The ionisation energy of an element is

Options

A. The same as the electron affinity of the element
B. Equal in magnitude but of opposite sign to the electron affinity of the element
C. The energy released when an electron is added to an atom of the element
D. The energy required to remove the outermost electron of an atom of the element

Answer

### Step-by-Step Text Solution:

1. **Understanding Ionization Energy**:
- Ionization energy is defined as the amount of energy required to remove an electron from a gaseous atom or ion.

2. **Analyzing the Options**:
- The question provides several options regarding the definition of ionization energy. We need to evaluate each option to determine which one correctly describes ionization energy.

3. **Evaluating Option 1**:
- Option 1 states that ionization energy is the same as electron affinity. This is incorrect because electron affinity refers to the energy change when an electron is added to an atom, not removed.

4. **Evaluating Option 2**:
- Option 2 claims that ionization energy is equal in magnitude but of opposite sign to electron affinity. This is misleading as they are not directly comparable in this way; they represent different processes.

5. **Evaluating Option 3**:
- Option 3 describes ionization energy as the energy released when an electron is added to an atom. This is also incorrect, as it describes electron affinity, not ionization energy.

6. **Evaluating Option 4**:
- Option 4 states that ionization energy is the energy required to remove the outermost electron of an atom of the element. This accurately describes ionization energy.

7. **Conclusion**:
- Based on the evaluations, the correct answer is Option 4: the energy required to remove the outermost electron of an atom of the element.

### Final Answer:
The ionization energy of an element is the energy required to remove the outermost electron of an atom of the element (Option 4).

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