Relative atomic mass for two isotopes can be calculated as follows
Isotope atomic mass abundance ²⁰³Tl 203 29.5% ²⁰⁵Tl 205 70.5%
Relative atomic mass = (mass of 1 isotope x % abundance) + (mass of 2 isotope x % abundance) = (203 x [tex] \frac{29.5}{100} [/tex]) + (205 x [tex] \frac{70.5}{100} [/tex]) = 204.41 amu
