Same element, different mass
Every atom of a given element has the same number of protons. That number, the atomic number, defines the element. But the number of neutrons in the nucleus can vary. Atoms of the same element with different neutron counts are called isotopes.
Carbon is the standard example. Every carbon atom has 6 protons. The most common isotope, carbon-12, has 6 neutrons. Carbon-13 has 7 and carbon-14 has 8. The number in the name, the mass number, is the total of protons and neutrons: 6 + 6 = 12, 6 + 7 = 13 and 6 + 8 = 14.
Since chemistry is largely governed by electrons, and all isotopes of an element have the same number of electrons when neutral, they behave almost identically in chemical reactions. The differences show up mainly in mass and in the stability of the nucleus. These pieces are part of the broader picture of subatomic particles, and each element’s place in the periodic table is set by its proton count, not its neutrons.
Examples
Hydrogen has three well-known isotopes. Ordinary hydrogen has one proton and no neutrons. Deuterium has one neutron and is sometimes called heavy hydrogen, and it forms “heavy water” when combined with oxygen. Tritium has two neutrons and is radioactive.
Uranium has several isotopes, with uranium-238 being the most common in nature and uranium-235 being a much smaller fraction. The less common uranium-235 is the one most associated with nuclear fission, which is why separating isotopes has historically been so important and difficult: they are chemically alike and differ only slightly in mass.
Stable and radioactive
Some combinations of protons and neutrons are stable and last indefinitely. Others are unstable, and their nuclei change spontaneously, emitting radiation in the process. These are radioactive isotopes, sometimes called radioisotopes. When an unstable nucleus changes, it becomes a different isotope or even a different element, a process called radioactive decay.
The pioneering study of radioactivity involved researchers such as Marie Curie, who worked on radioactive elements in the early twentieth century and helped establish the field.
Half-life
Decay is random for any one atom, but for a large number of atoms it follows a predictable pattern. The half-life is the time it takes for half of a sample of a radioactive isotope to decay. It is a fixed property of each isotope and ranges from fractions of a second to billions of years.
Carbon-14 has a half-life of about 5,730 years. A worked example: if a sample began with 100 units of carbon-14, after one half-life (about 5,730 years) about 50 would remain, after two (about 11,460 years) about 25, and after three (about 17,190 years) about 12.5. Because the pattern is regular, measuring what is left tells you how much time has passed.
Radiocarbon dating
Carbon-14 is formed continually in the upper atmosphere and mixes into carbon dioxide. Plants take it in through plant photosynthesis, and animals take it in by eating plants or other animals. While an organism is alive, the ratio of carbon-14 to carbon-12 in its body stays close to that of the atmosphere. When it dies, intake stops and the carbon-14 gradually decays.
By measuring the remaining ratio, scientists can estimate how long ago something died. The method works on organic materials such as wood, bone, charcoal and cloth, and is generally useful back to roughly 50,000 years, after which too little carbon-14 is left to measure reliably. Results are given with uncertainty, and scientists calibrate them against other records, such as tree rings, because atmospheric carbon-14 has varied over time. It has been used to date objects from sites around the world, including material related to the Egyptian pyramids and other ancient places.
Dating rocks and the age of the Earth
For much older things, scientists use isotopes with far longer half-lives. Uranium-238 decays through a series of steps to lead, with a half-life of billions of years, so uranium-lead dating can be used on ancient minerals. This kind of work underpins the estimate that Earth is about 4.5 billion years old. It also helps geologists understand the history recorded in the rock cycle. Methods like this are often checked by using more than one isotope system on the same sample.
Other uses
- Medicine: radioactive isotopes are used as tracers in imaging and, in some cases, in treatments. Doses and uses are decided by medical professionals and follow strict safety rules.
- Climate science: the ratio of oxygen isotopes in ice cores and shells gives clues about past temperatures, which helps in the study of ice ages.
- Food and forensics: isotope ratios can show where a plant or animal grew, because they vary slightly by environment.
- Energy and smoke detectors: some devices rely on the controlled behavior of particular radioisotopes.
Radioactive material can be harmful at high doses, but background radiation is a natural part of life, and careful handling and shielding manage the risks in medicine and industry.
Why the idea matters
Isotopes show that “element” and “atom type” are not quite the same thing. They also provide natural clocks and tracers, letting scientists read the history of objects from archaeological finds to the planet itself.