Chemistry · Ch 13 — Nuclear Chemistry and Radioactivity
Classification Based on Number of Nucleons
Classification Based on Number of Nucleons
A nuclide is a specific nucleus, characterised by its exact numbers of protons and neutrons, considered on its own without reference to the surrounding electron cloud. Comparing the proton and neutron counts of different nuclides gives five useful classification categories.
Isotopes are nuclides of the same element -- so they share the same number of protons () -- but with different numbers of neutrons, and hence different mass numbers. Sodium-22, sodium-23 and sodium-24 (written 22-11-Na, 23-11-Na, 24-11-Na), with 11, 12 and 13 neutrons respectively, are one such family.
Isobars are nuclides that happen to share the same mass number , even though they belong to different elements and so have different numbers of protons and neutrons. Carbon-14 and nitrogen-14 (14-6-C, with 8 neutrons, and 14-7-N, with 7 neutrons) are isobars; so are hydrogen-3 and helium-3 (3-1-H and 3-2-He).
Mirror nuclei are a special case of isobars: pairs where the proton count and the neutron count are simply swapped, and differ from each other by exactly 1. Hydrogen-3 (1 proton, 2 neutrons) and helium-3 (2 protons, 1 neutron) are mirror nuclei, as are carbon-13 (6 protons, 7 neutrons) and nitrogen-13 (7 protons, 6 neutrons).
Isotones are the mirror-image idea to isotopes: nuclides that share the same number of neutrons but have different numbers of protons, and therefore different mass numbers too. Carbon-13 and nitrogen-14 both have 7 neutrons, for example, so they are isotones of each other; so are sodium-23 and magnesium-24. …
Table 13.1 lists three families of isobars, each pair sharing the same mass number A but different proton/neutron counts. Isobars of A = 3: 3-1-H (N = 2) and 3-2-He (N = 1). Isobars of A = 14: 14-6-C (N = 8) and 14-7-N (N = 7). Isobars of A = 24: 24-11-Na and 24-12-Mg (N = 12). NOTE ON SOURCE FIDELITY: the printed table shows '24-11-Na (N = 32)', which is numerically impossible for a nuclide of mass number 24 (N cannot exceed A); the arithmetically correct value, cross-checked against the definition N = A - Z given earlier in this same chapter, is N = 24 - 11 = 13. This is flagged here a …