Question

What is the total number of inner transition elements in the periodic table ?
(A). ${\text{10}}$
(B). ${\text{14}}$
(C). ${\text{30}}$
(D). ${\text{28}}$

Answer 1

The inner transition elements consist of lanthanides and actinides. There are in total ${\text{14}}$ lanthanoids and ${\text{14}}$ actinides respectively that contribute to the inner transition element series.

Complete step by step answer:
The elements of f-block are also known as inner transition elements .
The elements comprising the f-block are those in which $4F$ and $5F$orbitals are filled. These elements are members of group ${\text{3}}$ formally but they are shown separately as f-block of the periodic table.
The two series of inner transition elements that are $4F$ and $5F$ are known as lanthanoids and actinoids. When the electron enter in one of the ${\text{4F}}$ orbital, then the element is of the first series of the inner transition elements and when the element is obtained by the filling of ${\text{5F}}$ orbitals successively, then they are said to belong to the actinides.
Due to similar outer shell configuration and respective energies of the f-orbital electrons, the lanthanides and actinides give the oxidation state of ${\text{ + 3}}$ without any exception. Even all the elements have the same properties due to the same size and similar outer shell configurations.
There are ${\text{14}}$ lanthanide. The elements are called lanthanides because they exist after lanthanum in the periodic table. There are ${\text{14}}$ actinides and these are known as actinides because of the same fact that they exist in the periodic table after the element named actinium. Hence, there are ${\text{28}}$ inner transition elements.
Therefore, the option D is correct.

Additional Information :-
The main characteristics of the inner transition elements are :
a. The third last shell is filled with electrons in the inner transition elements.
b. These all form colored ions.
c. The actinides are radioactive by nature.
d. These all show variable valencies.
e. The elements beyond atomic number $92$ up to $103$ are synthetic as well as radioactive.
f. Some of the inner transition elements are a good source of nuclear energy such as ${\text{U,}}\,{\text{Th,}}\,{\text{Pa}}$.

Note: The general electronic configuration of lanthanides is $4{f^{1 - 14}}5{p^6}5{d^{0 - 1}}6{s^2}$ and of actinides is $5{f^{1 - 14}}6{s^2}6{p^6}6{d^{0 - 1}}7{s^2}$ . The lanthanides start from cerium and end at lutetium whereas the actinides start from thorium and end at lawrencium.