How Many Hands Does Carbon Have? — Why Life Is Made of Carbon

The director's question

Once I learned that a single atom holds no memory, the next question came. If atoms remember nothing, what do they hold on to each other with, to become a tree or a person?

The picture in my head went like this. A carbon atom is four protons and four electrons; atoms stick together because they become ions; and neutrons sit between them like glue. I vaguely guessed that, just as gravity pulls things together, some force in the body pulls atoms together. Looking at it now, the picture needs fixing in several places. But those very places turned out to be the way into life's secret.

Science's answer — carbon has four hands

Numbers first. Carbon has six protons and six electrons. An atom's name is set by its protons; six means carbon.

Four comes from somewhere else. Of the six electrons, two live on the inner first floor and four on the top, second floor. That floor has eight seats, so four are empty. Carbon can neither give away four nor take four, so it shares four with neighbours. Carbon has four hands. Pick carbon on board 4 of the element tool and you see them.

With four hands carbon can form long chains, rings and branches, and hang other atoms on every side. Fats, proteins and DNA are all carbon chains with things attached. Almost no other atom builds such complex shapes so sturdily, which is why life on Earth is woven from carbon.

Bonded is not ionic

Carbon bonded to other atoms is not an ion. An ion is an atom that has lost or gained electrons and carries a charge. Salt is like that: sodium hands over an electron, chlorine takes it, and plus and minus attract. Carbon mostly does not trade electrons; it shares them with its neighbour. That is a covalent bond, and the shared electrons belong to neither atom but span the two.

The glue is electrons, not neutrons

What glues atom to atom is not the neutron but the electron: the electric force between electrons and nuclei holds atoms together. Neutrons are glue somewhere else, inside the nucleus. Protons are all positive and push each other apart; neutrons sit between them and hold them with the strong nuclear force. Outside the atom, electrons hold; inside, neutrons do.

At the size of atoms gravity is so weak it does almost nothing, weaker than the electric force beyond counting. We cannot walk through walls not because of gravity but because the electrons of hand and wall push each other away.

Those who asked first

The German chemist Kekulé was one of those who worked out that carbon has four hands. He is also famous for picturing benzene as a ring of six carbons. The story that he dreamed of a snake biting its own tail is his own, though whether it really happened no one can know.

The American chemist Linus Pauling spent his life on how atoms share electrons. He also explained why carbon's four hands point out at exactly equal angles.

A little further in

What we do not know yet

We know why life is made of carbon. We do not yet know how carbon-based matter first began to move and multiply on its own. The origin of life is one of science's great mysteries. We meet it again in part 7.

In the film

One of David's lines: the same energy, a different form. This is about energy, and it agrees with science: energy is never lost, only changes form. That is why light or heat comes out when atoms let go and take new hands. When wood burns, its carbon takes new hands with oxygen and the released energy becomes firelight and warmth. Atoms too are never lost, only change partners. The film's line speaks of energy and this piece of atoms, but both tell the same story: nothing disappears, it changes form.

The question that remainsWhose body was the carbon in mine in before I was born, and what was it holding on to?