For the following statements, write T for true and F for false :
(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral
(c) The mass of an electron is about `(1)/(2000)` times that of a proton
(d) A radioactive isotope of iodine is used for making tincture iodine, which is used as a medicine.