All atoms of an element have the same number of protons, but they can have different numbers of neutrons. Atoms of the same element with different numbers of neutrons are called isotopes.
Hydrogen has three isotopes. Protium (hydrogen-1) has no neutron, deuterium (hydrogen-2) has one and tritium (hydrogen-3) has two, so their mass numbers are 1, 2 and 3.
Almost all natural hydrogen is protium. Deuterium makes up only about 0.015% of it, and tritium occurs only in tiny traces.
Chlorine has two stable isotopes, chlorine-35 and chlorine-37. Both have 17 protons and 17 electrons, but chlorine-35 has 18 neutrons and chlorine-37 has 20.
Chemical reactions depend on the electrons, and the two isotopes have exactly the same arrangement of electrons. Isotopes of an element therefore have practically the same chemical properties.
Only properties that depend on mass, such as density, differ between isotopes, and the differences are largest for the light isotopes of hydrogen.
The isotopes of an element are found in nature in almost fixed proportions. In natural chlorine, about 76% of the atoms are chlorine-35 and about 24% are chlorine-37.
Natural carbon is about 98.9% carbon-12 and 1.1% carbon-13, with a tiny trace of radioactive carbon-14.
These proportions are used to work out the relative atomic mass of each element.
Some isotopes have unstable nuclei, which change into other nuclei by giving out radiation. They are called radioactive isotopes, or radioisotopes.
In alpha decay, the nucleus gives out an alpha particle made of two protons and two neutrons, so the mass number falls by 4 and the atomic number falls by 2. Radium-226, for example, becomes radon-222.
In beta decay, a neutron in the nucleus turns into a proton and an electron is given out, so the atomic number rises by 1 while the mass number stays the same. Carbon-14 becomes nitrogen-14 in this way.
Gamma rays are often given out along with these changes. They are electromagnetic waves and change neither the atomic number nor the mass number.
The time it takes for half of the nuclei of a radioactive isotope to decay is called its half-life. After one half-life, half of the nuclei are left, and after two half-lives, a quarter are left.
Carbon-14 has a half-life of about 5730 years. Living things keep taking in carbon, so the proportion of carbon-14 in their bodies stays about the same as in the carbon dioxide of the air.
When an organism dies, it stops taking in carbon, and the carbon-14 inside it slowly decays. Measuring how much is left shows roughly how long ago it died.