State one use of radioactive isotopes in medicine.

What is the general name of the elements having 8 electrons in the valence shell of their atoms?

Which noble gas has less than 8 electrons in the valence shell of its atom? What is this number?

Give one example of a radioactive isotope which is used as a fuel in the reactors of nuclear power plants.

Name the radioactive isotope which is used in the treatment of cancer.