Choose the correct pair regarding ionisation energy `(IE_(1))`

Options

A. `B gt Be`
B. `Tl gt Ga`
C. `N gt O`
D. `Li gt Na`

Answer

To solve the question regarding the correct pair of ionization energy (IE₁), we will analyze each option step by step.

### Step 1: Understanding Ionization Energy
**Definition**: Ionization energy is the energy required to remove an electron from the outermost shell of an isolated gaseous atom.

### Step 2: Analyzing Each Option
1. **Option 1: Boron (B) has greater ionization energy than Beryllium (Be)**
- Atomic number of Beryllium (Be) = 4; Electronic configuration: 1s² 2s²
- Atomic number of Boron (B) = 5; Electronic configuration: 1s² 2s² 2p¹
- **Comparison**: The penetration power of the s orbital is greater than that of the p orbital. Therefore, it is easier to remove an electron from Boron than from Beryllium.
- **Conclusion**: This option is **incorrect**.

2. **Option 2: Thallium (Tl) has greater ionization energy than Gallium (Ga)**
- Both elements belong to group 13 of the periodic table.
- **D-block contraction**: The presence of d-orbitals in the transition metals affects the ionization energy. Thallium experiences additional contraction due to the presence of f-block elements (actinoid contraction).
- **Conclusion**: Thallium has a greater ionization energy than Gallium due to these contractions. This option is **correct**.

3. **Option 3: Nitrogen (N) has greater ionization energy than Oxygen (O)**
- Atomic number of Nitrogen (N) = 7; Electronic configuration: 1s² 2s² 2p³
- Atomic number of Oxygen (O) = 8; Electronic configuration: 1s² 2s² 2p⁴
- **Comparison**: Nitrogen has a half-filled p subshell (2p³), which is more stable than the partially filled subshell of Oxygen (2p⁴). Thus, it requires more energy to remove an electron from Nitrogen.
- **Conclusion**: This option is **correct**.

4. **Option 4: Lithium (Li) has greater ionization energy than Sodium (Na)**
- Atomic number of Lithium (Li) = 3; Atomic number of Sodium (Na) = 11.
- **Trend**: Ionization energy decreases down a group due to increased atomic size and decreased effective nuclear charge on the outermost electron.
- **Conclusion**: Lithium has a greater ionization energy than Sodium. This option is **correct**.

### Final Conclusion
- The correct pairs regarding ionization energy are:
- **Option 2**: Thallium has greater ionization energy than Gallium.
- **Option 3**: Nitrogen has greater ionization energy than Oxygen.
- **Option 4**: Lithium has greater ionization energy than Sodium.