Covalent Networks
| Boron, carbon and silicon form networks rather than molecules. You should commit that fact to memory. |  |
| Look at the electron dot diagram for carbon. (This sequnce is also shown in example 22.) |
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| We can show the bond between this atom and another carbon in this way. But notice that this doesn't satisfy the octet rule for this first carbon atom. |
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| To do that we'd need another carbon atom and another and another. Then each of those carbon atoms would need another and another and another and another and another and so on. This pattern, which is shown here in two dimensions, actually exists in three dimensions. |
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| It is the diamond form of carbon, which is shown here. This model is also in the lab for you to look at when you are there. It has the same pattern of atoms as the diagram above, except that two colors are used to reperesent the carbon atoms. |
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| However, if you start taking a look at the different dimensions of this thing, you can see that quite a different set of patterns arise. The reason for the different colors in this model is to emphasize certain bonding patterns. You can see that there are planes or layers of atoms. |  |
| In the orientation shown to the right, you can see that the light ones are a little bit higher than the black ones. So you can see that it is a matter of position, rather than types of atoms. But that gives you a picture of what the diamond arrangement is like. Notice that each carbon atom is bonded to four other carbon atoms. |  |
| Another form of carbon is graphite and its arrangement can be envisioned by taking the puckered planes of atoms from the diamond form and making them flat. If you do that, then you end up with the graphite form of carbon. The atoms are arranged slightly differently. From the side you can see that the graphite has planes of carbon atoms that are somewhat more distant from the next plane than was the case with diamond. That is what gives graphite its slippery quality. This plane of atoms can slide on this plane of atoms because they are not quite so strongly bonded. |  |
| By changing the angle of view, you can see a hexagonal pattern. From this angle, it looks very much like the diamond. These are the two most common forms of carbon. Diamond and graphite. |  |
| Another form of the element carbon that is not as common but is making news are the Buckminsterfullerenes, also called Bucky balls, which contain clusters of carbon atoms. This particular model emphasizes the bonds. Each point where the bonds come together represents a carbon atom. These Bucky balls each contain about sixty carbon atoms, some contain 60, some 70, some 72. This is a form of carbon that has been making the news during the past several years. |  |
Models of these different forms of carbon are available for you to look at when you are in the lab.
Silicon has essentially the same bonding pattern as diamond. Diamond, graphite and silicon are important enough materials that you should specifically remember that they are covalent network materials.
Comparing Molecules and Networks
Another common covalent network material is the compound silicon dioxide, SiO2, also known as quartz. It is a covalent network material even though its formula is very similar to that of carbon dioxide which is a covalent molecular material. Why is the bonding arrangement so different?
| Take another look at carbon dioxide, CO2, which we've talked about before. (It is also shown in example 24-a.) Because a carbon atom is smaller and has a greater pull on electrons than a silicon atom, its four electrons can be concentrated into two double bonds to the oxygen atoms. | ··
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| Because silicon is larger and has less pull on its electrons, its electrons are spread out in four single bonds with four oxygen atoms. (Also shown in example 24-b.) This leaves each oxygen atom with the ability, actually, the necessity to bond to other atoms. Each oxygen atom shown here will bond to another silicon atom (shown in example 24-c), and each silicon atom will bond to more oxygen atoms. You can imagine that the network of covalently bonded silicon and oxygen atoms can continue indefinitely. | · |
| If you look carefully at this three-dimensional model of quartz, you can
see that the bonding is quite extensive. This three-dimensional model of quartz is in the lab so that you can look at it first hand when you are in the lab. At that time, you can also see that the alignment of the atoms parallels the faces of a quartz crystal. |
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| Let me summarize a few points about covalent materials. If you have covalent bonding, you may have elements or compounds. Also, you may have either network or molecular materials. Usually it will be molecular. The only examples of network covalent bonding that you have to worry about for this course are carbon, in the form of diamond and graphite, silicon, and silicon dioxide, SiO2, which is commonly known as quartz. Any material in this course which has just covalent bonding other than graphite, diamond, silicon, or quartz, will be a molecular material. |
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E-mail instructor: Eden Francis
Clackamas Community College
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