Which of the following pair form a dative bond?

Options

A. `NH_3 and H^(+)`
B. `HF and H_2O`
C. `NH_3 and O_2`
D. `CH_4 and AlCl_3`

Explanation

To determine which pair forms a dative bond, we need to understand what a dative bond is. A dative bond (or coordinate bond) is formed when one atom donates a pair of electrons to another atom that has an empty orbital.

### Step-by-Step Solution:

1. **Identify the pairs given in the options**: We need to analyze the pairs provided in the question to see if any can form a dative bond.

2. **Analyze Option A: NH3 and H+**:
- NH3 (ammonia) has a lone pair of electrons on the nitrogen atom.
- H+ (a hydrogen ion) has an empty orbital.
- The nitrogen atom can donate its lone pair to the H+ ion, forming a dative bond.
- **Conclusion**: NH3 and H+ can form a dative bond.

3. **Analyze Option B: HF and H2O**:
- HF (hydrofluoric acid) does not have a lone pair that can be donated to H2O.
- H2O (water) has lone pairs, but HF does not have an empty orbital to accept them.
- **Conclusion**: No dative bond is formed between HF and H2O.

4. **Analyze Option C: O2 and CH4**:
- O2 (oxygen) has a double bond between the two oxygen atoms and does not have a lone pair to donate.
- CH4 (methane) has no empty orbitals to accept a lone pair.
- **Conclusion**: No dative bond is formed between O2 and CH4.

5. **Analyze Option D: AlCl3 and another molecule**:
- AlCl3 (aluminum chloride) can act as a Lewis acid and accept electron pairs.
- However, without a specific molecule to pair with, we cannot determine if a dative bond can be formed.
- **Conclusion**: We cannot conclude about the dative bond without more information.

### Final Answer:
The pair that forms a dative bond is **Option A: NH3 and H+**.