Cesium metal, 5 g
Silver metal with a yellow tint, low melting and very active.
Availability of impurity of oxygen in cesium in the form of peroxy compounds makes metal darker and considerably lowers the melting point.
Natural cesium does not have radioactivity. The isotope cesium-137, which is radioactive, is produced by artificial nuclear reactions.
Cesium is easily oxidized in air, that can lead to spontaneous combustion. With water reacts with an explosion. When working with cesium an appropriate qualification of personnel is required.
Cesium metal is stored in special glass ampoules without contact with atmospheric air.
You can buy the purified metal cesium in sealed glass ampoules* at the price from 3 to 20 dollars per gram.