Animated Solution for Chemistry - Metallurgy: Sulphide ion is soft base and its ores are common for metals (A) Pb (B) Al (C) Ag (D) Mg Choose the correct answer from the options given below

Why Metals Choose Their Partners

Have you ever wondered why certain metals are found in nature combined with oxygen, while others are found combined with sulphur? It's not a random lottery. The Earth's crust is a giant chemical laboratory, and elements follow strict rules of attraction when forming minerals. This question beautifully ties together factual knowledge of metallurgy with a profound chemical concept: the Hard Soft Acid Base (HSAB) principle.

Hard and Soft

The question gives us a massive hint: "Sulphide ion is a soft base." But what does that mean?

In chemistry, we classify Lewis acids (electron acceptors, usually metal cations) and Lewis bases (electron donors, like anions) as either 'hard' or 'soft'.

Hard species are small, highly charged, and have tightly held electron clouds. They are not easily polarizable. Think of them like solid billiard balls. Examples include $O^{2-}$, $F^{-}$, $A{l}^{3+}$, and $M{g}^{2+}$. Soft species are larger, have lower charge densities, and possess electron clouds that are easily distorted or 'squishy'. They are highly polarizable. Think of them like water balloons. Examples include $S^{2-}$, $I^{-}$, $A{g}^{+}$, and $P{b}^{2+}$.

The golden rule of the HSAB principle is simple: Hard acids prefer to bond with hard bases, and soft acids prefer to bond with soft bases.

Let's apply this rule to the metals given in our question to see who pairs up with the soft sulphide ion ($S^{2-}$).

1. Lead (Pb) and Silver (Ag) Lead and Silver are relatively heavy transition (and post-transition) metals. When they form ions ($P{b}^{2+}$ and $A{g}^{+}$), they are large and have low charge densities. This makes them soft acids.

Following our HSAB rule, these soft acids will naturally seek out soft bases like the sulphide ion. And indeed, if we look at nature, the most common ores for these metals are sulphides: Lead: Found primarily as Galena, which is Lead Sulphide ($PbS$). Silver: Found primarily as Argentite, which is Silver Sulphide ($A{g}_{2}S$).

2. Aluminum (Al) and Magnesium (Mg) On the other hand, Aluminum and Magnesium are lighter elements. Their ions ($A{l}^{3+}$ and $M{g}^{2+}$) are small and carry a high charge. This makes them classic hard acids.

They will reject the soft sulphide ion and instead bond with hard bases like oxygen or halogens. Nature confirms this: Aluminum: Extracted from Bauxite, an oxide ore with the formula $A{l}_2{O}_3\cdot 2H_{2}O$. Magnesium: Often extracted from Carnallite, a halide ore with the formula $KCl\cdot MgC{l}_2\cdot 6H_{2}O$, or Magnesite ($MgC{O}_3$).

By understanding the chemical personalities of these elements, it becomes obvious why Lead (A) and Silver (C) are the ones commonly found as sulphide ores. They are the soft acids that perfectly match the soft sulphide base. Therefore, the correct choice is (A) and (C) only.