There are equal atoms of 20381Tl and 20581 Tl .Equal atoms of Superscript 203 Subscript; 81 Tl and Superscript 205 Subscript ; 81 Tl. There are___in a sample of thallium because the mass number of thallium, which is 204.38 a mu, is___the mass number of the isotope present in the highest percentage. In a sample of thallium because the mass number of thallium, which is 204.38 amu, is closer to.