An alkaline earth metal 'M' readily forms water soluble sulphate and water insoluble hydroxide. Its

Q: Why does sulphate solubility decrease down the group while hydroxide solubility increases?

It all comes down to the size of the anion! For large anions like sulphate ($SO_4^{2-}$), the hydration energy of the cation decreases much faster than the lattice energy as you go down the group, making them less soluble. For small anions like hydroxide ($OH^-$), the lattice energy decreases faster than the hydration energy, making them more soluble down the group.

Q: Are $BeSO_4$ and $MgSO_4$ really that soluble?

Yes! Because $Be^{2+}$ and $Mg^{2+}$ are very small, they have extremely high hydration enthalpies. When they dissolve, the energy released by water molecules surrounding these tiny, highly charged ions is more than enough to overcome their lattice energy.

Q: What exactly is a rock-salt structure?

A rock-salt structure is the crystal lattice adopted by sodium chloride (NaCl). In this structure, each cation is surrounded by six anions, and each anion is surrounded by six cations (an octahedral 6:6 coordination).

Q: Why doesn't BeO form a rock-salt structure like MgO?

The $Be^{2+}$ ion is exceptionally small. If it tried to fit into an octahedral hole surrounded by six relatively large $O^{2-}$ ions, the oxygen ions would clash into each other. Instead, it comfortably fits into a tetrahedral hole, forming a 4:4 coordinated Wurtzite structure.

Q: Is BeO amphoteric or basic?

Unlike the other Group 2 oxides which are purely basic, BeO is amphoteric. This is another classic example of Beryllium's anomalous behavior due to its high polarizing power!

Question

An alkaline earth metal '$M$' readily forms water soluble sulphate and water insoluble hydroxide. Its oxide $MO$ is very stable to heat and does not have rock-salt structure. $M$ is

Options:

(A) Sr

(B) Ca

(C) Be

(D) Mg

Sigma Percentile · Accepted Answer

### The Suspects: Group 2 Metals

Imagine you are a chemical detective, and you've just been handed a dossier on a mystery metal, $M$.

The file tells us that $M$ is an alkaline earth metal. This immediately narrows our list of suspects to the members of Group 2 in the periodic table: Beryllium ($Be$), Magnesium ($Mg$), Calcium ($Ca$), Strontium ($Sr$), and Barium ($Ba$).

To unmask the true identity of $M$, we need to carefully analyze three distinct chemical clues provided in the dossier. Let's break them down one by one.

### Clue 1: The Sulphate Solubility Mystery

The first clue states that our mystery metal readily forms a **water-soluble sulphate**, $MSO_4$.

In the world of inorganic chemistry, solubility is a constant tug-of-war between two opposing forces: **Lattice Energy** (the energy holding the solid crystal together) and **Hydration Energy** (the energy released when water molecules surround the ions).

For a salt to dissolve, the hydration energy must overcome the lattice energy. When we look at Group 2 sulphates, we are dealing with a very large anion, the sulphate ion ($SO_4^{2-}$).

Because the anion is so large, the lattice energy doesn't change much as we move down the group. However, the hydration energy drops drastically as the metal cation gets larger.

Therefore, the solubility of Group 2 sulphates **decreases** as we go down the group. Beryllium sulphate ($BeSO_4$) and Magnesium sulphate ($MgSO_4$) are highly soluble, while the others are sparingly soluble or completely insoluble.

This clue eliminates Calcium, Strontium, and Barium. Our suspect is either Beryllium or Magnesium!

### Clue 2: The Hydroxide Paradox

The second clue reveals that $M$ forms a **water-insoluble hydroxide**, $M(OH)_2$.

You might think the trend would be the same as the sulphates, but nature loves a plot twist! The hydroxide ion ($OH^-$) is quite small compared to the sulphate ion.

Because the anion is small, the lattice energy is highly sensitive to the size of the cation. As we move down the group and the metal cation gets larger, the lattice energy drops much faster than the hydration energy.

As a result, the solubility of Group 2 hydroxides **increases** down the group. Barium hydroxide is highly soluble, while Beryllium hydroxide ($Be(OH)_2$) and Magnesium hydroxide ($Mg(OH)_2$) are insoluble.

Wait a minute! Both Beryllium and Magnesium fit this clue perfectly as well. The suspense is building. We need a definitive tie-breaker.

### The Tie-Breaker: Crystal Structures

The final clue is the smoking gun: the oxide $MO$ is very stable to heat and **does not have a rock-salt structure**.

Most alkaline earth metal oxides, such as $MgO$, $CaO$, $SrO$, and $BaO$, crystallize in a rock-salt (Sodium Chloride, $NaCl$) structure. In this geometry, each metal cation is surrounded by six oxygen anions in an octahedral arrangement.

However, Beryllium is the rebel of Group 2. The $Be^{2+}$ ion is exceptionally tiny.

If this tiny ion tried to squeeze into an octahedral hole surrounded by six relatively large $O^{2-}$ ions, the oxygen ions would physically clash into each other, making the structure highly unstable.

Instead, Beryllium oxide ($BeO$) adopts a **Wurtzite** structure, where each Beryllium ion is comfortably surrounded by only four oxygen ions in a tetrahedral arrangement.

### The Verdict: Anomalous Beryllium

The evidence is undeniable. The only alkaline earth metal that forms a soluble sulphate, an insoluble hydroxide, and an oxide without a rock-salt structure is **Beryllium**.

This problem is a beautiful showcase of Beryllium's **anomalous behavior**. Because of its extremely small size and high charge density (polarizing power), it frequently breaks the rules of its own group.

In fact, Beryllium often shows more chemical similarities to Aluminum (a Group 13 element) than to its own Group 2 siblings—a phenomenon known as the **diagonal relationship**.

Always keep a close eye on the first element of any s or p-block group; they are the master rule-breakers of the periodic table!

Animated Solution for Chemistry - s and p-Block Elements: An alkaline earth metal ' $M$ ' readily forms water soluble sulphate and water insoluble hydroxide. Its oxide $M O$ is very stable to heat and does not have rock-salt structure. $M$ is

Select Answer:

Visualized Solution

\text{Alkaline Earth Metals}

\text{Sulphate Solubility Trend}

\text{Soluble Sulphates}

\text{Hydroxide Solubility Trend}

\text{Insoluble Hydroxides}

\text{Crystal Structure of Oxides}

\text{Anomalous Beryllium}

\text{Diagonal Relationship}

The Sigma Insight: Alkaline Metals

The Suspects

Clue 1

Clue 2

The Tie-Breaker

The Verdict

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