The decreasing basic order of the following compounds is:
i. `NH_(3)`, ii. `PH_(3)`, iii. `AsH_(3)`, iv. `SbH_(3)`

Options

A. `(i) gt (ii) gt (iii) gt (iv)`
B. `(iv) gt (iii) gt (ii) gt (i)`
C. `(ii) gt (i) gt (iii) gt (iv)`
D. `(ii) gt (i) gt (iv) gt (iii)`

Explanation

To determine the decreasing basic order of the compounds \( NH_3 \), \( PH_3 \), \( AsH_3 \), and \( SbH_3 \), we can analyze the basicity of these group 15 hydrides step by step.

### Step 1: Identify the Compounds
The compounds in question are:
1. \( NH_3 \) (Ammonia)
2. \( PH_3 \) (Phosphine)
3. \( AsH_3 \) (Arsenic Hydride)
4. \( SbH_3 \) (Antimony Hydride)

### Step 2: Understand Basicity
Basicity refers to the ability of a compound to donate a lone pair of electrons. In this case, we are dealing with Lewis bases, which can donate their lone pair to form bonds with protons (H⁺ ions).

### Step 3: Analyze the Central Atom
The central atom in these compounds is from group 15 of the periodic table:
- Nitrogen (N) in \( NH_3 \)
- Phosphorus (P) in \( PH_3 \)
- Arsenic (As) in \( AsH_3 \)
- Antimony (Sb) in \( SbH_3 \)

As we move down the group from nitrogen to antimony, the size of the central atom increases.

### Step 4: Consider the Effect of Atomic Size on Basicity
- **Electron Density**: The ability of the central atom to donate its lone pair is influenced by the size of the atom. Smaller atoms have higher electron density around the lone pair, making it easier to donate.
- **Trend**: As we go down the group from nitrogen to antimony, the atomic size increases, which leads to a decrease in electron density around the lone pair. This results in a decrease in basicity.

### Step 5: Order the Basicity
Based on the above analysis:
- \( NH_3 \) has the highest basicity due to its small size and high electron density.
- \( PH_3 \) is less basic than \( NH_3 \) but more basic than \( AsH_3 \).
- \( AsH_3 \) is less basic than \( PH_3 \).
- \( SbH_3 \) has the lowest basicity due to the largest atomic size and lowest electron density.

### Conclusion
The decreasing order of basicity for the compounds is:
\[ NH_3 > PH_3 > AsH_3 > SbH_3 \]

### Final Answer
The correct option for the decreasing basic order is:
1. \( NH_3 \)
2. \( PH_3 \)
3. \( AsH_3 \)
4. \( SbH_3 \)