The incorrect statement among the following is

Options

A. Most of the trivalent lanthanoid ions are colorless in the solid state
B. Lanthanoids are good conductors of heat and electricity
C. Actinoids are highly reactive metals, especially when finely divided
D. Actinoid contraction is greater for element to element than lanthanoid contraction

Explanation

To determine the incorrect statement regarding the lanthanides and actinides, let's analyze the statements provided in the question step by step.

### Step 1: Understand the Series
The lanthanides (4f series) and actinides (5f series) are two series of f-block elements. The lanthanides include elements from lanthanum (La) to lutetium (Lu), while the actinides range from actinium (Ac) to lawrencium (Lr).

**Hint:** Remember that the lanthanides are primarily known for their f-electron configuration, which influences their properties.

### Step 2: Analyze the First Statement
The first statement claims that "most of the trivalent lanthanides are colorless." This is incorrect because many trivalent lanthanides exhibit vibrant colors due to the presence of f-electrons that can absorb visible light.

**Hint:** Consider the electronic transitions that occur in f-block elements and how they relate to color.

### Step 3: Analyze the Second Statement
The second statement discusses the conductivity of lanthanides and states that "lanthanides are good conductors." This is generally true as lanthanides have metallic properties and good electrical conductivity.

**Hint:** Recall the general properties of metals and how they relate to conductivity.

### Step 4: Analyze the Third Statement
The third statement mentions that "the length of lanthanides and actinides is high." This is true as these elements have larger atomic and ionic sizes compared to other elements in the periodic table.

**Hint:** Think about the trends in atomic size across the periodic table.

### Step 5: Analyze the Fourth Statement
The fourth statement refers to "the shielding power of 5f being less than that of 4f." This is true, as the 4f orbitals provide better shielding than the 5f orbitals due to their shape and penetration.

**Hint:** Consider how the shape of orbitals affects their shielding capabilities.

### Conclusion
After analyzing all the statements, the incorrect statement is the first one: "most of the trivalent lanthanides are colorless."

### Final Answer
The incorrect statement among the following is: "most of the trivalent lanthanides are colorless."

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