This is a sample of cesium tribromide, prepared by adding finely powdered cesium bromide, CsBr, to liquid bromine and then gently heating to drive off excess bromine. The amount, shown here, is approximately 1.5 grams.

It is remarkable that this compound is much lighter than bromine. This compound is quite stable in air, it only has a very faint odour of bromine. As soon as it is made wet, it dissolves and then decomposes. The solution in water immediately gives off bromine vapor if it is made a little more concentrated.

Cesium tribromide is not a compound of cesium in the +3 oxidation state, but it is a complex of bromide and bromine, which is stabilized by the large Cs⁺ ion in its crystal lattice.

The picture below shows the sample in more detail.