transition metals display notable differences in chemical behavior. Like iron but unlike osmium, ruthenium can form aqueous cations in its lower oxidation states of +2 and +3. Ruthenium is the first in a downward trend in the melting and boiling points and atomization enthalpy in the 4d transition metals after the maximum seen at molybdenum, because the 4d subshell is more than half full and the electrons are contributing less to metallic bonding. (Technetium, the previous element, has an exceptionally low value that is off the trend due to its half-filled [Kr]4d55s2 configuration, though it is not as far off the trend in the 4d series as manganese in the 3d transition series.) Unlike the lighter congener iron, ruthenium is paramagnetic at room temperature, as iron also is above its Curie point. The reduction potentials in acidic aqueous solution for some common ruthenium ions are shown below: Isotopes Naturally occurring ruthenium is composed of seven stable isotopes. Additionally, 34 radioactive isotopes have been discovered. Of these radioisotopes, the most stable are 106Ru with a half-life of 373.59 days, 103Ru with a half-life of 39.26 days and 97Ru with a half-life of 2.9 days. Fifteen other radioisotopes have been characterized with atomic weights ranging from 89.93 u (90Ru) to 114.928 u (115Ru). Most of these have half-lives that are less than five minutes except 95Ru (half-life: 1.643 hours) and 105Ru (half-life: 4.44 hours). The primary decay mode before the most abundant isotope, 102Ru, is electron capture and the primary mode after is beta emission. The primary decay product before 102Ru is technetium and the primary decay product after is rhodium. 106Ru is a product of fission of a nucleus of uranium or plutonium. High concentrations of detected atmospheric 106Ru were associated with an alleged undeclared nuclear accident in Russia in 2017. Occurrence As the 78th most abundant element in Earth's crust, ruthenium is relatively rare, found in about 100 parts per trillion. This element is generally found in ores with the other platinum group metals in the Ural Mountains and in North and South America. Small but commercially important quantities are also found in pentlandite extracted from Sudbury, Ontario, Canada, and in pyroxenite deposits in South Africa. The native form of ruthenium is a very rare mineral (Ir replaces part of Ru in its structure). Ruthenium has a relatively high fission product yield in nuclear fission and given that its most