`PH_3` is less basic than `NH_3`.

Answer

To understand why PH₃ is less basic than NH₃, we can break down the explanation into several steps:

### Step 1: Understand Basicity
Basicity refers to the ability of a compound to donate a pair of electrons. A stronger base has a greater tendency to donate electrons.

### Step 2: Compare the Atoms
- **Nitrogen (N)** in NH₃ and **Phosphorus (P)** in PH₃ belong to the same group in the periodic table (Group 15).
- However, nitrogen is smaller in size compared to phosphorus.

### Step 3: Analyze Electron Density
- The smaller size of nitrogen allows it to have a higher electron density. This means that the electrons around nitrogen are held more tightly and are more available for donation.
- In contrast, phosphorus, being larger, has a lower electron density. The electrons are held less tightly and are less available for donation.

### Step 4: Tendency to Donate Electrons
- Because nitrogen has a higher electron density, it has a greater tendency to donate its lone pair of electrons compared to phosphorus.
- Therefore, NH₃ (ammonia) is more basic than PH₃ (phosphine) because nitrogen can more readily donate its electrons.

### Step 5: Conclusion
- Since NH₃ has a higher tendency to donate electrons due to its higher electron density, it is more basic than PH₃.

### Final Statement
Thus, we conclude that PH₃ is less basic than NH₃ because nitrogen has a higher electron density and a greater tendency to donate electrons compared to phosphorus.

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