Question

Knowing that the chemistry of (Ln) is dominated by its +3 oxidation state, which of the following statements is incorrect?
A. The ionic size of Ln (III) decreases in general with increasing atomic number.
B. Ln (III) compounds are generally colorless
C. Ln (III) hydroxide are mainly basic in character
D. Because of the large size of the Ln (III) ions the bonding in its compound is predominantly ionic in character.

Answer 1

Hint: Lanthanides belong to f-block transitions elements and there is difference in the properties of d-block transition elements. These elements are also known as rare earth elements. Most of the lanthanides are colored due to f-f transition. Actinides also belongs to f-block transition elements.

Complete answer:
From your chemistry lessons you have learned about the lanthanides and actinides.
Lanthanides having electronic configuration of $(n-2){{f}^{1-14}}(n-1){{d}^{0-1}}n{{s}^{2}}$ belongs to f- block transition elements. The +3 oxidation state of lanthanide is due to the loss of their electrons in the outermost s and d electrons. As you know that shielding or screening effects of d and f orbitals are very poor, so they are not able in protecting the outer electrons from the holding power of the nucleus.
Hence, the increase in the no. of electrons in f orbitals decreases the shielding effect and will increase the hold of the nucleus on the outermost electron. When the hold of nucleus on the outermost electron increases the size of the atom will decrease because the no. of electrons of electrons increases in the f-orbital.
So, when the atomic no. will decrease and the no. of electrons in f-orbital will increase and size will decrease.
Thus the option (A) will be correct because the ionic size of Ln decreases in general with increasing atomic number.
The color shown by most of the lanthanides is due to the f-f transition in this type of transition the electrons from the f orbital shows transition to the lower orbital of f orbital which will have lower energy. And due to this transition lanthanides have color .
Thus the incorrect one among the given options is (B) which states that Ln is colorless.

Therefore the correct answer will be option (B).

Note:
Like lanthanides, actinides also exist and actenoid series is the group of 15 metallic chemical compounds. In the periodic table the elements with the atomic number of 89-103 under actenoid. The name of this series is given on the name of element actinium because the properties of actinium and actenoid series are very much similar.