The elements in which the last electron enter the ante-penultimate energy level, i.e., (n - 2)f-orbital are called f-block elements. These elements have been termed as f-block elements as the last electron enters in one of the f-orbitals. These elements are also known as the inner transition elements. This is because the last electron in them enters into (n - 2)-orbital, i.e., inner to the penultimate energy level and they form a transition series within the transition series. The general electronic configuration is:

\[(n - 2)f^{1-14}(n - 1)d^{0-1}ns^2\]

Classification of f-block elements:

Depending upon whether the last electron enters a 4f-orbital or a 5f-orbital, the f-block elements have been divided into two series as follows: 

(i) Lanthanides:The elements in which the last electron enters one of the 4f-orbitals are called 4f-block elements or first inner transition series. These are also called lanthanides or lanthanons or lanthanoids because they come immediately after lanthanum.

(ii) Actinides: The elements in which the last electron enters one of the 5f-orbitals are called 5f-block elements or second inner transition series. These are also called actinides or actinons or actinoids because they come immediately after actinium.

Which of the following statements is not correct?

The tendency of d-block elements to form complexes is less than that of f-block elements

The correct answer is option 3. The tendency of d-block elements to form complexes is less than that of f-block elements.

The incorrect statement is: The tendency of d-block elements to form complexes is less than that of f-block elements.

This statement is not correct because the d-block elements (transition metals) are well-known for their strong tendency to form complexes. This is due to their ability to exhibit variable oxidation states, the presence of vacant d-orbitals, and their comparatively high charge densities, which make them highly effective at attracting ligands and forming stable coordination compounds.

Explanation of the Other Statements:

1. The oxides and hydroxides of d-block elements are less basic than those of f-block elements:

This is generally correct. The oxides and hydroxides of f-block elements (lanthanoids and actinoids) are more basic due to their larger ionic sizes and lower charge densities compared to d-block elements.

2. The sizes of the atoms and ions of d-block elements are relatively smaller than those of the f-block elements:

This is correct. F-block elements (especially the lanthanoids) are larger in size compared to d-block elements due to the presence of electrons in the f-orbitals, which are less effective at shielding the nuclear charge. However, it's important to note that after the lanthanoid contraction, the sizes of the later lanthanoids can be comparable to those of the d-block elements.

4. f-block elements are called inner transition elements:

This is correct. The f-block elements, which include the lanthanoids and actinoids, are referred to as inner transition elements because their f-orbitals are being filled in the penultimate energy levels (4f and 5f), in contrast to d-block elements where the d-orbitals are being filled.