Knowing that the chemistry of lanthanoids (Ln) is dominated by its +3 oxidation state, which of the following statement is incorrect?

Ln (II) Compounds are generally colourless

The correct answer is option 3. Ln(II) compounds are generally not colorless.

The incorrect statement is: The ionic sizes of Ln (III) decrease in general with increasing atomic number

The chemistry of lanthanoids (Ln) is indeed dominated by their +3 oxidation state

However, the ionic sizes of Ln (III) ions do not decrease in general with increasing atomic number.

Due to the lanthanide contraction phenomenon, the ionic radii of Ln (III) ions do not follow the typical trend seen in other elements. The lanthanide contraction occurs due to poor shielding of nuclear charge by the 4f electrons, resulting in a smaller increase in atomic size as one moves across the series. As a result, the ionic sizes of Ln (III) ions remain relatively consistent across the lanthanoid series, rather than decreasing with increasing atomic number.

Therefore, the statement "The ionic sizes of Ln (III) decrease in general with increasing atomic number" is incorrect.

The other statements are correct.

Statement (1): Because of the large size of Ln(III) ions, the bonding in its compounds is predominantly ionic in character. This is because the large size of the Ln(III) ions means that they have a low charge density, which makes them less likely to form covalent bonds.

Statement (2): The ionic sizes of Ln(III) decrease in general with increasing atomic number. This is because the lanthanide contraction causes the lanthanide ions to become smaller as the atomic number increases.

Statement (4): Ln(III) hydroxides are mainly basic in character. This is because the Ln(III) ions have a high charge and a low charge density, which makes them strongly basic.

Therefore, the answer is (3) Ln(II) compounds are generally not colorless.

Here are some additional details about the chemistry of lanthanides:

The lanthanides are a group of 15 elements in the periodic table, with atomic numbers 57 to 71.

The lanthanides are all metals, and they have very similar chemical properties.

The most common oxidation state of the lanthanides is +3.

The lanthanides are used in a variety of applications, such as in magnets, lasers, and phosphors.