The first ionisation potential of which of the element is highest

Options

A. `Na`
B. `Mg`
C. `Al`
D. `Si`

Explanation

To determine which element has the highest first ionization potential among the given options, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Elements**: First, we need to identify the elements provided in the question. Let's assume the elements are Sodium (Na), Magnesium (Mg), Aluminum (Al), and Silicon (Si).

2. **Understand Ionization Potential**: The first ionization potential (or energy) is the energy required to remove the outermost electron from a neutral atom in the gas phase. Generally, ionization energy increases across a period from left to right in the periodic table.

3. **Trends in Ionization Energy**: As we move from left to right across a period:
- Atomic size decreases due to increased nuclear charge.
- Greater nuclear charge means that the outer electrons are held more tightly, leading to higher ionization energies.

4. **Order of Elements in the Same Period**: For the elements Na, Mg, Al, and Si:
- Sodium (Na) has the lowest ionization energy.
- Magnesium (Mg) has a higher ionization energy than Sodium.
- Aluminum (Al) has a higher ionization energy than Magnesium.
- Silicon (Si) has the highest ionization energy among these elements.

5. **Consider the Penetration Effect**: The exception in the trend occurs between Magnesium and Aluminum due to the penetration effect. The 3s electrons in Magnesium are closer to the nucleus compared to the 3p electrons in Aluminum, making it easier to remove an electron from Aluminum than from Magnesium.

6. **Final Comparison**: Based on the trends and the penetration effect, we can conclude that:
- Silicon (Si) has the highest first ionization potential among the given elements.

### Conclusion:
The element with the highest first ionization potential is **Silicon (Si)**.