Define isotopes, isobars and isotones. Give one example for each.

1. 1 · Open with the key terms. Atomic number Z = number of protons. Mass number A = protons + neutrons. Neutrons = A − Z.
2. 2 · Isotopes. Atoms of the same element with the same Z but different A. Example: Cl-35 and Cl-37 (Z = 17).
3. 3 · Isobars. Atoms of different elements with the same A but different Z. Example: Ar-40 (Z = 18) and Ca-40 (Z = 20).
4. 4 · Isotones. Atoms of different elements with the same number of neutrons. Example: C-13 and N-14, with 7 neutrons each.
5. 5 · Draw a comparison table. [Table: term, what is the same, what is different, example]
6. 6 · Real-life link. Isotopes such as iodine-131 and cobalt-60 are used to treat thyroid disease and cancer.
7. 7 · Close in one line. Thus the three terms differ in which number, Z, A or neutrons, is the same.

Scoring tip: Write Z and A next to every example. An example without numbers often gets only half the mark.