Composition of the nucleus: protons, neutrons and isotopes
A nucleus holds Z protons and N neutrons, together called nucleons, with mass number A = Z + N. Atomic masses are measured in atomic mass units, u.
What is inside a nucleus
A nucleus contains protons (charge +e) and neutrons (no charge). Together they are called nucleons. The atomic number Z is the number of protons. The neutron number N is the number of neutrons. The mass number A = Z + N is the total number of nucleons. The charge on the nucleus is +Ze, and a neutral atom has Z electrons outside it. Almost all the mass of the atom is in the nucleus. James Chadwick found the neutron in 1932, when he bombarded beryllium with alpha particles and detected a neutral radiation.
Atomic mass unit and isotopes
Atomic masses are tiny in kilograms, so we use the atomic mass unit (u): 1/12 of the mass of a carbon-12 atom, 1.66 × 10⁻²⁷ kg. Atomic masses are measured with a mass spectrometer. Many elements have average atomic masses that are not whole numbers, such as chlorine (about 35.5 u). The reason is that they are mixtures of isotopes: atoms with the same Z but different numbers of neutrons.
Isotopes, isobars and isotones
| Term | Same | Different | Example |
|---|---|---|---|
| Isotopes | Atomic number Z | Mass number A (and N) | ¹₁H, ²₁H, ³₁H (protium, deuterium, tritium); ³⁵Cl and ³⁷Cl |
| Isobars | Mass number A | Atomic number Z | ³₁H and ³₂He |
| Isotones | Neutron number N | Z and A | ¹⁹⁸₈₀Hg and ¹⁹⁷₇₉Au (both have N = 118) |
Worked example: average atomic mass of chlorine
Chlorine has two isotopes: ³⁵Cl (mass 34.98 u, 75.4%) and ³⁷Cl (mass 36.98 u, 24.6%). Find the average atomic mass of chlorine.
- Average mass = (fraction × mass) for each isotope, added together.
- ³⁵Cl: 0.754 × 34.98 u = 26.375 u.
- ³⁷Cl: 0.246 × 36.98 u = 9.097 u.
- Average = 26.375 u + 9.097 u = 35.47 u.
About 35.47 u, which is why chlorine's atomic mass is not a whole number
Worked example: counting nucleons
For the nucleus ²³⁵₉₂U, find the number of protons, neutrons and nucleons, and the charge on the nucleus. (e = 1.6 × 10⁻¹⁹ C)
- Protons Z = 92; nucleons A = 235.
- Neutrons N = A − Z = 235 − 92 = 143.
- Charge = Ze = 92 × 1.6 × 10⁻¹⁹ C = 1.47 × 10⁻¹⁷ C.
92 protons, 143 neutrons, 235 nucleons; charge ≈ 1.47 × 10⁻¹⁷ C
Deuterium, the isotope of hydrogen with one neutron, forms heavy water (D₂O). India makes its own heavy water and uses it in its pressurised heavy water reactors that generate electricity.
2 marks: “Define isotopes and isobars, giving one example of each.” Isotopes: same Z, different A, e.g. ³⁵Cl and ³⁷Cl. Isobars: same A, different Z, e.g. ³₁H and ³₂He.
1 marks: “How many neutrons are there in the nucleus ¹⁹⁷₇₉Au?” N = A − Z = 197 − 79 = 118.
2 marks: “Two nuclei have the same number of neutrons but different mass numbers. What are they called? Give an example.” Isotones, e.g. ¹⁹⁸₈₀Hg and ¹⁹⁷₇₉Au, both with N = 118.
IsotoPes: same Protons; IsobArs: same A; IsotoNes: same Neutrons.