Basic strength order `NH_(3) gtPH_(3) gtAsH_(3) gtBiH_(3)` .

Explanation

To determine the basic strength order of the hydrides of the nitrogen family (NH₃, PH₃, AsH₃, BiH₃), we need to analyze the factors that influence basicity in these compounds.

### Step-by-Step Solution:

1. **Identify the Hydrides**: The hydrides we are considering are NH₃ (ammonia), PH₃ (phosphine), AsH₃ (arsine), and BiH₃ (bismuthine). These compounds are formed by elements from Group 15 of the periodic table.

2. **Understand Basicity**: Basicity refers to the ability of a compound to donate a lone pair of electrons to an acid. In the case of these hydrides, the central atom (N, P, As, Bi) has a lone pair of electrons that can be donated.

3. **Consider Atomic Size**: The size of the central atom increases as we move down the group from nitrogen to bismuth. The order of atomic size is:
- N < P < As < Bi

4. **Lone Pair Availability**: As the size of the central atom increases, the lone pair of electrons becomes more delocalized and less available for donation. This is because larger atoms have their electrons spread over a larger volume, which reduces the electron density around the lone pair.

5. **Analyze Electron Density**: The basicity of these hydrides is directly related to the availability of the lone pair for donation. A higher electron density means a stronger base. Therefore, as we move down the group:
- NH₃ has the highest electron density due to the small size of nitrogen.
- PH₃ has lower electron density than NH₃ but is still more basic than AsH₃ and BiH₃.
- AsH₃ is less basic than PH₃ due to the larger size of arsenic.
- BiH₃ has the lowest basicity because bismuth is the largest atom among them, leading to the most delocalized lone pair.

6. **Establish the Basic Strength Order**: Based on the analysis of atomic size and electron density, we can conclude that the basic strength order is:
- NH₃ > PH₃ > AsH₃ > BiH₃

### Final Conclusion:
The order of basic strength for the hydrides of the nitrogen family is indeed NH₃ > PH₃ > AsH₃ > BiH₃.

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