In which of the following hydrides, hydrogen exists in negative oxidation state?

Options

A. `HCl`
B. `NaH`
C. `CaH_(2)`
D. `HI`

Explanation

To determine in which of the given hydrides hydrogen exists in a negative oxidation state, we will calculate the oxidation state of hydrogen in each compound provided.

### Step-by-Step Solution:

1. **Identify the Compounds**: The compounds we need to analyze are HCl, NaH, CaH2, and HI.

2. **Calculate Oxidation State in HCl**:
- Let the oxidation state of hydrogen be \( X \).
- The oxidation state of chlorine (Cl) is \(-1\).
- The equation for HCl can be set up as:
\[ X + (-1) = 0 \]
- Solving for \( X \):
\[ X = +1 \]
- Therefore, in HCl, hydrogen has a positive oxidation state.

3. **Calculate Oxidation State in NaH**:
- Let the oxidation state of hydrogen be \( X \).
- The oxidation state of sodium (Na) is \( +1 \).
- The equation for NaH can be set up as:
\[ +1 + X = 0 \]
- Solving for \( X \):
\[ X = -1 \]
- Therefore, in NaH, hydrogen has a negative oxidation state.

4. **Calculate Oxidation State in CaH2**:
- Let the oxidation state of hydrogen be \( X \).
- The oxidation state of calcium (Ca) is \( +2 \).
- The equation for CaH2 can be set up as:
\[ +2 + 2X = 0 \]
- Solving for \( X \):
\[ 2X = -2 \implies X = -1 \]
- Therefore, in CaH2, hydrogen also has a negative oxidation state.

5. **Calculate Oxidation State in HI**:
- Let the oxidation state of hydrogen be \( X \).
- The oxidation state of iodine (I) is \(-1\).
- The equation for HI can be set up as:
\[ X + (-1) = 0 \]
- Solving for \( X \):
\[ X = +1 \]
- Therefore, in HI, hydrogen has a positive oxidation state.

### Conclusion:
From the calculations:
- In **HCl**, hydrogen is \( +1 \).
- In **NaH**, hydrogen is \( -1 \).
- In **CaH2**, hydrogen is \( -1 \).
- In **HI**, hydrogen is \( +1 \).

Thus, the hydrides where hydrogen exists in a negative oxidation state are **NaH and CaH2**.

### Final Answer:
The correct options are **NaH and CaH2**.