Isotopes, isobars and isotones
Isotopes share the same atomic number, isobars share the same mass number, and isotones share the same number of neutrons.
Compare the three
| Term | What is the same | What is different | Examples |
|---|---|---|---|
| Isotopes | Atomic number Z (number of protons); same element | Mass number A (number of neutrons) | H-1, H-2, H-3 (protium, deuterium, tritium; Z = 1); Cl-35 and Cl-37 (Z = 17) |
| Isobars | Mass number A | Atomic number Z; they are different elements | Ar-40 (Z = 18), K-40 (Z = 19), Ca-40 (Z = 20) |
| Isotones | Number of neutrons (A − Z) | Both Z and A | C-13 (6 p, 7 n) and N-14 (7 p, 7 n); Na-23 (11 p, 12 n) and Mg-24 (12 p, 12 n) |
How to sort any pair of atoms
- 1Compare the atomic numbers (Z). If Z is the same, the atoms are isotopes.
- 2If Z differs, compare the mass numbers (A). If A is the same, they are isobars.
- 3If both differ, find neutrons = A − Z for each. If the neutrons are equal, they are isotones.
Worked example: K-39 and Ca-40
Potassium-39 has Z = 19. Calcium-40 has Z = 20. Find their neutrons and name the pair.
- Formula: neutrons = A − Z
- K-39: 39 − 19 = 20 neutrons
- Ca-40: 40 − 20 = 20 neutrons
- Z differs (19 and 20) and A differs (39 and 40), but the neutrons are equal.
K-39 and Ca-40 are isotones, with 20 neutrons each.
Properties and uses
- Isotopes have the same chemical properties (same number of electrons) but slightly different physical properties such as mass and density.
- Isobars and isotones are different elements, so their chemical properties differ.
- Uses of isotopes: cobalt-60 treats cancer, iodine-131 treats thyroid disease, carbon-14 dates old objects, uranium-235 is a nuclear fuel.
Hospitals use iodine-131 to test and treat thyroid problems and cobalt-60 to treat cancer. Heavy water, made with deuterium (H-2), is used in the reactors of the Madras Atomic Power Station at Kalpakkam near Chennai.
1 marks: “C-13 and N-14 both have 7 neutrons. Atoms like these are called ______.” Isotones.
2 marks: “Define isobars with an example.” Atoms of different elements with the same mass number but different atomic numbers, e.g. Ar-40 (Z = 18) and Ca-40 (Z = 20).
4 marks: “Distinguish between isotopes, isobars and isotones with one example each.” A three-row table: what is the same, what differs and one example each. Add neutrons = A − Z and one line on properties.
isotoPes: same Protons. isobArs: same A (mass number). isotoNes: same Neutrons.