Carbon-14 from the sky

Section 2 of 5

Carbon-14 is made all the time, high in the air, from the nitrogen you breathe.

Earth is showered with cosmic rays: very fast particles from space, mostly protons. Most come from far outside the solar system. When one crashes into an atom at the top of the air, it shatters the nucleus and sends neutrons flying.

Nitrogen makes up 78% of the air, so a flying neutron often meets a nitrogen-14 nucleus, with 7 protons and 7 neutrons. The nucleus takes the neutron in and throws out a proton. It is left with 6 protons and 8 neutrons: it has become carbon-14. Carbon-14 forms fastest 9 to 15 km up, around the height where airliners fly and a little above.

A neutron turns nitrogen-14 into carbon-14 On the left, a neutron flies in and hits a nitrogen-14 nucleus, which has 7 protons and 7 neutrons. An arrow leads to a carbon-14 nucleus on the right, with 6 protons and 8 neutrons: the nucleus has kept the neutron, ringed, and a proton flies out of it. neutron + + + + + + + + + + + + + + proton nitrogen-14 7 protons 7 neutrons carbon-14 6 protons 8 neutrons
A neutron hits a nitrogen-14 nucleus. The nucleus keeps the neutron (ringed) and throws out a proton, and 6 protons with 8 neutrons is carbon-14.

Into the air and into living things

The new carbon atom soon joins with oxygen, first as carbon monoxide and then as carbon dioxide, and winds spread it round the world. Plants take in carbon dioxide and, using the energy of sunlight, build its carbon into sugars such as glucose. A carbon dioxide molecule with carbon-14 in it bonds like any other, so it goes in with the rest. Animals eat the plants, other animals eat them, and the carbon-14 passes into every living thing.

While a plant or animal is alive, its carbon keeps changing over: plants take in fresh carbon dioxide, and animals eat fresh food and breathe out carbon dioxide. So its share of carbon-14 stays very close to the air's, about one carbon atom in a trillion.

Carbon dioxide, CO₂One carbon atom joined by double bonds to two oxygen atoms, in a straight line.OCO
Carbon dioxide, CO₂: about one molecule in a trillion holds a carbon-14 atom, with the same straight O=C=O as the rest.
Glucose, C₆H₁₂O₆A sugar. Five carbon atoms and one oxygen atom make a six-membered ring, and a sixth carbon hangs off the ring. Four ring carbons and the outside carbon each carry an -OH group.OOHOHOHOHOH
Glucose, C₆H₁₂O₆, as a skeletal formula: plants build it from carbon dioxide, so its carbon holds carbon-14 in almost the same share as the air.

Quick check

Carbon-14 forms when a neutron hits which atom high in the air?