Thallium has two isotopes thallium-203 and thallium-205 find which isotope is more abundant\abundance

Let we have X of Tl-205 so the percent of Tl-203 will be 100-X

Average mass of  Thallium is 204.4

Mass of  Tl-205=  

Mass of  Tl-203=  

So,

205X +20300-203X=20440

X=70

So the percent or abundance of  Tl-205 is 70 while that of Tl-203 will be 30.

hypostyle hall;