Ruthenium, «roo THEE nee uhm», is a rare, silver-white metallic element. It is used mainly in jewelry as a hardener of the metals platinum and palladium. Alloys of ruthenium with those metals are highly resistant to wear and are used for electrical contacts in the ignition systems of some aircraft engines. Karl Klaus, a Russian chemist, discovered ruthenium in 1844. Ruthenium has an atomic number (number of protons in its nucleus) of 44. Its relative atomic mass is 101.07. An element’s relative atomic mass equals its mass (amount of matter) divided by the 1/12 of the mass of carbon 12, the most abundant form of carbon. Ruthenium melts at about 2300 °C and boils at about 4000 °C. Its chemical symbol is Ru.