Which of the following lanthanoids is rediocative ?

Options

A. Lutetium
B. Cerium
C. Promethium
D. Thulium

Answer

C

Explanation

The lanthanides are not particularly rare. Cerium is about as abundant are more abundant than iodine. The abundance of the elements and the number of naturally atomic number (i.e. an even number of protons in the nucleus) are more abundant than their neighbours with odd atomic numbers also have more abundant than their neighours with odd atomic numbers (Harkins rule). Elements with even atomic numbers also have more stable isotopes. Elements with odd atomic number never have more than two stable isotopes. Throughout the periodic table the stability of a nucleus is related to both the number of neutrons and the number of protons in the nucleus. Element promethium (Z = 61) does not occur naturlly. It was first made in 1946. Absence of Pm is explained by Mattauch's rule which states that if two elements with consecutive atomic numbers each have an isotope of the same weight, one of the istopes will be unstable. Since elements 60 and 62 have seven isotopes each, there are not many stable mass numbers available for promethium (Z = 16). According to Mattauch's rule, if Pm is to have a stable isotope, it must have a mass number outside the range 142 - 150. The onl.y isotopes of Pm which have been made so far are radioactive.