Every carbon atom has 6 protons. The number of neutrons beside them can differ, and that small difference lets scientists tell the age of old wood and bone.
You met these at the very start of the course: most carbon atoms have 6 neutrons, about 1 in 100 has 7 and a very rare few have 8. The 6 protons, carbon's atomic number, stay the same. Atoms of one element with different numbers of neutrons, which carry no charge, are isotopes of that element. Each is named by its mass number, its protons and neutrons added together: carbon-12 has 6 + 6, carbon-13 has 6 + 7 and carbon-14 has 6 + 8.
Same chemistry
Chemistry is about electrons, and the neutrons have no charge to pull on them. All three isotopes have 6 electrons, with 4 in the outer shell, so all three make the same four bonds with the same partners. A carbon dioxide molecule built round a carbon-14 atom is straight, has two C=O bonds and reacts like any other. The difference is weight: a carbon-14 atom is about a sixth heavier than a carbon-12 atom.
| Isotope | Share |
|---|---|
| carbon-12 | 98.9% |
| carbon-13 | 1.1% |
| carbon-14 | about 1 in a trillion |
Carbon-12 and carbon-13 are stable: their nuclei stay as they are. Carbon-14 is radioactive: each carbon-14 nucleus breaks down sooner or later. That is why there is so little of it, and why it works as a clock.