What are the constituents of a nucleus? Define atomic number and mass number, and explain isotopes, isobars and isotones with one example each.

How to lay out the answer. Follow the frame, then write it in your own words.

1 · Name the constituents. A nucleus contains protons (+e) and neutrons (no charge), together called nucleons.
2 · Define Z and A. Atomic number Z = number of protons; mass number A = Z + N = number of nucleons.
3 · Explain isotopes. Same Z, different A: ³⁵Cl and ³⁷Cl.
4 · Explain isobars and isotones.
- Isobars: same A, different Z: ³₁H and ³₂He
- Isotones: same N: ¹⁹⁸₈₀Hg and ¹⁹⁷₇₉Au (N = 118)
5 · Real life. Deuterium, an isotope of hydrogen, makes heavy water for Indian reactors.
6 · Close in one line. So, a nucleus is fully described by its Z and A.

Scoring tip: Write each example with both A and Z marked on the symbol; examiners check that the numbers match the definition.