Atomic Mass of Thallium (Tl)
The relative atomic mass of Thallium is 204.38 u — the weighted average of its naturally occurring isotopes, on the carbon-12 scale.
Looking for Thallium's atomic mass without wading through a whole chapter? Here is the direct answer first, then the tables and derivations examiners want to see. Every number on this page comes from the same engine that powers the interactive table. Need the whole picture instead — history, characteristics, uses, exam questions? The full Thallium page is one click away.
Thallium by the numbers
Z = 81 fixes the protons; the neutron count can vary between isotopes, which is why the atomic mass is an average (204.38 u) rather than a round number.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 81 |
| Relative atomic mass | 204.38 u |
| Protons | 81 |
| Neutrons (commonest isotope) | 123 |
| Electrons | 81 |
Atomic mass vs mass number
Mass number (A) counts particles — protons + neutrons in one specific atom, always a whole number. Relative atomic mass is the experimentally measured average over all natural isotopes, so it comes out fractional (204.38). Exams love this one-mark distinction.
Students also ask
Is the atomic mass of Thallium a whole number?
No. 204.38 u is the average of Thallium's natural isotopes, which is why it lands between whole numbers. The mass number of a single isotope, though, is always whole.
How many neutrons does Thallium have?
Neutrons = mass number − atomic number. For Thallium's commonest isotope that is about 204 − 81 = 123, though the exact count differs between isotopes.