There are two naturally occurring isotopes of thallium: ²⁰³Tl and ²⁰⁵Tl. Use the atomic mass of thallium listed on the periodic table to identify the more prevalent isotope.

In order for a high temperature boiler or steam engine to produce superheated water, or steam: the heat source must be greater than 100°c the water must be permitted to evaporate quickly the system must be sealed and become pressurized above atmospheric pressure the vapor pressure must be kept below 760 mm(hg)