Which of the following order of ionisation energy is correct?

Options

A. `BeltBltCltNltO`
B. `BltBeltCltOltN`
C. `BegtBgtCgtNgtO`
D. `BltBeltNltCltO`

Explanation

To determine the correct order of ionization energy among the given elements, we will follow these steps:

### Step 1: Understand Ionization Energy
Ionization energy is defined as the minimum energy required to remove an electron from an atom in its gaseous state. It is influenced by the atomic size and the effective nuclear charge experienced by the outermost electrons.

### Step 2: Identify the Trends in Ionization Energy
1. **Down a Group**: Ionization energy decreases as you move down a group in the periodic table. This is because the outermost electrons are farther from the nucleus and are shielded by inner electrons, making them easier to remove.
2. **Across a Period**: Ionization energy increases as you move from left to right across a period. This is due to the increase in nuclear charge, which attracts the electrons more strongly, making them harder to remove.

### Step 3: Analyze the Given Elements
The elements mentioned are:
- Beryllium (Be)
- Boron (B)
- Carbon (C)
- Nitrogen (N)
- Oxygen (O)

These elements are arranged in the following order based on their position in the periodic table:
- Beryllium (Group 2, Period 2)
- Boron (Group 13, Period 2)
- Carbon (Group 14, Period 2)
- Nitrogen (Group 15, Period 2)
- Oxygen (Group 16, Period 2)

### Step 4: Determine the Ionization Energies
Based on the trends:
- Beryllium has the lowest ionization energy because it is an S-block element and has a larger atomic radius compared to the others.
- As we move to Boron, Carbon, Nitrogen, and Oxygen, the ionization energy increases due to the increasing nuclear charge and decreasing atomic radius.

### Step 5: Establish the Correct Order
Thus, the order of ionization energy from lowest to highest is:
**Be < B < C < N < O**

### Conclusion
The correct order of ionization energy is **Be < B < C < N < O**.