Select the incorrect statement(s).

Options

A. `IE_1` of nitrogen atom is less than `IE_1` of oxygen atom
B. Electron gain enthalpy of oxygen is less negative than selenium
C. Electronegativity on pauling scale is 2.8 times the electronegativity on Mulliken scale
D. `Cr^(6+)` is smaller than `Cr^(3+)`

Explanation

To solve the question of selecting the incorrect statements, we will analyze each statement one by one based on the information provided in the video transcript.

### Step-by-Step Solution:

1. **Statement 1: First ionization energy of nitrogen is less than first ionization energy of oxygen.**
- **Analysis:**
- The electronic configuration of nitrogen (N) is 1s² 2s² 2p³, and for oxygen (O), it is 1s² 2s² 2p⁴.
- Nitrogen has a half-filled p subshell (2p³), which is more stable and requires more energy to remove an electron compared to oxygen, which has one more electron in the p subshell (2p⁴).
- Therefore, the first ionization energy of nitrogen is greater than that of oxygen.
- **Conclusion:** This statement is **incorrect**.

2. **Statement 2: Electron gain enthalpy of oxygen is less negative than that of selenium.**
- **Analysis:**
- As we move down the group from oxygen to selenium, the atomic size increases, leading to increased electron-electron repulsions in the smaller oxygen atom.
- This results in a less negative electron gain enthalpy for oxygen compared to selenium.
- **Conclusion:** This statement is **true**.

3. **Statement 3: Electronegativity on Pauling scale is 2.8 times electronegativity on Mulliken scale.**
- **Analysis:**
- The relationship between electronegativity on the Pauling scale and the Mulliken scale is actually the opposite; the electronegativity on the Mulliken scale is approximately 2.8 times that on the Pauling scale.
- **Conclusion:** This statement is **incorrect**.

4. **Statement 4: Cr6+ is smaller than Cr3+.**
- **Analysis:**
- As the positive charge on a cation increases, the effective nuclear charge experienced by the outermost electrons increases, leading to a decrease in atomic size.
- Therefore, Cr6+ will indeed be smaller than Cr3+ due to the higher positive charge.
- **Conclusion:** This statement is **true**.

### Final Selection of Incorrect Statements:
- The incorrect statements are:
- Statement 1: First ionization energy of nitrogen is less than that of oxygen.
- Statement 3: Electronegativity on Pauling scale is 2.8 times electronegativity on Mulliken scale.