The covalent alkaline earth metal halide (X = Cl, Br, I) is
(a) SrX2
(b) CaX2
(c) MgX2
(d) BeX2

Q: Why do ionic compounds have covalent character at all?

In reality, no bond is 100% ionic. When a positive cation approaches a negative anion, it naturally attracts the anion's electron cloud. This distortion causes some electron density to shift between the two nuclei, which is the fundamental definition of electron sharing, or covalent character.

Q: What exactly is polarizing power?

Polarizing power is the ability of a cation to distort the electron cloud of a neighboring anion. It heavily depends on the cation's charge density—the higher the charge and the smaller the size, the stronger the polarizing power.

Q: Why does Beryllium have the highest polarizing power here?

All the given cations ($Be^{2+}, Mg^{2+}, Ca^{2+}, Sr^{2+}$) have the same +2 charge. However, Beryllium is at the top of Group 2, making it the smallest in size. This tiny size concentrates its +2 charge, giving it the highest charge density and thus the highest polarizing power.

Q: Does the anion affect the covalent character too?

Absolutely! Larger anions have their outer electrons further from the nucleus, making them easier to distort. This ease of distortion is called 'polarizability'. However, in this specific question, the anion ($X^-$) is kept constant across all options, so it doesn't affect our comparison.

Q: Are all Beryllium compounds covalent?

Most of them are predominantly covalent. Due to Beryllium's exceptionally small size and high ionization energy, it is very difficult for it to completely lose its electrons to form a true $Be^{2+}$ ion in a crystal lattice without significant electron sharing occurring.

Question

The covalent alkaline earth metal halide ($X = 	ext{Cl, Br, I}$) is

Options:

(A) $	ext{SrX}_2$

(B) $	ext{CaX}_2$

(C) $	ext{MgX}_2$

(D) $	ext{BeX}_2$

Sigma Percentile · Accepted Answer

### The Secret of Covalent Character in Ionic Bonds: Fajan's Rule

Have you ever wondered why some ionic compounds don't behave like typical ionic compounds? They might have lower melting points or dissolve in organic solvents. This happens because no bond is 100% ionic. There is always a secret sharing of electrons happening behind the scenes, and this is what we call **covalent character**.

To unlock this secret, we use a powerful tool called **Fajan's Rule**. Fajan's rule tells us that the covalent character of an ionic bond is directly proportional to the **polarizing power** of the cation.

But what exactly is polarizing power? Think of it as the cation's ability to pull the anion's electron cloud towards itself. Mathematically, it is directly proportional to the charge on the cation and inversely proportional to its size:

$$ 	ext{Polarising Power} \propto \frac{	ext{Charge}}{	ext{Size}} $$

### Analyzing the Group 2 Halides

Let's look at the options provided in our problem: $SrX_2$, $CaX_2$, $MgX_2$, and $BeX_2$.

Notice a pattern? In all these compounds, the anion is the exact same halide ion ($X^-$). Furthermore, all these metals belong to Group 2 (Alkaline Earth Metals), meaning they all carry a constant $+2$ charge.

Since the charge and the anion are constant, the entire game of polarizing power now rests solely on the **size of the cation**.

### The Dramatic Effect of Size

Let's take a trip down Group 2 of the periodic table. As we move from Beryllium down to Barium, extra electron shells are added, causing the atomic and ionic radii to increase.

The size order is:

$$ Be^{2+} < Mg^{2+} < Ca^{2+} < Sr^{2+} < Ba^{2+} $$

Because $Be^{2+}$ is the smallest ion in this group, its $+2$ charge is concentrated over a very tiny volume. This gives it an exceptionally high **charge density**.

### The Phenomenon of Polarization

Imagine this tiny, highly charged $Be^{2+}$ ion approaching a large, fluffy halide ion ($X^-$). The intense positive electric field of the Beryllium ion strongly attracts the loosely held outer electrons of the halide.

This attraction distorts the normally spherical electron cloud of the halide, pulling it into the space between the two nuclei. This distortion is called **polarization**.

Because the electron cloud is now pulled into the region between the atoms, the electrons are effectively being shared. And what do we call the sharing of electrons? A covalent bond!

Therefore, due to its incredibly small size and massive polarizing power, $Be^{2+}$ causes the maximum distortion, leading to the highest covalent character among all alkaline earth metal halides.

**The correct answer is $BeX_2$.**

Animated Solution for Chemistry - s and p-Block Elements: The covalent alkaline earth metal halide ( $X = Cl, Br, I$ ) is

Select Answer:

Visualized Solution

$Fajan’s Rule$

$Analyzing the Halides$

$Size of Cations$

$Polarizing Power$

$Polarization of Anion$

$Final Answer$

The Sigma Insight: Alkaline Metals

The Secret of Covalent Character in Ionic Bonds

Analyzing the Group 2 Halides

The Dramatic Effect of Size

The Phenomenon of Polarization

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Animated Solution for Chemistry - s and p-Block Elements: .math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } The covalent alkaline earth metal halide (X=Cl, Br, I) is