Inside the atom
An atom has a central nucleus containing protons (positive charge) and neutrons (no charge), with electrons (negative charge) moving around it. The proton number (atomic number) tells us which element the atom is – every carbon atom has 6 protons, every oxygen atom has 8. The nucleon number (mass number) is the total number of protons and neutrons in the nucleus.
What are isotopes?
Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. Because the proton number is unchanged, they are still the same element; because the neutron number differs, their nucleon numbers (and therefore their masses) are different.
Key idea
Same protons → same element and same chemical behaviour. Different neutrons → different mass. Number of neutrons = nucleon number − proton number.
Examples
- Carbon: carbon-12, carbon-13 and carbon-14 all have 6 protons; carbon-14 has 8 neutrons (14 − 6).
- Hydrogen: protium (1 proton, 0 neutrons), deuterium (1 proton, 1 neutron) and tritium (1 proton, 2 neutrons).
- Chlorine: chlorine-35 and chlorine-37.
Properties and uses
Isotopes of an element have the same chemical properties because chemical reactions depend on the electrons, and isotopes have the same electron arrangement. They differ only in physical properties linked to mass, such as density. Some isotopes are radioactive and are very useful:
- Carbon-14 is used to estimate the age of ancient wood, bone and fossils (radiocarbon dating).
- Iodine-131 and cobalt-60 are used in medicine to study or treat the body.
- Radioactive tracers help engineers find leaks in underground pipes.
Example
A hospital in Malaysia may use iodine-131 to check how well a patient's thyroid gland is working. It behaves chemically like ordinary iodine, but its radiation can be detected from outside the body.
Remember
Isotopes are never different elements – the proton number is the fingerprint of an element, and it stays the same.