This is the average atomic mass, which takes into account the natural abundance of its two stable isotopes, Thallium-203 and Thallium-205.

You're asking for the atomic mass of each isotope. Here's what we know:

* Thallium-203: Its atomic mass is 202.97234 u.

* Thallium-205: Its atomic mass is 204.97440 u.

Important Note: The "203" and "205" in the names of the isotopes refer to their mass number, which is the total number of protons and neutrons in the nucleus.