Shapes trigonal

Boron trifluoride has a plane trigonal shape a 2p orbital on each fluorine atom overlaps with a boron sp2 hybrid. In general, we can expect that all molecules in which a central atom uses three equivalent sp2 hybrid orbitals will exhibit plane trigonal geometry, since this represents the most symmetrical, and hence equivalent , arrangement of the three bonds. 

The polarisation along the bond is characterised by both its magnitude and its direction, i.e. it is a vector. This may be illustrated by considering a molecule of boron trichloride, which has a symmetrical planar trigonal shape. First, consider an isolated B-Cl bond and determine the direction of polarisation,... 

Surrounding boron are three space occupying groups, the three fluorines. Boron does not have an octet of electrons. Therefore it assumes a trigonal shape and is sp2 hybridized. 

Make molecular models of ethane and ethene. Notice the tetrahedral shape and 109° bond angles around each carbon of ethane and the trigonal shape and 120° bond angles around each carbon of ethene. Also notice that you can rotate the single bond of ethane but not the double bond of ethene. 

Hybridization and symmetry are intimately connected because of the relationships seen above Atomic centers in molecules that have certain hybridization have specific shape. Atoms that make sp hybrid orbitals impart a linear shape about that atomic center (which might suggest Coov or D h point groups, for simple molecules). Atoms that have sp hybrid orbitals make bonds in a threefold, or trigonal, shape. 

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