Which of the following metals directly combine with hydrogen gas to give a hydride ?

Options

A. Au
B. Ni
C. Ca
D. Cu

Explanation

To determine which of the given metals directly combine with hydrogen gas to form a hydride, we will analyze the reactivity of each metal provided: Au (Gold), Ni (Nickel), Ca (Calcium), and Cu (Copper).

### Step-by-Step Solution:

1. **Identify the Metals**: The metals in question are Au, Ni, Ca, and Cu.

2. **Understand Reactivity**: Reactivity of metals generally increases as you move down the group in the periodic table. Alkali and alkaline earth metals are known to react with hydrogen to form hydrides.

3. **Evaluate Each Metal**:
- **Au (Gold)**: Gold is a noble metal and is not reactive enough to combine with hydrogen to form a hydride.
- **Ni (Nickel)**: Nickel is also less reactive and does not readily combine with hydrogen to form a hydride.
- **Ca (Calcium)**: Calcium is an alkaline earth metal. It is quite reactive and can easily combine with hydrogen.
- **Cu (Copper)**: Copper is a transition metal and does not react with hydrogen to form a hydride.

4. **Determine the Reaction**: The reaction for calcium with hydrogen is:
\[ \text{Ca} + \text{H}_2 \rightarrow \text{CaH}_2 \]
This shows that calcium can directly combine with hydrogen to form calcium hydride.

5. **Conclusion**: Among the given options, only Calcium (Ca) directly combines with hydrogen gas to give a hydride.

### Final Answer:
The metal that directly combines with hydrogen gas to give a hydride is **Calcium (Ca)**.

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