Across the lanthanide series,the basicity of the lanthanoide hydroxides:

Options

A. increases
B. decreases
C. first increases and then decreases
D. does not change

Explanation

To determine the trend of basicity of lanthanide hydroxides across the lanthanide series, we can follow these steps:

### Step 1: Understand the Lanthanide Series
The lanthanide series consists of 15 elements from Cerium (Ce) to Lutetium (Lu), with atomic numbers 58 to 71.

### Step 2: Recognize the Effect of Atomic Number on Ionic Size
As we move across the lanthanide series from Ce to Lu, the atomic number increases. This leads to an increase in the number of protons in the nucleus, which increases the effective nuclear charge (Z effective) experienced by the electrons.

### Step 3: Consider Lanthanide Contraction
The lanthanide contraction refers to the phenomenon where the ionic radii of the lanthanides decrease as we move from Ce to Lu. This is primarily due to the poor shielding effect of the 4f electrons, which results in a stronger attraction between the nucleus and the outer electrons.

### Step 4: Analyze the Impact on Basicity
As the size of the cation decreases (due to lanthanide contraction), the cations become more polarizing. A more polarizing cation can attract electron density from nearby anions more effectively, leading to an increase in covalent character of the hydroxides.

### Step 5: Relate Covalent Character to Basicity
Basicity is inversely related to covalent character. As the covalent character of the hydroxides increases (due to the increasing polarizing power of the cations), the basicity of the hydroxides decreases.

### Step 6: Identify the Trend
From the above analysis, we can conclude that the basicity of lanthanide hydroxides decreases as we move from Cerium hydroxide (most basic) to Lutetium hydroxide (least basic).

### Conclusion
Thus, the trend in basicity of lanthanide hydroxides across the lanthanide series is that it **decreases**.

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