Keychain. Barium.

<div>   </div> <div> <strong> Atomic number   <span class = "Apple-tab-span" style = "white-space: pre;"> </span> 56 </strong> </div> <div> <strong> Atomic mass   <span class = "Apple-tab-span" style = "white-space: pre;"> </span>   137.33 </strong> < / div> <div> <strong> Density, kg / m? <span class = "Apple-tab-span" style = "white-space: pre;"> </span>   3500 </strong> </ div> <div> <strong> Temperature</strong></div> <div> <strong> melting, ° С       <span class = "Apple-tab-span" style = "white-space: pre;"> </span>729</strong></div> <div>   </div> <div > Barium is a soft, malleable, silvery-white alkaline earth metal. has a high chemical activity. The nuclear fission of uranium and plutonium produces the radioactive isotope Ba, which is used as a radioactive indicator. Barium was discovered by the Swedish chemist K. Scheele (1774) in the form of BaO oxide, called "heavy earth", or barite (from the Greek. Barys - heavy). Metallic barium was obtained by the English chemist G. Davy (1808) by electrolysis of wet hydroxide with a mercury cathode. The content of barium in the earth's crust is 0.05% by weight, in a free state it does not occur in nature. Of the barium minerals, barite (heavy spar) and less common witherite are of industrial importance. </div> <div>   </div>