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

Hint: Lanthanides are $f - $ block transition elements but their properties differ from $d - $ block transition elements. These elements are called rare earth elements. Like lanthanide there exist actinides as well. Actinides are also $f - $ block transition elements.

Complete step by step answer:
We know lanthanides are $f - $ block transition elements. General configuration of lanthanides is $\left( {n - 2} \right){f^{1 - 14}}\left( {n - 1} \right){d^{0 - 1}}n{s^2}$. Lanthanide elements generally lose their electrons in outermost $s$ and $d$ electrons. Due to this reason these have $ + 3$ oxidation state. We know that the $d$ and $f$ electrons have poor shielding effects. This means these elements do not protect outermost electrons from the hold of the nucleus. Therefore as the number of $f$ electrons increases the shielding effect decreases and hold of the nucleus on the outermost electron will increase. As the hold of the nucleus is increasing, the size of the atom will decrease on the increasing number of $f$ electrons. As the atomic number will decrease, the number of $f$ electrons will increase and size will decrease. This means option A is correct which states that the ionic size of $Ln\left( {{\rm I}{\rm I}{\rm I}} \right)$ decreases in general with increasing atomic number. Most of the lanthanides are colored because of the $f - f$ transition. In this transition, there is a transition of $f$ electrons which are in lower orbital to the $f$ orbital which has lower energy. This transition is the cause of the color of lanthanides. This means statement two is false which states that $Ln\left( {{\rm I}{\rm I}{\rm I}} \right)$ compounds are generally colorless. We had to find the incorrect option and option B is incorrect. Therefore the correct answer is option B.

Note:
Actenoid series is the series of $15$ metallic chemical compounds. Atomic numbers of these elements are from $89 - 103$. Properties of elements belonging to actenoid series is similar to the element actinium due to which this series is named as actenoid series.