This means that the nucleus of Cerium has 58 protons in it . The atomic mass of Cerium is 140.12 . The remaining Î cerium starts to change to Î cerium when cooled to â `` 172 Â C , and the transformation is complete at â `` 269 Â C. Î Cerium has a density of 8.16 ; Î cerium exists above 726 Â C. At atmospheric pressure , liquid cerium is more dense than its solid form at the melting point .