Name the ion which is used for balancing the hydrogen atom in acidic medium.

Answer

To answer the question, "Name the ion which is used for balancing the hydrogen atom in acidic medium," we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Context**: In redox reactions, especially in acidic medium, we often need to balance the number of hydrogen atoms that appear in the half-reactions.

2. **Identifying the Ion**: In acidic medium, the ion that is commonly used to balance hydrogen atoms is the hydronium ion, represented as \( \text{H}_3\text{O}^+ \) or simply \( \text{H}^+ \).

3. **Application in Balancing**: When balancing redox reactions in acidic solutions, we add \( \text{H}^+ \) ions to the side of the equation that requires hydrogen atoms. This helps to ensure that the number of hydrogen atoms is equal on both sides of the equation.

4. **Example for Clarity**: For instance, if we consider the reaction of \( \text{MnO}_2 \) in acidic medium, it can be represented as:
\[ \text{MnO}_2 + \text{H}_2\text{O} \rightarrow \text{Mn}^{2+} + \text{H}^+ \]
Here, to balance the hydrogen atoms, we would add \( \text{H}^+ \) ions.

5. **Final Answer**: Therefore, the ion used for balancing the hydrogen atom in acidic medium is \( \text{H}^+ \) (hydronium ion).