Thallium is a soft, malleable, grayish post-transition metal. It can be cut with a knife at room temperature. It melts at a low temperature, 304°C. This is typical of a post-transition metal. Thallium has 25 isotopes and two stable (nonradioactive) ones. It is extremely toxic.
Thallium is a moderately reactive metal. It corrodes easily in air with a blue-gray color that is similar to lead. If it is kept in air for a long time, a large amount of thallium(I) oxide will build up. It corrodes in the presence of water to make the hydroxide. It burns with a greenish flame. It reacts with most acids.
Thallium makes chemical compounds in two oxidation states: +1 and +3. The +1 state is more common and less reactive. Its chemical compounds are very similar to potassium or silver compounds. It makes a
hydroxide that in a strong base when dissolved in water. Most other transition metal and post-tranansition metal hydroxides do not dissolve in water. This reacts with carbon dioxide to make thallium(I) carbonate, which is also water-soluble and very heavy. It is the only heavy metal carbonate that can dissolve in water. Other compounds are similar to silver compounds. Thallium(I) bromide turns yellow when exposed to light, similar to silver(I) bromide. Thallium(I) sulfide is black, similar to silver(I) sulfide. The +3 state compounds are oxidizing agents. The black oxide, thallium(III) oxide and the hydroxide, thallium(III) hydroxide, are the only stable +3 compounds. They break down to oxygen and thallium(I) oxide when heated. Thallium and its compounds are rare because they are toxic and polluting.
+1 compounds are quite unreactive. It is the more common oxidation state. They are made when thallium dissolves in acids or corrodes in air.
- Thallium(I) bromide, yellowish solid, turns black when in light
- Thallium(I) carbonate, colorless solid, dissolves in water
- Thallium(I) chloride, colorless solid, does not dissolve in water
- Thallium(I) fluoride, colorless solid, dissolves in water
- Thallium(I) hydroxide, yellow solid, strong base, dissolves in water
- Thallium(I) iodide, yellow or red solid, does not dissolve in water
- Thallium(I) oxide, black solid, dissolves in water
- Thallium(I) sulfate, colorless solid, dissolves in water
- Thallium(I) sulfide, black solid, does not dissolve in water
+3 compounds are oxidizing agents. They are quite rare.