Correct order of ionisation enthalpy is

Options

A. `Be lt B lt C lt N lt O`
B. `BltBeltCltNltO`
C. `BeltBltCltOltN`
D. `BltBeltCltOltN`

Answer

To determine the correct order of ionization enthalpy for the elements Beryllium (Be), Boron (B), Carbon (C), Nitrogen (N), and Oxygen (O), we need to analyze their electronic configurations and the general trends in ionization energy.

### Step-by-Step Solution:

1. **Understanding Ionization Enthalpy**:
Ionization enthalpy (or ionization energy) is the energy required to remove an electron from an atom in the gaseous state. Generally, ionization energy increases across a period (from left to right) due to increasing nuclear charge and decreasing atomic radius.

2. **Identify the Electronic Configurations**:
- Beryllium (Be): 1s² 2s²
- Boron (B): 1s² 2s² 2p¹
- Carbon (C): 1s² 2s² 2p²
- Nitrogen (N): 1s² 2s² 2p³
- Oxygen (O): 1s² 2s² 2p⁴

3. **Analyze the Trends**:
- **Beryllium to Boron**: Beryllium has a full 2s subshell, while Boron has one electron in the 2p subshell. Removing an electron from Boron is easier than from Beryllium because of the increased shielding effect.
- **Boron to Carbon**: The ionization energy increases as we move from Boron to Carbon because Carbon has a higher nuclear charge and more electrons in the same shell.
- **Carbon to Nitrogen**: Nitrogen has a half-filled 2p subshell (2p³), which is relatively stable. Therefore, it has a higher ionization energy than Carbon.
- **Nitrogen to Oxygen**: Oxygen has one more electron than Nitrogen, which leads to increased electron-electron repulsion in the 2p subshell, making it easier to remove an electron compared to Nitrogen.

4. **Establish the Correct Order**:
Based on the analysis:
- Nitrogen (N) has the highest ionization energy due to its stable half-filled configuration.
- Oxygen (O) follows due to increased repulsion.
- Carbon (C) comes next.
- Beryllium (Be) has a higher ionization energy than Boron (B) because it has a filled 2s subshell.
- Boron (B) has the lowest ionization energy among these elements.

Therefore, the correct order of ionization enthalpy is:
**N > O > C > Be > B**

### Final Order:
The correct order of ionization enthalpy is:
**Nitrogen (N) > Oxygen (O) > Carbon (C) > Beryllium (Be) > Boron (B)**