Which statement is false? multiple choice fission occurs among the heaviest isotopes, whereas fusion occurs more readily for light isotopes. the mass defect (δm) for a fission reaction is negative, whereas δm for fusion is positive. in order for fusion reactions to occur, temperatures must be in the millions of degrees. the fission of pu-239 atoms produces isotopes of many different elements. neutron-induced fission processes can occur at room temperature, rather than at millions of degrees.