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

Options

A. The ionic sizes of `Ln(III)` decrease in general with increasing atomic number.
B. `Ln(III) ` compounds are generally colourless
C. `Ln(III)` hydroxides are mainly basic in character
D. Because of the large size of the `Ln (III)` ions of the bonding in its compounds is predominantly ionic in character.

Explanation

To solve the question regarding the incorrect statement about lanthanoids, we will analyze each statement one by one based on the chemistry of lanthanides, which is primarily characterized by the +3 oxidation state.

### Step-by-Step Solution:

1. **Statement 1: The ionic size of lanthanide +3 decreases in general with the increase in atomic number.**
- **Analysis:** This statement is correct. As we move from cerium (Ce) to lutetium (Lu) in the lanthanide series, the ionic size decreases. This is due to two reasons:
- **Increased Nuclear Pull:** As the atomic number increases, the nuclear charge also increases, pulling the electrons closer to the nucleus.
- **Lanthanide Contraction:** The poor shielding effect of the 4f electrons leads to a stronger attraction between the nucleus and the outer electrons, resulting in a decrease in ionic size.

2. **Statement 2: Ln +3 compounds are generally colorless.**
- **Analysis:** This statement is incorrect. Lanthanide compounds are generally colorful due to f-f transitions. Unlike transition metals that show d-d transitions, lanthanides exhibit f-f transitions, which can absorb visible light and thus impart color to their compounds. Therefore, this statement is the incorrect one.

3. **Statement 3: Ln3+ hydroxides are mainly basic in nature.**
- **Analysis:** This statement is correct. Lanthanide hydroxides are basic because they are ionic in nature. The larger ionic size of Ln3+ makes it easier for these ions to dissociate in solution, leading to the formation of hydroxide ions (OH-) and thus exhibiting basic properties.

4. **Statement 4: Because of the large size of the Ln3+ ions, the bonding in its compounds is predominantly ionic in nature.**
- **Analysis:** This statement is also correct. The large size of the Ln3+ ions results in a lower effective nuclear charge experienced by the outermost electrons, making it easier for these ions to form ionic bonds. Hence, the bonding in lanthanide compounds is predominantly ionic.

### Conclusion:
- The only incorrect statement among the options provided is **Statement 2: Ln +3 compounds are generally colorless.**