WBCS Preliminary Examination 2024 — General Studies · Question 179 of 200

Answer: (D) All of the above

All three halogens have radioactive isotopes although none belongs to the natural radioactive decay series (the uranium, thorium and actinium series), so PSC's answer is 'All of the above'. Fluorine-18 is the positron emitter used in PET scans; iodine-131 is used to diagnose and treat thyroid disorders and iodine-129 is a long-lived fission product; chlorine-36 (half-life about 3 lakh years) is used to date old groundwater. In fact almost every element has at least one radioisotope, for example carbon-14 (radiocarbon dating), potassium-40, phosphorus-32, cobalt-60 (cancer radiotherapy) and tritium, which is why choosing iodine alone would be incomplete. The halogens are fluorine, chlorine, bromine, iodine and astatine, the last having no stable isotope at all.