Isotopes, isobars and isotones
Isotopes share Z, isobars share A and isotones share the neutron number N = A − Z. Only isotopes are the same element with the same chemistry.
Isotopes, isobars and isotones
| Term | Same | Different | Examples |
|---|---|---|---|
| Isotopes | Atomic number Z | Mass number A and neutron number N | ¹₁H, ²₁H, ³₁H; ¹²₆C and ¹⁴₆C |
| Isobars | Mass number A | Z and N | ³₁H and ³₂He; ⁷⁶₃₂Ge and ⁷⁶₃₄Se |
| Isotones | Neutron number N | Z and A | ²⁷₁₃Al and ²⁸₁₄Si (N = 14); ¹⁴₆C and ¹⁶₈O (N = 8) |
Isotopes and isobars: do not mix them up
Isotopes of hydrogen
Hydrogen has three isotopes. Protium (¹₁H) has no neutron. Deuterium (²₁H) has one neutron and forms heavy water, D₂O. Tritium (³₁H) has two neutrons and is radioactive. All three have one proton and one electron, so they react in the same way chemically.
Worked example: sorting a list
Sort these into a pair of isotopes, a pair of isobars and a pair of isotones: ³⁵₁₇Cl, ³⁷₁₇Cl, ¹⁴₆C, ¹⁴₇N, ²³₁₁Na, ²⁴₁₂Mg.
- Write Z, A and N = A − Z for each: Cl-35 (17, 35, 18); Cl-37 (17, 37, 20); C-14 (6, 14, 8); N-14 (7, 14, 7); Na-23 (11, 23, 12); Mg-24 (12, 24, 12).
- Same Z: ³⁵₁₇Cl and ³⁷₁₇Cl are isotopes.
- Same A: ¹⁴₆C and ¹⁴₇N are isobars.
- Same N: ²³₁₁Na and ²⁴₁₂Mg are isotones (N = 12).
Isotopes: ³⁵Cl and ³⁷Cl; isobars: ¹⁴C and ¹⁴N; isotones: ²³Na and ²⁴Mg
Carbon-14, an isotope of carbon, is used to find the age of old wood and bones. Deuterium, an isotope of hydrogen, makes the heavy water used in India's nuclear power reactors.
2 marks: “Distinguish between isotopes and isobars, giving one example of each.” Isotopes: same Z, different A (³⁵₁₇Cl, ³⁷₁₇Cl); same element, same chemistry. Isobars: same A, different Z (⁴⁰₁₈Ar, ⁴⁰₂₀Ca); different elements.
1 marks: “What are isotones? Give one example.” Nuclei with the same number of neutrons, such as ¹⁴₆C and ¹⁶₈O (N = 8).
1 marks: “Which number is the same for all isotopes of an element?” The atomic number Z.
IsotoPes: same Protons; IsobArs: same A; IsotoNes: same Neutrons.