, will be trigonal pyramidal.