Naturally occurring thallium have two stable isotopes, thalium-203 and thalium-205. A mercury-thallium alloy, which forms a eutectic at 8.5% thallium, is reported to freeze at -60C, some 20 degrees below the freezing point of mercury. Freshly cut thallium has a metallic lustre that dulls to bluish gray upon Thus For 12 C the atomic mass is exactly 12u, since the atomic mass unit is defined from it. There are twenty-five isotopes of thallium. Twenty five isotopic forms of thallium, with atomic masses ranging from 184 to 210 are recognized. To estimate the atomic mass of Thallium, you have to sum the mass of each isotope (203 or 205), multiplied … Tl-203 - 202.9723 amu - 29.524% Whoa!. Name: Thallium Symbol: Tl Atomic Number: 81 Atomic Mass: 204.3833 amu Melting Point: 303.5 °C (576.65 K, 578.3 °F) Boiling Point: 1457.0 °C (1730.15 K, 2654.6 °F) Number of Protons/Electrons: 81 Number of Neutrons: 123 Classification: Other Metals Crystal Structure: Hexagonal Density @ 293 K: 11.85 g/cm 3 Color: bluish Atomic Structure The unit of measure for mass is the atomic mass unit (amu).One atomic mass unit is equal to 1.66 x 10-24 grams. Isotopes of Thallium (click to see decay chain): 176 Tl 177 Tl 178 Tl 179 Tl 180 Tl 181 Tl 182 Tl 183 Tl 184 Tl 185 Tl 186 Tl 187 Tl 188 Tl 189 Tl 190 Tl 191 Tl 192 Tl 193 Tl 194 Tl 195 Tl 196 Tl 197 Tl 198 Tl 199 Tl 200 Tl 201 Tl 202 Tl 203 Tl 204 Tl 205 Tl 206 Tl 207 Tl 208 Tl 209 Tl 210 Tl 211 Tl 212 Tl Like lead, thallium is a soft, low-melting element of low tensile strength. For a precise answer you should not simply assume that the mass number is the actual mass. Thallium is a chemical element with the symbol Tl and atomic number 81. Next, you must determine the actual masses of the two isotopes. Weights of atoms and isotopes are from NIST article. So you are referring to Tl-203 and Tl-205. Molecular mass (molecular weight) is the mass of one molecule of a substance and is expressed in the unified atomic mass units (u). Explanation: TI-203 and TI-205 are isotopes of Thallium, it means elements that differs in the number of neutrons they have. Thallium (Tl), chemical element, metal of main Group 13 (IIIa, or boron group) of the periodic table, poisonous and of limited commercial value. You have to get product of mass number and percentage abundance and then obtain the sum of the two values to get the relative atomic mass (), then divide everything by 100 to remove percentage. Thallium atomic mass is 204.41. where m₁= mass number of thallium 205, a₁= abundance of thallium 205. and m₂= mass of thallium 203 and a₂= abundance of thallium 203. These two isotopes make all naturally occurring thallium. One unified atomic mass unit is approximately the mass of one nucleon (either a single proton or neutron) and is numerically equivalent to 1 g/mol. Twenty five isotopic forms of thallium, with atomic masses ranging from 184 to 210 are recognized. A mercury-thallium alloy, which forms a eutectic at 8.5% thallium, is reported to freeze at -60C, some 20 degrees below the freezing point of mercury. (1 u is equal to 1/12 the mass of one atom of carbon-12) Molar mass (molar weight) is the mass of one mole of a substance and is expressed in g/mol. Atomic structure of Thallium includes atomic number, atomic weight, electron configuration That will give only an approximation of the average mass. Th is thorium. Thallium is Tl, not Th. When isolated, thallium resembles tin, but discolors when exposed to air.Chemists William Crookes and Claude-Auguste Lamy discovered thallium independently in 1861, in residues of sulfuric acid production. . Natural thallium is a mixture of two isotopes. It is a gray post-transition metal that is not found free in nature. Their atomic masses range from 184 to 210. Natural thallium is a mixture of two isotopes.