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 stability of their outermost electrons.

### Step-by-Step Solution:

1. **Identify the Elements and Their Electronic Configurations:**
- Beryllium (Be): Atomic number 4, electronic configuration: 1s² 2s²
- Boron (B): Atomic number 5, electronic configuration: 1s² 2s² 2p¹
- Carbon (C): Atomic number 6, electronic configuration: 1s² 2s² 2p²
- Nitrogen (N): Atomic number 7, electronic configuration: 1s² 2s² 2p³
- Oxygen (O): Atomic number 8, electronic configuration: 1s² 2s² 2p⁴

2. **Understand Ionization Enthalpy:**
- Ionization enthalpy is the energy required to remove an electron from an atom in the gas phase. The higher the ionization enthalpy, the more energy is needed to remove an electron.

3. **Analyze the Stability of the Outer Electrons:**
- The stability of the outermost electrons affects the ionization enthalpy.
- Nitrogen has a half-filled p-orbital configuration (2p³), which is particularly stable. This stability means that removing an electron from nitrogen requires more energy compared to the others.

4. **Order the Elements Based on Ionization Enthalpy:**
- Generally, as we move from left to right across a period in the periodic table, ionization enthalpy increases due to increased nuclear charge and decreased atomic radius.
- Based on the electronic configurations and the stability of half-filled orbitals, the order of ionization enthalpy from highest to lowest is:
- Nitrogen (N) > Oxygen (O) > Carbon (C) > Boron (B) > Beryllium (Be)

5. **Final Answer:**
- The correct order of ionization enthalpy is: **N > O > C > B > Be**.