The correctorder of base strength among the following is:

Q: The correct order of base strength among the following is:

The correct order of base strength among the following is:

Question

Accepted Answer

The correct order of base strength among the given hydrides is:

1. $NH_{3} > PH_{3} > AsH_{3} > SbH_{3} > BiH_{3}$

### Explanation for Option 1:

1. **Basicity and Electron Pair Availability:**
   - The basicity of a compound is determined by the availability of a lone pair of electrons for donation. In the case of these hydrides, the central atom (N, P, As, Sb, Bi) has a lone pair of electrons.
   - As we move down the group in the periodic table from nitrogen to bismuth, the size of the central atom increases. This increase in size results in a decrease in the electron density around the central atom, making the lone pair less available for donation.
   - Therefore, the basicity decreases as we move down the group: $NH_{3}$ (ammonia) has the most available lone pair, making it the strongest base, followed by $PH_{3}$, $AsH_{3}$, $SbH_{3}$, and $BiH_{3}$.

2. **Electronegativity:**
   - Nitrogen is the most electronegative element among the group 15 elements, which means it holds onto its electrons more tightly. However, due to its small size, the electron pair is more concentrated and more available for bonding, enhancing its basicity.
   - As we move down the group, the electronegativity decreases, further reducing the basicity of the hydrides.

3. **Bond Strength and Stability:**
   - The N-H bond in $NH_{3}$ is stronger and more stable compared to the P-H, As-H, Sb-H, and Bi-H bonds. This stability contributes to the higher basicity of ammonia.

### Explanation for Why Other Options are Incorrect:

- **Option 2: $BiH_{3} > SbH_{3} > AsH_{3} > PH_{3} > NH_{3}$**
  - This option suggests that $BiH_{3}$ is the strongest base, which contradicts the trend of decreasing basicity down the group. $BiH_{3}$, being the heaviest and having the least electronegative central atom, is actually the weakest base.

- **Option 3: $NH_{3} > SbH_{3} > BiH_{3} > AsH_{3} > PH_{3}$**
  - This option incorrectly places $SbH_{3}$ and $BiH_{3}$ above $PH_{3}$ and $AsH_{3}$. As explained, the basicity decreases down the group, so $PH_{3}$ and $AsH_{3}$ should be stronger bases than $SbH_{3}$ and $BiH_{3}$.

- **Option 4: $BiH_{3} > NH_{3} > PH_{3} > SbH_{3} > AsH_{3}$**
  - This option incorrectly places $BiH_{3}$ as the strongest base, which is not consistent with the trend of decreasing basicity down the group. $NH_{3}$ should be the strongest base, not $BiH_{3}$.

In conclusion, Option 1 correctly reflects the trend of decreasing basicity from $NH_{3}$ to $BiH_{3}$, based on the availability of the lone pair, electronegativity, and bond strength considerations.