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Dec 27

Great Show But Was the Chemistry Quite Right?

by Mark | Categories: general chemistry, Uncategorized | (51470 Views)

A guest blog post by Dr Nigel Young, an inorganic chemist at the University of Hull.

I have just watched the first of the 2012 RI Christmas Lectures entitled “Air: The Elixir of Life”  by Dr Peter Wothers on BBC4. Whilst there were many excellent demonstrations and explanations (some of which I may make use of) and it is very good to see chemistry portrayed in an enthusiastic manner I am a little concerned about the way that chemical bonding was dealt with which was confusing, and almost certainly incorrect. He had a terrific contraption representing a diatomic molecule, and by adding electrons into a holder between the atoms they got pulled together to represent bonding orbitals. When electrons were put into anti-bonding holders the atoms moved further apart. This all sounds fine, but the alarm bells began to ring  when the consequences of considering that there are both 2s and 2p electrons/orbitals involved in the bonding were ignored, and especially the fact that the bonding and anti-bonding orbitals derived from the 2s set are filled before getting to the 2p ones. The approach used in the programme predicts that Be2 has a considerable bond energy/strength as it has four bonding electrons, whereas experimentally and computationally it is very weakly bound if at all, and this is because it has an equal number of bonding and anti-bonding electrons. It was also stated that C2 has the strongest bond energy, whereas amongst the diatomics this is found for N2. He seemed to show a chart of increasing bond energy/strength from Li, through Be, B to C and then decreasing to Ne,  but this does not seem to correlate with other available data. The high carbon bond energy was then used to explain the hardness of diamond, but what about the slipperiness of graphite? Simplifying concepts is certainly necessary in these sort of activities, but presenting an inaccurate picture and more importantly an erroneous prediction of bond strengths seems less than ideal. It seems to me that this has suffered from the over use of hybridisation (which in general is a bad way to explain bonding) and  lack of understanding of molecular orbital theory (which is a much better way to explain bonding, although admittedly there is the complication of explaining the difference between the ordering in O2 and N2) which has been compounded by extrapolating from diatomics to solid state compounds.

Did no one else spot this?

UPDATE. Posted by Mark:

Peter Wothers has very kindly taken the time to explain his thinking behind his model.

Having come up with this idea of a model, perhaps I should clarify.

What I was trying to get across was the idea of bonding orbitals and antibonding orbitals. The confusion comes from the model suggesting that it ONLY refers to diatomics whereas the data in the chart is for essentially the enthalpy of atomisation of the period two elements in their standard states. The graph used real data which clearly shows carbon with the greatest enthalpy of atomisation. (As an aside, let’s not forget graphite is not slippery in a vacuum!)

Of course the simplified diagrams given in the diagrams quoted are approximately correct for diatomics, but the picture is more complicated for solid states when band theory would provide a better starting point. However, whether in solids or diatomics, the idea that some electrons help to bond atoms together whilst some actively pull them apart is a key one which the model tried to illustrate. This is not an idea commonly met with at schools.

Personally, I would not go into hybrid orbitals, but in order to understand the bonding in the standard state of the elements lithium to fluorine, we should use the 2s orbitals and three 2p orbitals. The maximum bonding is reached for carbon since each carbon atom supplies enough electrons (4) to the bulk to fill completely the bonding levels this was what the bonding model was meant to show.  Sadly, this clearly misled since some people thought it was talking about the bonding in C2 molecules.

I accept that the model is not perfect (how could it be?) but I personally thought it better than “dot and cross” diagrams and might introduce young students to a new idea.

Sorry if it’s confused people instead.

Peter

6 comments

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  1. Peter Wothers

    Having come up with this idea of a model, perhaps I should clarify. What I was trying to get across was the idea of bonding orbitals and antibonding orbitals. The confusion comes from the model suggesting that it only refers to diatomics whereas the data in the chart is for essentially the enthalpy of atomisations of the period two elements in their standard states. I’m afraid the graph used real data which clearly shows carbon with the greatest enthalpy of atomisation. Of course the simplifed diagrams above are approximately correct for diatomics, but the picture is more complicated for solid states when band theory would provide a better starting point. However, (solids and diatomics) the idea that some electrons help to bond atoms together whilst some actively pull them apart is a key one which the model tried to illustrate. I accept that it’s not perfect (how could it be?) but I thought it better than “dot and cross” and might introduce young students to a new idea.

  2. Nigel Young

    Peter,

    Thanks for the clarification. I got led down the wrong path by the sight of your pairs of volunteers putting the requisite number of electrons into what looked like a diatomic molecule and this was reinforced by having recently done MO diagrams of the diatomics in first year workshops.

    I certainly applaud you for trying to introduce the idea of bonding and anti-bonding electrons. I guess the problem is how to deal with the bonding in Li to Ne in about 5 minutes for 12-16 year olds whilst still maintaining their attention. This is made all the more complicated by the need to use a simple/single thermodymanic parameter to explain the subtle and not so subtle changes in structural motifs as one goes across the elements. Explaining this could be the subject of a whole lecture course!

    The bonding machine was certainly a very good try and much better than abstract dots and crosses. If it is going to have another outing would it be worth showing/explaining/animating more explicitly how the whizzing electrons you had in your graphics can be thought to have shapes in the form of orbitals in the atoms and that when combined these either result in electrons between the nuclei that act as the bonding glue, or result in the electrons on the outside, with therefore no glue to hold the positive nuclei together? This might also help to dispel the solar system model of atomic structure that seems to be very embedded.

    Keep up the good work!

    Best wishes

    Nigel Young

  3. dan @ Free Chemistry Online

    Thanks Peter for the clarification, and thanks also to Nigel for pointing out the seeming confusion.

  4. anon

    Anyone know how to access the lecture in the US? Would love to watch it, but it seems to be UK only

  5. Mark

    It may be available on the ri channel in the near future. http://www.richannel.org/

  6. Mark

    The Christmas lectures are now available on the RI channel where they should be accessible by everyone.

    http://www.richannel.org/the-modern-alchemist-air

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