Assertion: There is a continuous decrease in size among lanthanoids.
Reason : Lanthanoids show lanthanoid contraction.

Options

A. Assertion and reason both are correct statements and reason is correct explanation for assertion.
B. Assertion and reason both are correct statements but reason is not correct explanation for assertion.
C. Assertion is correct statement but reason is wrong statement.
D. Assertion is wrong statement but reason is correct statement

Answer

To solve the given question regarding the assertion and reason about lanthanoids, we can break it down into a step-by-step analysis:

### Step 1: Understand the Assertion
The assertion states that there is a continuous decrease in size among lanthanoids. The lanthanoid series consists of elements with atomic numbers from 57 (Lanthanum, La) to 71 (Lutetium, Lu). As we move from La to Lu, the atomic radius decreases.

**Hint:** Look at the periodic trends in atomic size as you move across a series.

### Step 2: Analyze the Reason
The reason provided is that lanthanoids show lanthanoid contraction. Lanthanoid contraction refers to the gradual decrease in the size of the lanthanoid ions as we move from La to Lu due to the increasing nuclear charge without a corresponding increase in shielding effect.

**Hint:** Consider how the electron configuration and shielding effect influence atomic size.

### Step 3: Connect the Assertion and Reason
Both the assertion and reason are correct. The decrease in size (assertion) is indeed due to the lanthanoid contraction (reason). The contraction occurs because the 4f electrons do not shield the increasing nuclear charge effectively, leading to a stronger attraction between the nucleus and the outer electrons, which results in a decrease in size.

**Hint:** Think about how electron shielding affects the effective nuclear charge experienced by outer electrons.

### Step 4: Conclusion
Since both the assertion and reason are correct and the reason correctly explains the assertion, we can conclude that the answer to the question is:

**Assertion is correct, Reason is correct, and Reason is the correct explanation of Assertion.**

### Final Answer:
Assertion: True
Reason: True
Explanation: The reason explains the assertion correctly.

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