To determine the incorrect statement among the given options regarding the hydrides of group 15 elements, we will analyze each statement step by step.
### Step 1: Understand Group 15 Hydrides
Group 15 elements include nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi). The hydrides of these elements are NH₃ (ammonia), PH₃ (phosphine), AsH₃ (arsine), SbH₃ (stibine), and BiH₃ (bismuthine).
### Step 2: Analyze the First Statement
**Statement 1:** "Reducing characters of hydrides of group 15 increases down the group."
- As we move down the group from nitrogen to bismuth, the size of the atoms increases, and the bond dissociation enthalpy decreases. This means that it becomes easier to release hydrogen (H) from the hydrides.
- Therefore, the reducing character does indeed increase down the group. This statement is **correct**.
### Step 3: Analyze the Second Statement
**Statement 2:** "Basicity of hydrides of group 15 increases down the group."
- Basicity refers to the ability to donate a lone pair of electrons. As we move down the group, the size of the atoms increases, leading to a decrease in electron density around the lone pair.
- Consequently, the basicity decreases down the group because larger atoms have less effective overlap for bond formation. This statement is **incorrect**.
### Step 4: Analyze the Third Statement
**Statement 3:** "Phosphorus and arsenic can form p-pi d-pi bonds but not nitrogen."
- Nitrogen does not have d-orbitals available for bonding, while phosphorus and arsenic, being in higher periods, do have d-orbitals and can form p-pi d-pi bonds.
- This statement is **correct**.
### Step 5: Analyze the Fourth Statement
**Statement 4:** "NCl₅ does not exist."
- Nitrogen cannot expand its valency to form five bonds because it lacks d-orbitals. Therefore, NCl₅ does not exist.
- This statement is **correct**.
### Conclusion
After analyzing all the statements, the incorrect statement is:
**Answer:** The basicity of hydrides of group 15 increases down the group (Statement 2).
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