Thallium has two isotopes: thallium-203 and thallium 205. thallium's atomic number is 81 and its atomic mass is 204.38 amu. which statement about the thallium isotopes is true? a. there is more thallium-203 in nature. b. atoms of both isotopes have 81 neutrons. c. thallium-205 isotopes have fewer neutrons than thallium-203 isotopes. d. all atoms of thallium have 81 protons.