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The Sound of Science - "Hard as Diamond"


K: I’m Kate Powers from NIU STEM Outreach and I’m in the studio with Jeremy Benson. This is the Sound of Science on WNIJ.

J: Kate, we have a question today from Sara. She wants to know why diamonds are so hard?

K: Sara’s right, diamonds are one of the hardest substances on earth. They are very important for industrial tools and scientific instruments.

J: Wait, that must be very expensive to make a tool out of diamonds!

K: Well, it is but not as expensive as you think. Diamond tools use the very smallest and lowest quality diamonds. You probably wouldn’t even recognize them as diamonds. But the reason that diamonds are so useful in tools is because of how hard they are. 

J: I knew about that but have always been confused about why that is. Aren’t diamonds made up of carbon? Aren’t we mostly made up of carbon? And isn’t pencil lead, graphite, made up of carbon?! None of these things seem like the other!

K: You’re right! Carbon is a very versatile atom! In our body carbon is bonded with all sorts of other elements like hydrogen, nitrogen, and oxygen. These atoms form many types of compounds and none of those compounds are hard. But diamond is just carbon, no other types of elements are present.

J: I thought graphite is also just carbon. But I don’t think my girlfriend would be impressed with pencil tip earrings for her birthday.

K: Yeah, I can’t recommend that Jeremy. Graphite is only carbon, but it is very different from diamond. First off let’s talk about carbon-carbon chemical bonds; they are very, very strong. In graphite and diamond, carbon likes to form six membered rings that are all linked up together - like a honeycomb structure. In graphite, large numbers of carbon atoms form honeycomb like sheets. Millions of the sheets stack up on one another. This give a hard substance that feels very slippery. 

J: Oh, that’s why a pencil slips across a notebook page, it is leaving behind tiny sheets of carbon?

K: That’s right. But with diamond, these sheets of carbon are put under very high pressure underground for a very long time. The high pressure lowers the amount of energy needed to form even more carbon-carbon chemical bonds. More bonds are formed between the honeycomb sheets, creating a strong lattice of honeycomb shaped carbon-carbon bonds. Each carbon atom is strongly interacting with all of its neighbors, creating an impenetrable network of carbon atoms. This complete and perfect lattice is what makes diamonds so hard.

J: That’s really interesting. Thanks for the answer, Kate!

K: Keep the questions coming to stemoutreach@niu.edu. This has been the Sound of Science on WNIJ.

J: Where you learn something new every day.

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