What Are Isotopes?
Isotopes are different forms of the same element. They have:
- the same number of protons (same atomic number — same element)
- different numbers of neutrons (different mass numbers)
Because the number and arrangement of electrons is the same, isotopes behave identically in chemical reactions. They only differ in mass.
Example: Carbon Isotopes
Carbon has three naturally occurring isotopes:
| Isotope | Protons | Neutrons | Mass number |
|---|---|---|---|
| Carbon-12 (¹²C) | 6 | 6 | 12 |
| Carbon-13 (¹³C) | 6 | 7 | 13 |
| Carbon-14 (¹⁴C) | 6 | 8 | 14 |
Carbon-12 and carbon-13 are stable. Carbon-14 is radioactive — its nucleus is unstable and it slowly breaks down over time.
Radiocarbon dating uses carbon-14. Living things constantly take in carbon. When an organism dies, its carbon-14 slowly decays at a known rate. Scientists can measure how much carbon-14 remains to estimate how long ago the organism died — useful for dating fossils up to about 50,000 years old.
Relative Atomic Mass
Since most elements are mixtures of isotopes, the relative atomic mass (Ar) is a weighted average of all the isotopes, taking into account how common each one is.
For example, chlorine exists as:
- 75% chlorine-35
- 25% chlorine-37
This is why the periodic table shows 35.5 for chlorine, not a whole number.
Radioactive Isotopes in Everyday Life
Radioactive isotopes (radioisotopes) have many uses:
- Medicine: cancer treatment, medical imaging (e.g. iodine-131 for thyroid)
- Archaeology: carbon-14 dating of organic material
- Energy: uranium-235 in nuclear power stations
Real world: Radiocarbon dating of ancient artefacts
When a tree is cut down, it stops absorbing carbon-14. Scientists measure the ratio of carbon-14 to carbon-12 in wooden artefacts, furniture, or charcoal from ancient fires. Since carbon-14 has a half-life of 5,730 years, calculating how much has decayed tells archaeologists how old the object is. The Turin Shroud was dated to medieval times this way.
Real world: Nuclear energy from uranium isotopes
Natural uranium contains mostly uranium-238 (99.3%) and a small amount of uranium-235 (0.7%). Only U-235 is fissile — it can sustain a nuclear chain reaction. Nuclear power stations use enriched uranium (with more U-235) as fuel. The energy released from splitting U-235 nuclei heats water to drive turbines and generate electricity.
Real world: Medical isotopes in cancer treatment
Iodine-131 (¹³¹I) is a radioactive isotope used to treat thyroid cancer. The thyroid gland naturally absorbs iodine, so a patient given iodine-131 concentrates it in their thyroid. The radiation it emits destroys cancer cells from within. This targeted use of a single isotope exploits the difference between a stable (iodine-127) and radioactive (iodine-131) form of the same element.
Where do you see this in real life? Carbon dating in museums, nuclear power on the electricity grid, and medical imaging in hospitals all rely on specific isotopes. The same element, with just different numbers of neutrons, can be life-saving medicine or fuel for a power station.