Which of the following transformations is considered in defining ionization energy ?

Options

A. `A(g) + aq to A^(+) (aq) + e^(-)`
B. `A(g) + e^(-) to A^(-) + e^(-)`
C. `A(g) to A^(+)(g) + e^(-)`
D. `A(aq) to A^(+)(aq) + e^(-)`

Explanation

To solve the question regarding which transformation is considered in defining ionization energy, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Ionization Energy**:
- Ionization energy is defined as the minimum amount of energy required to remove the most loosely bound electron (usually a valence electron) from an isolated gaseous atom to form a cation.

2. **Identify the Key Characteristics**:
- The transformation must involve:
- An isolated gaseous atom.
- The removal of an electron.
- The formation of a cation (positively charged ion).

3. **Analyze the Options**:
- Look at each option provided in the question and check if they meet the criteria for ionization energy.
- Specifically, check if the transformation involves a gaseous atom and results in a cation.

4. **Evaluate Each Option**:
- **Option A**: If it describes a gaseous atom losing an electron to form a cation, it is a candidate.
- **Option B**: Similar evaluation as above.
- **Option C**: If it describes an anion or involves an aqueous state, it does not fit the definition.
- **Option D**: If it describes an aqueous solution, it does not fit the definition.

5. **Select the Correct Option**:
- After evaluating all options, choose the one that correctly represents the transformation of a gaseous atom losing an electron to form a cation.

### Conclusion:
- The correct option that defines ionization energy is the one that describes a gaseous atom losing an electron to form a cation.