Ionisation energy increases in the order :

Options

A. `Be lt B lt C lt N`
B. `B le Be lt C lt N`
C. `C lt N lt Be lt B`
D. `N lt C lt Be lt B`

Answer

To determine the increasing order of ionization energy among the elements Beryllium (Be), Boron (B), Carbon (C), and Nitrogen (N), we need to analyze the trends in ionization energy based on their electronic configurations and positions in the periodic table.

### Step-by-Step Solution:

1. **Understanding Ionization Energy**:
- Ionization energy is the energy required to remove the most loosely bound electron from an atom in its gaseous state.

2. **Trends in Ionization Energy**:
- **Down a Group**: Ionization energy decreases as you move down a group because the number of electron shells increases, which leads to a greater distance between the nucleus and the outermost electron. This reduces the effective nuclear charge felt by the outermost electron.
- **Across a Period**: Ionization energy increases as you move from left to right across a period. This is due to the increase in the number of protons (nuclear charge) without a significant increase in shielding effect, which pulls the electrons closer to the nucleus.

3. **Identifying the Elements**:
- The elements in question are:
- Beryllium (Be) - Atomic number 4
- Boron (B) - Atomic number 5
- Carbon (C) - Atomic number 6
- Nitrogen (N) - Atomic number 7

4. **Electronic Configurations**:
- Be: 1s² 2s²
- B: 1s² 2s² 2p¹
- C: 1s² 2s² 2p²
- N: 1s² 2s² 2p³

5. **Analyzing Ionization Energies**:
- **Beryllium (Be)** has a filled 2s subshell, making it relatively stable and requiring more energy to remove an electron.
- **Boron (B)** has one electron in the 2p subshell, which is less stable than Be, so it has a lower ionization energy than Be.
- **Carbon (C)** has two electrons in the 2p subshell, which is still stable but less so than Be. Therefore, it has a higher ionization energy than B.
- **Nitrogen (N)** has three electrons in the 2p subshell, and it is half-filled, which gives it additional stability, resulting in a higher ionization energy than C.

6. **Final Order of Ionization Energy**:
- Based on the analysis:
- Be > B > C > N
- Therefore, the increasing order of ionization energy is:
- B < C < N < Be

### Conclusion:
The correct increasing order of ionization energy for the elements is:
**Boron (B) < Carbon (C) < Nitrogen (N) < Beryllium (Be)**.