Composites and the Periodic Table

So, why am I writing about the Periodic Table? I needed to bring this back to the basics a little bit and talk about where composites come from and why we have found them to be such useful materials – basically what’s all the fuss about.

First a little chemistry lesson to get us started. When Dmitry Mendeleev put together his arrangement of what were the “known elements” in 1864, he arranged them by Atomic Number which is the number of protons in the nucleus of each atom of each element. Since that time more elements have been discovered, but Mendeleev’s original table and the “Periodic Law” that he came up with have remained the same. Interestingly, in Mendeleev’s table it becomes apparent that Atomic Number closely follows Atomic Weight. And we have found that Atomic Weight fairly closely follows density – especially for those elements that are solids at room temperature. Carbon and Silicon as pure substances are fairly light weight, whereas Iron (Fe) and Lead (Pb) are quite heavy.

What does this mean? The most common elements that are used to make composite materials are Carbon, Hydrogen, Oxygen, Nitrogen, and Silicon, all of which are in the first three rows of the Periodic Table. This means that they are all elements with a low Atomic Number and a low Atomic Weight, so solids and liquids that are made of these elements are in general light weight substances. This is especially true of Carbon which is the most useful and most highly used element in composites. There are Carbon fibers of course, but all resins are organic compounds which are all compounds of Carbon, Hydrogen, Oxygen, and Nitrogen – with a few other things sprinkled into the mix from time to time.

And now a little history of the universe for you. The first three elements made just after the Big Bang were Hydrogen, Helium, and Carbon – in that order. And, Carbon is the fourth most abundant element in the Universe behind Hydrogen, Helium, and Oxygen. So, there’s lots of Carbon around, and when planets form from the intergalactic dust there is quite a bit of carbon that ends up in the mix of stuff that makes up a planet.

Carbon is also the stuff of Organic Chemistry, and is the backbone of life itself. This is partly because of the abundance of Carbon around, but it is also because of the unique chemistry of Carbon. Carbon sits right in the middle of the second row of Mendeleev’s table which means that it has 4 “Valence Electrons”. There are a potential 8 valence electrons in that row with Lithium having one valence electron and Florine having 7. Both of these elements are very chemically reactive – lithium is more than happy to give up its one electron, and Florine is desperately seeking an electron to fill out its electron shell. The reasons that this is true have to do with energy levels and some atomic physics that we don’t need to get into here.

So, with Carbon in the middle of this row, it has 4 valence electrons, and can make a chemical bond with up to 4 different atoms. Methane (natural gas) is one Carbon and 4 Hydrogens and is a very stable compound. It is also one of the worst greenhouse gases because it is lighter than air and sticks around in the upper atmosphere as long as CO2 or longer. Carbon also makes covalent bonds when it forms compounds which are a stronger bond type than ionic bonds (think table salt – when it dissolves in water the sodium and chlorine dissociate and become ions floating around in the water).

Carbon also bonds to itself, and one of the more common forms of pure Carbon is a structure with Carbon atoms arranged neatly in layers of 6 Carbon ring structures. This is Graphite – the stuff of your pencil lead – literally “rock that writes”. And this 6-Carbon ring structure is the backbone of all biology on this planet, and also forms the backbone of both Carbon Fiber and all of the resins that are used to make composites. That’s why I make such a big deal out of Carbon whenever I talk about composites.

In an upcoming post I’m going to talk a bit about the glues (resins) in composites, what they are made out of and why you might see some of the same names in the food you eat. All of it is organic chemistry which is really the chemistry of life.