To determine which of the given orders is incorrect, we will analyze each option step by step.
### Step 1: Analyze the first order (Acidic nature of NH3, PH3, AsH3)
- **Order Given**: NH3 < PH3 < AsH3
- **Explanation**: As we move down the group in the periodic table, the acidic character of hydrides increases due to the decreasing bond strength of the hydrogen atom with the central atom. Therefore, the order is correct as the acidic character increases from NH3 to AsH3.
### Step 2: Analyze the second order (First Ionization Potential of Li, Be, B, C)
- **Order Given**: Li < Be < B < C
- **Explanation**: The first ionization potential generally increases across a period due to increasing nuclear charge. However, there is an exception between Be and B. Beryllium (Be) has a full 2s subshell (1s² 2s²), while Boron (B) has one electron in the 2p subshell (1s² 2s² 2p¹). The presence of a half-filled p subshell in Boron makes it less stable than Beryllium, which has a fully filled s subshell. Therefore, the correct order should be: Li < B < Be < C. Thus, the second order is incorrect.
### Step 3: Analyze the third order (Basic nature of oxides)
- **Order Given**: Al2O3 < MgO < Na2O < K2O
- **Explanation**: The basic character of oxides increases down the group. Sodium oxide (Na2O) and potassium oxide (K2O) are more basic than magnesium oxide (MgO) and aluminum oxide (Al2O3). Therefore, this order is correct.
### Step 4: Analyze the fourth order (Cation and ionic radius of Li⁺, Na⁺, K⁺, Cs⁺)
- **Order Given**: Li⁺ < Na⁺ < K⁺ < Cs⁺
- **Explanation**: As we move down the group, the ionic radius increases due to the addition of electron shells. Therefore, this order is also correct.
### Conclusion
The only incorrect order is the second one regarding the first ionization potential of Li, Be, B, and C. The correct order should be:
**Li < B < Be < C**
### Final Answer
The wrong order is the second one: Li < Be < B < C.
