If diamonds and pencils are made out of carbon, how is it that pencils can write?

“Can I get a pencil?”

“Yes of course,” and the lady at the counter hands me a cigarette.

I awkwardly resist and say “I actually do need a pencil.” She judges me a bit and hands me one, probably wondering why an adult might need this. As it turns out, ‘pencil’ is one of many words young adults use to refer to cigarettes, which is unfortunate because a pencil should just mean a pencil.

While tables and smartphones are all the fad, some of us still prefer the pencil. They are a technological marvel and very resourceful. They can help push in electrical sockets, keep your teeth fit (if you chew on them), and punch open a food packet. And of course they can help you write.

Each pencil has a black-grey core surrounded by wood. When you sharpen the pencil, the core becomes more exposed, and the pencil can write better.

We regularly use pens to write but they have a thick liquid ink inside. This is essentially a coloured liquid that flows like any other liquid, be it water or oil. Since it flows on paper and has a colour, it leaves its footprints behind as it flows, and in this way pens write. But the core of a pencil is solid, like a metal spoon. If we move a steel spoon on paper, no amount of metal spills on the paper.

How then does a pencil write?

Carbon and its phases

The core of the pencil is made of carbon — the same carbon most of us are primarily made of. Carbon is one of the most common elements on our planet. It can come in different phases and forms.

In ambient conditions, water is a liquid. When it’s hotter, it becomes vapour, a gas. When it’s cold enough, it becomes a solid called ice. These phases of matter have completely different properties. You could sit (with some inconvenience) on a block of ice but you shouldn’t risk sitting on a pool of water. Yet both are made of the same molecules of hydrogen and oxygen: H2O. The difference is in how these molecules connect to each other.

Similarly, when carbon atoms are arranged and stacked in different ways, they have completely different properties. Imagine each carbon atom has four hands. Each hand is a restless electron. If the hand forms a bond with another carbon, the atoms share the electron and it becomes calmer. In this way, if the carbon atoms arrange themselves in the form of an Egyptian pyramid-like structure, they form a diamond.

Diamonds are shiny, transparent, and the hardest material known in nature. This is why it’s often used to cut other metals. No wonder diamonds are expensive and, for no fault of their own, often confused with expressions of love. The real magic lies with the carbon in a pencil.

Graphite and sandwiches

Pencil cores are made of carbon, too, but a different form called graphite.

Unlike in diamonds, the carbons atoms in graphite are arranged in sheets, like slices of bread in a sandwich. Each layer contains carbon atoms connected to each other with three hands — that’s a very strong bond. However, the bonds between sheets are weak. Each layer in this arrangement is called graphene.

When graphene is stacked in multiple layers, it creates graphite — which is at the core of a pencil.

If you like, imagine the pencil’s core to be a tower of your favourite sandwich, containing millions of layers of bread with some cheese, patties, and/or sauce in between. If both graphite and diamond are made of carbon atoms, why can’t we write with diamonds?

Writing by sliding

When you write using a pencil, you are sliding the core of the pencil on paper. In the process, you’re forcing the carbon atoms of the pencil to slide on the atoms of the paper.

If you slide a hard material like diamond or a steel spoon on paper, the atoms of the material are so tightly bound to each other they have absolutely no interest in leaving and moving to the surface of the paper.

But something amazing happens when you slide graphite. Graphite has layers of graphene. And just as it is slightly harder to remove a slice of bread from the middle of the sandwich than from the top, sliding graphite on paper sloughs off graphene-like layers onto the paper.

This is how scientists first discovered graphene, too. They stuck cellophane tape on graphite and then ripped it off. When they looked at the tape under a microscope, they found thin layers of carbon stuck to it, i.e. graphene.

As you go on wiggling the pencil, more and more layers of carbon come off. Each of these layers is black and shiny and is readily apparent to the human eye.

This is how a pencil can write even though it lacks liquid ink.

Condensed matter physics

Carbon atoms arranged in one way behave so differently from when they’re arranged in a different way — even though each carbon atom itself behaves the same way. Diamond is whitish transparent while graphite is shiny and black. These differences arise based on how the electrons the carbon atoms share behave even though, again, all electrons are the same.

The underlying physics here is similar to why birds form patterns in the evening sky or how even humans behave so differently when they are in large groups. We may not be able to predict the behaviour of even a single bird or a person in general, but when they get together, they can acquire altogether new behaviours. The same is true for cars in traffic and ants in a colony.

This field of study is called condensed matter physics. If you want to explore it more, consider taking an undergraduate degree in physics.

And the next time you have some time to spare on a casual evening, consider getting yourself a pencil. Sharpen it well and sketch. As you find your lines shaping up your thoughts, don’t forget to thank those hundreds of carbon atoms selflessly sliding off at your will.

Adhip Agarwala is an assistant professor of physics at IIT Kanpur.