Which one has least ionisation potential?

Options

A. Ne
B. N
C. O
D. F

Explanation

To determine which element has the least ionization potential among the given options (Neon, Nitrogen, Oxygen, and Fluorine), we can follow these steps:

### Step 1: Understand Ionization Potential
Ionization potential (or ionization energy) is the energy required to remove an electron from an isolated atom in its gaseous state. Generally, elements with a stable electronic configuration (like noble gases) have high ionization potentials, while those with less stable configurations have lower ionization potentials.

**Hint:** Remember that noble gases have full valence shells, making them very stable and requiring more energy to remove an electron.

### Step 2: Analyze Each Element
1. **Neon (Ne)**:
- Atomic Number: 10
- Electronic Configuration: 1s² 2s² 2p⁶
- Neon has a complete octet (8 electrons in its outer shell), making it very stable. Thus, it has a high ionization potential.

**Hint:** Noble gases like Neon have the highest ionization energies due to their stable electron configuration.

2. **Nitrogen (N)**:
- Atomic Number: 7
- Electronic Configuration: 1s² 2s² 2p³
- Nitrogen has a half-filled p orbital, which is relatively stable, but it still requires a significant amount of energy to remove an electron.

**Hint:** Half-filled orbitals are stable, but not as stable as full orbitals.

3. **Oxygen (O)**:
- Atomic Number: 8
- Electronic Configuration: 1s² 2s² 2p⁴
- Oxygen is just one electron short of a half-filled p orbital. Removing one electron from oxygen will lead to a more stable half-filled configuration, which means it requires less energy to ionize compared to nitrogen.

**Hint:** Elements that can achieve a more stable configuration by losing an electron tend to have lower ionization potentials.

4. **Fluorine (F)**:
- Atomic Number: 9
- Electronic Configuration: 1s² 2s² 2p⁵
- Fluorine is very close to having a full octet and actually prefers to gain an electron rather than lose one. Therefore, it has a high ionization potential.

**Hint:** Elements that want to gain electrons (like Fluorine) will have higher ionization energies.

### Step 3: Compare Ionization Potentials
Based on the stability of their electronic configurations:
- Neon has the highest ionization potential.
- Nitrogen has a relatively high ionization potential due to its half-filled p orbital.
- Fluorine has a high ionization potential as it prefers to gain an electron.
- Oxygen has the least ionization potential among these options because removing one electron leads to a more stable half-filled configuration.

### Conclusion
The element with the least ionization potential among the given options is **Oxygen (O)**.

**Final Answer:** Oxygen has the least ionization potential.