Select the correct statement(s) about chemical behavior of lanthanoids:

Options

A. `II^(nd)` I.E. are comparable with those of calcium.
B. Earlier members of the series are quite reactive, similar of calcium.
C. With increasing atomic number, they behave more like aluminium.
D. The metals combine with hydrogen when gently heated in the gas.

Explanation

To solve the question regarding the chemical behavior of lanthanides, we will analyze each statement provided and determine their correctness based on the known properties of lanthanides.

### Step-by-Step Solution:

1. **Understanding the Electronic Configuration**:
- First, we need to know the electronic configuration of calcium (Ca) and a representative lanthanide, cerium (Ce).
- Calcium: \( \text{[Ne]} 3s^2 \) (or \( 1s^2, 2s^2, 2p^6, 3s^2 \))
- Cerium: \( \text{[Xe]} 4f^1 5d^1 6s^2 \)

2. **Analyzing the First Statement**:
- The first statement claims that the second ionization energy of lanthanides is equal to that of calcium.
- The second ionization energy of calcium is approximately 1100 kJ/mol. For cerium, it is also around the same value, indicating that this statement is **correct**.

3. **Analyzing the Second Statement**:
- The second statement suggests that early members of lanthanides are quite reactive, similar to calcium.
- Since both calcium and cerium have two electrons in their outermost s-orbital (4s for Ca and 6s for Ce), they exhibit similar reactivity. Thus, this statement is also **correct**.

4. **Analyzing the Third Statement**:
- The third statement posits that as the atomic number increases, lanthanides start behaving like aluminum.
- Aluminum is in the p-block, and as we move across the lanthanides, they do exhibit some behavior similar to p-block elements due to the filling of f-orbitals. Therefore, this statement is also **correct**.

5. **Analyzing the Fourth Statement**:
- The fourth statement states that when lanthanides combine with hydrogen, they form hydrides gently.
- Lanthanides do react with hydrogen to form hydrides, and this reaction is typically mild. Hence, this statement is also **correct**.

### Conclusion:
After analyzing all statements, we conclude that **all statements regarding the chemical behavior of lanthanides are correct**.

### Final Answer:
All statements about the chemical behavior of lanthanides are correct.

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