19. cerium is the most abundant rare earth metal. pure cerium ignites when
scratched by even a soft object. it has four known isotopes: "ce (atomic mass
= 135.907 amu), ce (atomic mass = 137.905 amu), 4°сe (atomic mass =
139.905 amu), and "ce (atomic mass = 141.909 amu). ce-140 and ce-142 are
fairly abundant. which is the more abundant isotope?

Ais the correct answer

1. why does the same side of the moon always face earthc.the rotational period of the moon is equal to one revolution around earth2. which is greater the moons period of rotation or its period of revolution  b.the moons revolution period around earth3.what was the major accomplishment of the apollo missions  d.landing on and exploring the moon4. which of these space missions involved the deployment of lunar rovers d.apollo5. which scientists are going to explore next on kepler-186fd. evidence of existence of life6.which mission is not going to explore the outside of the solar systemb.safir7.which technology limit the future of space exploration b. scientist must first develop certain technologies before missions can be completed.