Animated Solution for Chemistry - Periodicity in Properties: The set in which compounds have different nature is

When we talk about the "nature" of a compound in inorganic chemistry, we are usually referring to its acid-base behavior. Is it an acid, a base, or can it act as both (amphoteric)? This question asks us to identify the pair of compounds that do not share the same chemical nature.

To solve this, we need to evaluate each compound individually. The nature of hydroxides and oxides is a classic periodic trend: as we move from left to right across a period, the nature shifts from strongly basic to amphoteric to acidic. Let's break down the options.

Option (a): ${\text{B(OH)}}_3$ and ${\text{H}}_3{\text{PO}}_3$ At first glance, ${\text{B(OH)}}_3$ might look like a base because of the hydroxide (${\text{OH}}^{-}$) groups. However, Boron is a non-metal/metalloid and is electron-deficient. Instead of releasing ${\text{OH}}^{-}$, it accepts an electron pair from water's oxygen, releasing an ${\text{H}}^{+}$ ion in the process. Thus, ${\text{B(OH)}}_3$ (orthoboric acid) is a Lewis acid.

On the other hand, ${\text{H}}_3{\text{PO}}_3$ is phosphorous acid. It is a well-known diprotic Bronsted acid. Since both are acidic, they have the same nature.

Option (c): $\text{NaOH}$ and ${\text{Ca(OH)}}_2$ Sodium ($\text{Na}$) is an alkali metal, and Calcium ($\text{Ca}$) is an alkaline earth metal. Both are highly electropositive elements located on the far left of the periodic table. Their hydroxides readily dissociate in water to give ${\text{OH}}^{-}$ ions. Therefore, both $\text{NaOH}$ and ${\text{Ca(OH)}}_2$ are strong bases. They share the same basic nature.

Option (d): ${\text{Be(OH)}}_2$ and ${\text{Al(OH)}}_3$ This pair is a classic example of the diagonal relationship in the periodic table. Beryllium ($\text{Be}$) and Aluminum ($\text{Al}$) share remarkably similar chemical properties due to their similar charge-to-size ratios. One of their defining shared traits is that their oxides and hydroxides are amphoteric. This means they can react with both strong acids and strong bases. Since both are amphoteric, they have the same nature.

Option (b): ${\text{B(OH)}}_3$ and ${\text{Al(OH)}}_3$ Now we come to the winning pair. We have already established that ${\text{B(OH)}}_3$ is acidic (a Lewis acid). We have also just seen that ${\text{Al(OH)}}_3$ is amphoteric.

One is acidic, and the other is amphoteric. They possess completely different chemical natures!

By systematically analyzing the acid-base behavior of each compound based on their position in the periodic table and specific chemical properties, we can confidently conclude that the set with different natures is ${\text{B(OH)}}_3$ and ${\text{Al(OH)}}_3$.