Allotropes

The soft graphite in a pencil and the diamond, the hardest gemstone on Earth, are made of just one and the same element: carbon (C). The basic ingredient is exactly the same. Yet one crumbles onto paper under the pressure of your fingers, while the other is hard enough to scratch glass.

The reason for such a dramatic difference is how the atoms hold hands with each other. In graphite, carbon atoms form flat sheets shaped like honeycombs, and these sheets are stacked in thin layers. The force holding one layer to the next is weak, so when you write, the sheets slide right off and leave a mark on the paper.

In diamond, on the other hand, each carbon atom bonds tightly with four other carbon atoms, building a rigid three-dimensional network. So even with the same element, a different arrangement of atoms gives very different properties. Substances like these, made of the same element but with different structures and properties, are called allotropes.

Allotropes aren't something special to carbon. The oxygen gas (O₂) we breathe in every moment and the ozone (O₃) that blocks harmful ultraviolet light in the stratosphere are also classic allotropes of the element oxygen. When two oxygen atoms bond, you get the oxygen gas that life depends on. When three clump together, you get ozone, which has a sharp smell and is highly reactive.

The carbon family of allotropes also plays a starring role in modern high technology. Graphene, a single layer of carbon peeled from graphite, is hundreds of thousands of times thinner than paper. A flawless single layer is known to be about 200 times stronger than steel of the same thickness, and it conducts electricity very well.

Fullerenes, such as C60, in which 60 carbon atoms clump into a soccer-ball shape, and carbon nanotubes, which are rolled into long, tiny straws, also belong to the carbon allotrope family. Depending on how atoms are put together, amazing new materials that no one imagined keep being born.

Because the names sound alike, many people mix up "allotropes," "isotopes," and "isomers." But in science, the three work at completely different levels.

Allotropes are differences in how atoms are arranged and structured when many atoms come together to form a substance. Isotopes, on the other hand, are about the inside of a single atom. They have the same number of protons, so their chemical properties are the same, but they have different numbers of neutrons in the nucleus, so their masses differ.

Finally, isomers are compounds made of several kinds of elements, where the order in which atoms are connected, or their arrangement in space, is different. So here's how to keep them straight: allotropes is a special name that can only be used for a substance made of just one kind of element.