Isotopes.

Step 1 of 5

One more neutron

In the last Lesson the atomic number decided the element. But the number of neutrons is not fixed: atoms of one element can have different numbers of neutrons in their nuclei. Such atoms are called isotopes of the element.

The Scene shows a carbon nucleus: six protons and six neutrons. Predict what you get by adding one neutron to it, then look.

Step 2 of 5

Naming an isotope

An isotope is named by its element and its mass number: carbon-12, carbon-13, carbon-14. In the symbol the mass number goes at the top: $^{12}_{6}\mathrm{C}$⁠, $^{13}_{6}\mathrm{C}$⁠, $^{14}_{6}\mathrm{C}$⁠. The name counts every particle in the nucleus, not the neutrons: carbon-14 has eight neutrons.

Chlorine, $Z = 17$⁠, has two stable isotopes in nature: chlorine-35 and chlorine-37. Use Protons and Neutrons to build the nucleus of chlorine-37.

Step 3 of 5

The same chemistry

Every isotope of carbon has six protons, so a neutral atom also has six electrons, arranged the same way. An atom’s chemistry, what it bonds to and how, is decided by its electrons. So carbon-12 and carbon-14 build the same molecules, and your body holds a little carbon-14 in every molecule that has carbon in it.

The difference is in the nucleus. The Scene shows carbon-14, with Look inside open at Light nuclei, the map of the nuclei: the stable nuclei sit on the floor of a valley, and carbon-14 stands on its wall. It is radioactive: in time one of its neutrons turns into a proton, and the atom becomes nitrogen. That decay belongs to the Unit on modern physics; for chemistry it is enough that the isotopes of an element react in the same way.

Step 4 of 5

The natural mixture

Almost every element occurs in nature as a fixed mixture of its isotopes. Out of every hundred chlorine atoms, on average $75.8$⁠ are chlorine-35 and $24.2$⁠ are chlorine-37, whether the chlorine came from sea salt or from rock.

So the periodic table does not print the mass number of any one atom but the relative atomic mass, $A_r$⁠: the mean mass of the element’s atoms as they occur in nature, in units in which carbon-12 has a mass of exactly $12$⁠. It is a weighted mean: each isotope counts by how common it is,

$A_r = \frac{75.8 \times 35 + 24.2 \times 37}{100}$$A_r =$$\frac{75.8 \times 35 + 24.2 \times 37}{100}$⁠

Work out chlorine’s relative atomic mass.

Step 5 of 5

Reading the mean

The weighted mean also tells you something the other way round: if an element’s $A_r$⁠ is close to one whole number, most of its atoms are probably that isotope. Carbon’s $A_r$⁠, $12.011$⁠, has a reason not to be exactly $12$⁠: one atom in a hundred is carbon-13.