Problem 36 - A Complete Resource Book in Chemistry for JEE Main

Question

Property of alkaline earth metals that increases with their atomic number is (a) ionization energy (b) solubility of their hydroxides (c) solubility of their sulphates (d) electronegativity

Step-by-Step Solution

Answer

(b) Solubility of their hydroxides increases.

Step 1: Understanding the Question

We need to identify a property of alkaline earth metals that increases as we move down the group in the periodic table, meaning as their atomic number increases. Alkaline earth metals include elements like beryllium, magnesium, calcium, strontium, barium, and radium.

Step 2: Analyzing Each Option

Let's consider each option: (a) Ionization energy generally decreases down a group because the outer electrons are further from the nucleus. (b) Solubility of hydroxides increases down the group as lattice energies decrease faster than hydration energies. (c) Solubility of sulphates actually decreases down the group because lattice energy decrease is offset by a larger decrease in hydration energy. (d) Electronegativity decreases down the group because valence electrons are further from the nucleus.

Step 3: Identifying the Correct Answer

From the analysis, the only property that increases with atomic number within the alkaline earth metals is the solubility of their hydroxides.

Key Concepts

Ionization Energy

Ionization energy refers to the energy needed to remove an electron from an atom in its gaseous state. For alkaline earth metals, the ionization energy decreases as you move down the group in the periodic table. This is because atomic size increases due to the addition of electron shells.

As a result, the outermost electrons are farther away from the nucleus. The nuclear attraction is weaker, making it easier to remove an electron.

• Ionization energy is higher at the top of the group.
• Decreases with an increasing atomic number.

In essence, it takes less energy to ionize atoms of heavier alkaline earth metals, meaning they are more reactive than lighter ones.

Solubility of Hydroxides

The solubility of hydroxides of alkaline earth metals increases as you go down the group. This trend is interesting because it is connected to the behavior of lattice and hydration energies.

Lattice energy is the energy released when ions form a solid crystal structure. Hydration energy is the energy released when an ion interacts with water.

• Down the group, lattice energies greatly decrease.
• Hydration energy decreases but at a slower rate.

This means, for heavier alkaline earth metals, dissolving their hydroxides in water becomes easier. Hence, hydrolysis is more pronounced with elements like barium compared to magnesium.

Solubility of Sulphates

Alkaline earth metal sulphates show a decrease in solubility as you move down the group. This occurs due to changes in lattice and hydration energies as well.

In this case, the decrease in hydration energy is more significant than the decrease in lattice energy. As a result, the sulphates of elements like barium are more insoluble compared to those of magnesium.

• Lattice energies decrease slightly.
• Hydration energies decrease more significantly.

The net effect is a decreased ability to dissolve in water, making the sulphates of heavier elements less soluble.

Electronegativity

Electronegativity is the ability of an atom to attract electrons in a chemical bond. For alkaline earth metals, this property decreases down the group due to increased atomic radius.

Valence electrons are further from the nucleus, reducing the effective nuclear charge experienced by the electrons. As a result, their ability to attract bonding electrons decreases.

• High electronegativity at the top of the group.
• Decreases as atomic number increases.

Thus, elements like beryllium hold electrons more strongly than heavier elements like barium.