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.

 

credit: chemeddl.org

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





Water H2O

10 05 2012

Water is something we can’t live without; roughly 70% of an adult human body is composed of water. I will be writing a followup post about the properties, but for now lets just focus on the molecular structure.

I made the model below to show its shape. The red centre is oxygen, the two whites are hydrogen, and the two empty red prongs represent two lone pairs.

The presence of two lone pairs means lots of repulsion between them, which pushes the two sets of bonded pairs closer together, so the two atoms of hydrogen are forced closer together to form a V shape.

Based on this we can predict the shape for hydrogen sulfide (H2S), bent for the same reasons H2O is bent, see below:

and also the oxonium ion H3O+

This oxonium ion is pyramidal as it is isoelectronic with ammonia, see previous post.

I highly recommend visiting Wolfram Alpha, simply type in a formula, name or equation and a treasure of information comes up.





Ammonia

10 05 2012

Ammonia NH3 contains 3 electron pair bonds (N-H) and one lone pair, represented on the model I made below as the empty tip of the pyramid shape. The blue centre represents Nitrogen, and the 3 white bits are Hydrogen.

The pyramidal shape is a result of the electron pairs (bonded and lone) repelling each other. Remember where the most repulsion occurs:

Most electron repulsion between:       lone pairlone pair

Strong repulsion between:                      lone pairbonded pair

Least repulsion between:                         bonded pairbonded pair

These repulsion rules explain why the bonded pairs are pushed downwards to form the base of the pyramid, they don’t want to be anywhere near the lone pair at the tip of the pyramid. But they also don’t want to be near each other, so they go as far as they can away without breaking their bonds to the nitrogen centre.

credit: wikimedia.org

What shape do you think the Ammonium ion NH4+ would have?

Ammonium is isoelectronic CH4 (Methane) so has the same shape, tetrahedral. Remember ‘iso’ in Greek means ‘equal’, so in chemistry it means ‘the same’ or ‘no change’, remember we studied Isotopes of elements in Lesson 2.

Methane (credit: wikimedia.org)

Here’s an ammonium ion I drew below:

NH3 + H+  = NH4+

Dative bond: Both electrons of a shared pair are donated by one of the bonded atoms.

In the example of ammonium this symmetrical ion has a dative bond that is hard to distinguish from its other covalent (electron sharing) bonds because of its symmetry. But as you can see there are two dots instead of dot cross, and the + sign atop the brackets surrounding the ion represents the electron the hydrogen atoms gives away. Electrons are negative so giving them away means you become more positive.

Ammonium is a positive ion so is called a cation. Here’s a model of amonium I made:

 

Methane: