The element which has highest 2nd ionisation energy is

Options

A. Na
B. Mg
C. Ca
D. Ar

Answer

To determine the element with the highest second ionization energy among sodium, magnesium, calcium, and argon, we will analyze the electronic configurations and the implications of removing electrons from these elements.

### Step-by-Step Solution:

1. **Understanding Ionization Energy**:
- Ionization energy is the energy required to remove an electron from a neutral isolated gaseous atom. The second ionization energy specifically refers to the energy required to remove a second electron after the first has already been removed.

2. **Identify the Electronic Configurations**:
- Sodium (Na): Atomic number 11 → Electronic configuration: 2, 8, 1
- Magnesium (Mg): Atomic number 12 → Electronic configuration: 2, 8, 2
- Calcium (Ca): Atomic number 20 → Electronic configuration: 2, 8, 8, 2
- Argon (Ar): Atomic number 18 → Electronic configuration: 2, 8, 8

3. **First Ionization Energy Analysis**:
- When the first electron is removed from sodium (Na), it achieves a stable noble gas configuration (2, 8). The configuration becomes 2, 8 after removing one electron.
- For magnesium (Mg) and calcium (Ca), upon removing one electron, their configurations become 2, 8, 1 and 2, 8, 8, 1 respectively, which are not as stable as sodium's configuration after its first ionization.

4. **Second Ionization Energy Consideration**:
- For sodium, removing the second electron from the stable configuration (2, 8) is very difficult, resulting in a high second ionization energy.
- For magnesium, after the first ionization, the configuration becomes 2, 8, 1, and removing the second electron is less difficult compared to sodium.
- For calcium, the configuration becomes 2, 8, 8, 1 after the first ionization, which is also less stable than sodium's configuration.
- Argon is already a noble gas (2, 8, 8) and does not have a second ionization energy in the same sense, as it is very difficult to remove an electron from a stable noble gas configuration.

5. **Conclusion**:
- Among the given elements, sodium has the highest second ionization energy because removing the second electron from its stable noble gas configuration (after the first ionization) requires significantly more energy compared to the other elements.

### Final Answer:
The element which has the highest second ionization energy is **Sodium (Na)**.