## Chemistry and Chemical Reactivity (9th Edition)

$^{205}Tl$ is more abundant.
Assuming their atomic weights are approximately equal to their mass numbers, with x being the first isotope's abundance (meaning $1-x$ is the second one's abundance): $203\times x + 205 \times (1-x) = 204.4$ $x=0.3$ $^{205}Tl$ is more abundant.