The Mass of Thallium (Tl) on the Periodic Table Is

Multiple Choice

The mass of thallium (Tl) on the periodic table is given as 204.3833 without any units. There are 47 isotopes of thallium, but only two are stable and abundant, thallium-203, with a mass of 202.9723 amu, and thallium-205, with a mass of 204.9744 amu. What is the percentage of each of these isotopes in naturally occurring thallium?

A) 29.5% ²⁰³Tl and 70.5% ²⁰⁵Tl
B) 70.5% ²⁰³Tl and 29.5% ²⁰⁵Tl
C) 25.5% ²⁰³Tl and 74.5% ²⁰⁵Tl
D) 74.5% ²⁰³Tl and 25.5% ²⁰⁵Tl
E) 32.5% ²⁰³Tl and 67.5% ²⁰⁵Tl