Isotope
Isotopes are atoms of the same element that have the same atomic number but different mass numbers.[1][2] The atomic number (Z) is the number of protons in the nucleus, which identifies the element; the mass number (A) is the number of protons plus neutrons.[3] Isotopes of an element therefore have the same number of protons but different numbers of neutrons.
Naming isotopes
Isotopes are named using the element's name and the mass number.[4] For example, chlorine (atomic number 17) has two natural isotopes: chlorine-35, which has 18 neutrons, and chlorine-37, which has 20.[2]
Isotopes and atomic weight
Some elements have only one natural isotope, such as fluorine and gold, while tin has ten.[4][2] The atomic weight of an element is the average mass of all its naturally occurring isotopes, taking into account how common each one is.[4] About three-quarters of chlorine atoms are chlorine-35, so the average mass of a chlorine atom, about 35.45, is closer to 35 than to 37.[4][5]
Chemical behaviour
Isotopes of an element take part in the same chemical reactions. Their reaction rates differ very slightly, but the difference only matters for the lightest elements.[2] Because isotopes behave alike chemically, they can be used as "tracers". For example, when acetic acid is made with heavy water, which contains the heavy hydrogen isotope deuterium, the results show that only one of its hydrogen atoms is acidic.[6]
Radioactive isotopes
If a nucleus has too many or too few neutrons compared with protons, it is unstable and radioactive. Tritium (hydrogen-3), for example, turns a neutron into a proton by giving out a beta particle, becoming helium-3.[6] Each radioactive isotope has its own half-life, which is not affected by its surroundings: about 12.4 years for tritium and 4.5 billion years for uranium-238.[6] Radioactive isotopes have practical uses, for example americium-241 in household smoke detectors.[7]
Uses in science
The ratios of different isotopes, such as those of hydrogen, carbon, nitrogen and oxygen, give information about the past, including past sea temperatures and climate from ice cores, and ancient diets.[6]
History
The chemists Soddy and Fajans put forward the idea of isotopes, which explained why atomic weights seemed so irregular.[8] After Moseley's work on atomic number and Rutherford's on the charge of the nucleus, atomic weights could be redefined as an average of the masses of an element's isotopes.[9]
See also
References
- ↑ Young, Susan M.; Vining, William J.; Day, Roberta; Botch, Beatrice (2018). General Chemistry: Atoms First. Boston: Cengage Learning. pp. 42–43. ISBN 978-1-337-61229-6.
{{cite book}}:|access-date=requires|url=(help) - ↑ 2.0 2.1 2.2 2.3 Mackay, K. M.; Mackay, R. A.; Henderson, W. (2002). Introduction to Modern Inorganic Chemistry (6th ed.). Cheltenham: Nelson Thornes. section 2.2: Isotopes. ISBN 0-7487-6420-8. Retrieved 2026-10-23.
- ↑ Young, Susan M.; Vining, William J.; Day, Roberta; Botch, Beatrice (2018). General Chemistry: Atoms First. Boston: Cengage Learning. p. 40. ISBN 978-1-337-61229-6.
{{cite book}}:|access-date=requires|url=(help) - ↑ 4.0 4.1 4.2 4.3 Young, Susan M.; Vining, William J.; Day, Roberta; Botch, Beatrice (2018). General Chemistry: Atoms First. Boston: Cengage Learning. p. 43. ISBN 978-1-337-61229-6.
{{cite book}}:|access-date=requires|url=(help) - ↑ Young, Susan M.; Vining, William J.; Day, Roberta; Botch, Beatrice (2018). General Chemistry: Atoms First. Boston: Cengage Learning. p. 44. ISBN 978-1-337-61229-6.
{{cite book}}:|access-date=requires|url=(help) - ↑ 6.0 6.1 6.2 6.3 Mackay, K. M.; Mackay, R. A.; Henderson, W. (2002). Introduction to Modern Inorganic Chemistry (6th ed.). Cheltenham: Nelson Thornes. section 2.3: Radioactive isotopes and tracer studies. ISBN 0-7487-6420-8. Retrieved 2026-10-23.
- ↑ Mackay, K. M.; Mackay, R. A.; Henderson, W. (2002). Introduction to Modern Inorganic Chemistry (6th ed.). Cheltenham: Nelson Thornes. section 1.3: Recent advances. ISBN 0-7487-6420-8. Retrieved 2026-10-23.
- ↑ Brock, William H. (1992). The Fontana History of Chemistry (ePub ed.). London: Fontana Press. Chapter 9: Principles of Chemistry, section Manufacturing Elements / Mendeleev's Principles (one of these). ISBN 9780007394968.
{{cite book}}:|access-date=requires|url=(help) - ↑ Brock, William H. (1992). The Fontana History of Chemistry (ePub ed.). London: Fontana Press. Chapter 9: Principles of Chemistry, section Conclusion. ISBN 9780007394968.
{{cite book}}:|access-date=requires|url=(help)