Radioactive isotopes actinoids

The actinoid elements (or actinides An) constitute a series of 14 elements which are formed by the progressive filling of the 5/ electron shell and follow actinium in the periodic table (atomic numbers 90-103). All of the isotopes of the actinide elements are radioactive and only four of the primordial isotopes, Th, and " " Pu, have a sufficient long half-life for there to be any of these left in nature. 

The lanthanoids resemble each other much more closely than do the members of a row of t/-block metals. The chemistry of the actinoids is more complicated, and in addition, only Th and U have naturally occurring isotopes. Studies of the transuranium elements (those with Z > 92) require specialized techniques. The occurrence of artihcial isotopes among the /-block elements can be seen from Appendix 5 all the actinoids are unstable with respect to radioactive decay (see Section 24.9), although the half-lives of the most abundant isotopes of thorium and uranium ( Th and t = 1.4 x lO and 4.5 x 10 yr respectively) are... 

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