Question

The first element of actinide series is:
A.Thorium
B.Cerium
C.Lanthanum
D.Actinium

Answer 1

Basically, actinides are the elements with atomic number 90-103 following the element Actinium in the periodic table. This series includes naturally occurring elements like thorium, uranium and protactinium and eleven transuranic i.e. the elements that are artificially produced by nuclear reaction. So, to solve this question, we need to know the actinide series.

Complete step by step answer:
-The actinide series has basically been derived from the first element of the series i.e. actinium. Its symbol is An and is used while referring to any of the actinide series elements which range in the periodic table from atomic numbers 89-103. Moreover, all the actinide series elements are radioactive in nature and they release a large amount of energy on radioactive decay.
-So, the general electronic configuration of actinides is $[Rn]5{f^{1 - 14}}6{d^{0 - 1}}7{s^2}$ . These are the second series of elements of the f-block in the periodic table. The energy of 5f and 6d electrons are close to each other and hence the electrons enter into the 5f orbital.
The elements of these series are actinium, Thorium, Protactinium, Uranium, Neptunium, Plutonium, Americium, Curium, Berkelium, Californium, Einsteinium, Fermium, Mendelevium, Nobelium and Lawrencium. So, the first element is Actinium.
-Now, let’s talk about some chemical and physical properties of these elements. These are highly electropositive and radioactive in nature. These are very dense metals with distinctive structures. Moreover, they react with boiling water or dilute acid to release hydrogen gas. These are malleable and ductile and paramagnetic in nature.

Hence, option D is correct.

Note:
Generally, the actinides show variable oxidation states because of the smaller energy gap between 5f,6d and 7s orbitals. Though +3 is the most stable oxidation state, the other oxidation states are also possible due to the good shielding of f- electrons.