Boron Trichloride

13 05 2012

Molecules based around a triangle shape:

A molecule with no lone pairs and 3 electron pair bonds (3 pairs of bonded electrons), the shape is planar instead of pyramidal. Plane means a flat or level surface, so planar means a flat two-dimensional shape.

Planar shape allows 120° angle between bonds, so the separation between electrons is greater than in the example we used in a previous post, ammonia (NH3), which had a trigonal pyramidal shape.

Boron trichloride, BCl3 is an example of a molecule with a trigonal planar shape. If you build a model like the one I built below you notice it forms an equilateral triangle, this is because it is made up of 3 bonded pairs of electrons, one for each chlorine atom, and there is no lone pair adding extra repulsion.

Write out the electronic configuration and dot cross structure of phosphorous trichloride (PCl3), based on this you will see how many pairs of electrons are bonded or lone. Using this information you can make a model like the one I made below:

How would you describe the shape? It has a pyramidal structure like ammonia. You can predict this from drawing out its electron configuration:

We have established pyramidal shape in previous posts, and explored trigonal in this current post, but some molecules have a trigonal bipyramid shape.

Phosphorous pentachloride (PCl5) has 5 repelling bonds. Look again at the model we made before of boron trichloride, if you add another piece through the middle with two chlorine atoms attached you see that this constructs the model of PCl5. This helps you see that it has a trigonal planar section but with an added dimension creating a trigonal bipyramidal shape.

The trigonal bipyramid shape looks like 2 pyramids joined base to base.

Sulfur tetrafluoride (SF4) and chlorine trifluoride (ClF3) are further examples of trigonal bipyramid shaped molecules. ClF3 is T-shaped with 2 ‘equatorial’ lone pairs. Try making models of these yourself.

Molecules with an octahedron shape:

Sulfur hexafluoride (SF6) has 6 repelling bonds connecting to the fluorine corners of an octahedron. There is a 90° angle between its adjacent bonds.

Draw the electronic structure of BrF5, from this we can guess that its shape would be a square pyramid.



To summarise:

3 bonds – triangular molecule

4 bonds – tetrahedral molecule

3 bonds + 1 lone pair – pyramidal molecule

2 bonds + 2 lone pairs – bent molecule

5 bonds – trigonal bipyramid

6 bonds – octahedral molecule




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