Question

A major difference between the two rows of inner transition elements is that the lanthanides _____.


a.are radioactive and the actinides are not
b.occur in nature while most of the actinides are synthetic
c.are gases at room temperature and the actinides are solids
d.have no uses while the actinides have many

Answer 1

B. The so-called "rare-earth" elements, the lanthanides, are not very rare at all. But aside from thorium and uranium, the actinides all have too short half-lives to be found in nature.

As for the other choices: promethium (Pm), a lanthanide, is radioactive, so that rules out A. All of the lanthanides and actinides are transition metals, so that rules out C. In fact, very few elements other than the noble gases are gases at room temperature. (Only the halogens, oxygen, nitrogen and hydrogen). And, finally, the lanthanides do have some important uses, as catalysts, for example. Additionally, "rare earth" magnets are made from neodymium or samarium, usually, two lanthanides.