Thallium is extremely soft and malleable and can be cut with a knife at room temperature. It has a metallic luster, but it quickly tarnishes with a bluish-grey tinge that resembles lead. It may be preserved by keeping it under oil. A heavy layer of oxide builds up on thallium if left in air. In the presence of water, thallium hydroxide is formed.
Thallium has 25 isotopes which have atomic masses that range from 184 to 210. 203Tl and 205Tl are the only stable isotopes, and 204Tl is the most stable radioisotope, with a half-life of 3.78 years.
202Tl (half life 12.23 days) can be made in a cyclotron, while 204Tl (half life 3.78 years) is made by the neutron activation of stable thallium in a nuclear reactor.
201Tl (half-life 73 hrs), decays by electron capture, emitting Hg x-rays (~ 70–80 keV), and photons of 135 and 167 keV in 10% total abundance; therefore it has good imaging characteristics without excessive patient radiation dose. It is the most popular isotope used for thallium nuclear cardiac stress tests
The two main oxidation states of thallium are +1 and +3. In the oxidation state +1 most compounds closely resemble the potassium or the silver compounds (The ionic radius of thallium(I) is 1.47 Å while that of potassium is 1.33 Å and that of silver is 1.26 Å). For example the water soluble and very basic thallium(I) hydroxide reacts with carbon dioxide forming water soluble thallium carbonate this carbonate is the only water soluble heavy metal carbonate. The similarity with silver compounds is observed with the halide, oxide, and sulphide compounds. Thallium(I) bromide is a photosensitive yellow compound very similar to the silver bromide, while the black thallium(I) oxide and thallium(I) sulphide are very similar to the silver oxide and silver sulphide. The compounds with oxidation state +3 resemble the aluminium(III) compounds. They are moderately strong oxidizing agents, as illustrated by the reduction potential of +0.72 volts for Tl3+ + 3 e- → Tl(s). The thallium(III) oxide is a black solid which decomposes above 800 °C, forming the thallium(I) oxide and oxygen.
The base value of each unit of ranges between 5 and 20Ð per unit, with up to 3 units being found at any one time.
Presence on Mars: Rare
|Group 1 | Group 2 | Group 3 | Group 4 | Group 5 | Group 6|
|Group 3|||Antimony | Astatine | Barium | Bismuth | Cesium | Francium | Hafnium | Indium | Iodine | Iridium | Lanthanum | Lead | Mercury | |Osmium | Platinum | Polonium | Radium | Radon | Rhenium | Tantalum | Tellurium | Thallium | Tin | Tungsten | Xenon||