Define the term isotope.

Determine the number of neutrons in one atom of iodine-131.

atoms which have same atomic number but different mass number / atoms of the same element which have different numbers of neutrons / atoms with the same number of protons but different numbers of neutrons / atom of an element with a fixed number of protons but a number of neutrons which can be variable;

78;

In (a)(i), the word atoms was frequently omitted from the definition; it is accepted that it would have been preferable to ask for the definition of isotopes of an element as specified in the syllabus.

The answer in (ii) was generally correct.