Question

Among the Lanthanides, the one obtained by synthetic method is:
A. Lu
B. Pr
C. Pm
D. Gd

Answer 1

Hint: The lanthanide series consists of fifteen elements starting from lanthanum to lutetium. The elements of this series have no partly filled 4f- subshell but have electrons in 5d-shell. Studying the properties of the given lanthanoids, we can determine which one is obtained synthetically.

Complete Solution :
To solve the question, let us know about the given lanthanoids.
The properties of the given Lanthanides are:
1. Lutetium (Lu):
Atomic number: 71
Electronic configuration: [Xe] \[4{{f}^{14}}5{{d}^{1}}6{{s}^{2}}\]
 Mass number: 174.97
Characteristic: shiny white metal; corrosion resistant in dry air but not in moist air; has the highest density, melting point, and hardness of the Lanthanides.
It is the last member of the lanthanide series. Due to lanthanide contraction, the lutetium atom is the smallest among the Lanthanides. It occurs naturally on the Earth in the form of two isotopes: lutetium-175 and lutetium-176.

2. Praseodymium (Pr):
Atomic number: 59
Electronic configuration: [Xe] \[4{{f}^{3}}6{{s}^{2}}\]
Mass number: 140.91
Characteristic: soft, silvery, ductile, malleable metal valued for its magnetic and electric properties.
It is the third member of the lanthanide series. Praseodymium occurs naturally together with the other rare-earth metals. It has only one stable and naturally occurring isotope, Pr-141.

3. Promethium (Pm):
Atomic number: 61
Electronic configuration: [Xe] \[4{{f}^{5}}6{{s}^{2}}\]
Mass number: 146.92
Characteristic: pink or red in colour; has a double hexagonal close packed structure.
Promethium-146 does not occur naturally, and is manufactured synthetically in particle accelerators by bombarding uranium-235 with thermal neutrons. The other two isotopes of promethium, Pr-145 and Pr-147, occur in trace amounts.

4. Gadolinium (Gd):
Atomic number: 64
Electronic configuration: [Xe] \[4{{f}^{7}}5{{d}^{1}}6{{s}^{2}}\]
Mass number: 157.25
Characteristic: silvery white; slightly malleable and ductile rare-earth metal.
It is found in nature only in an oxidized form. Naturally occurring gadolinium is composed of six isotopes: Gd-154, Gd-155, Gd-156, Gd-157, Gd-158, Gd-160.

Thus, we can conclude from these properties that Promethium (Pr) is the only lanthanoid among the given that is obtained by synthetic method.
So, the correct answer is “Option C”.

Note: Some isotopes of the elements gadolinium, lutetium, promethium are obtained synthetically, but they are in traces and have very short-half lives. Promethium is obtained in abundance by synthetic method only.