Which is the correct increasing order of ionisation energy ?

Options

A. `Li lt B lt Be`
B. `Be lt B lt Li`
C. `Li lt Na lt K`
D. `O lt N lt F`

Explanation

To determine the correct increasing order of ionization energy, we need to understand the factors that influence ionization energy and the trends in the periodic table. Here’s a step-by-step solution:

### Step 1: Understand Ionization Energy
Ionization energy is the energy required to remove an electron from an atom in its gaseous state. It is influenced by two main factors:
- **Effective Nuclear Charge (Z_eff)**: The net positive charge experienced by an electron in a multi-electron atom.
- **Atomic Size**: As the atomic size increases, the outermost electrons are farther from the nucleus, making them easier to remove.

### Step 2: Analyze Trends in the Periodic Table
- **Across a Period**: As we move from left to right across a period, the atomic size decreases due to an increase in Z_eff, leading to an increase in ionization energy.
- **Down a Group**: As we move down a group, the atomic size increases because additional electron shells are added, resulting in a decrease in ionization energy.

### Step 3: Compare Specific Elements
1. **Lithium (Li)**, **Beryllium (Be)**, and **Boron (B)**:
- Li: 1s² 2s¹
- Be: 1s² 2s²
- B: 1s² 2s² 2p¹
- Ionization Energy Trend: Li < B < Be (Beryllium has a higher ionization energy than Boron due to the penetration effect).

2. **Sodium (Na)** and **Potassium (K)**:
- Na: 1s² 2s² 2p⁶ 3s¹
- K: 1s² 2s² 2p⁶ 3s² 3p¹
- Ionization Energy Trend: Na < K (Ionization energy decreases down the group).

3. **Oxygen (O)**, **Nitrogen (N)**, and **Fluorine (F)**:
- O: 1s² 2s² 2p⁴
- N: 1s² 2s² 2p³
- F: 1s² 2s² 2p⁵
- Ionization Energy Trend: N > O > F (Nitrogen has a higher ionization energy than Oxygen due to its half-filled p subshell).

### Step 4: Establish the Correct Order
Based on the analysis:
- For Li, Be, and B: Li < B < Be
- For Na and K: Na < K
- For O, N, and F: N > O > F

### Conclusion
The correct increasing order of ionization energy for the elements discussed is:
- Li < B < Be < Na < K < O < N < F