Question

The property of the alkaline earth metals that increases with their atomic number is

• A. Ionisation enthalpy
• B. Electronegativity
• C. Solubility of their hydroxide in water
• D. Solubility of their sulphate in water

Answer 1

The Correct Option is C

To solve this problem, we need to understand how the properties of alkaline earth metals change as we move down the group (increasing atomic number).

1. Analyzing the Trend of Alkaline Earth Metals:
The alkaline earth metals (Group 2) include elements like beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). As we move down the group, certain properties change due to the increasing atomic number.

2. Ionization Enthalpy:
Ionization enthalpy (or ionization energy) decreases as we move down the group. This is because the outermost electrons are farther from the nucleus and are more shielded by inner electrons, making it easier to remove them. Hence, ionization enthalpy does not increase with atomic number.

3. Electronegativity:
Electronegativity decreases as we move down the group. This is because the atomic size increases, and the outer electrons are less tightly held, making the atom less electronegative. Thus, electronegativity does not increase with atomic number.

4. Solubility of Hydroxide in Water:
The solubility of the alkaline earth metal hydroxides in water increases as we move down the group. For example, beryllium hydroxide (Be(OH)₂) is insoluble in water, but calcium hydroxide (Ca(OH)₂) is more soluble, and the solubility increases further down the group.

5. Solubility of Sulphate in Water:
The solubility of the alkaline earth metal sulphates in water generally decreases as we move down the group. For instance, barium sulfate (BaSO₄) is insoluble, while magnesium sulfate (MgSO₄) is highly soluble. Therefore, solubility of sulphates does not increase with atomic number.

Final Answer:
The property of alkaline earth metals that increases with their atomic number is (C) "Solubility of their hydroxide in water."

Answer 2

As we move down the alkaline earth metals group (Group 2 of the periodic table), their properties change as follows:

• Ionisation enthalpy: Ionisation enthalpy decreases as we move down the group. This is because the atomic size increases and the outermost electron is farther from the nucleus, thus easier to remove.
• Electronegativity: Electronegativity decreases as we move down the group because the atomic size increases, which results in a weaker attraction for the bonding electrons.
• Solubility of their hydroxide in water: The solubility of hydroxides increases down the group. For example, magnesium hydroxide is only sparingly soluble, while calcium hydroxide is more soluble, and barium hydroxide is highly soluble in water.
• Solubility of their sulphate in water: The solubility of sulphates decreases as we move down the group. For example, while magnesium sulphate is highly soluble, barium sulphate is almost insoluble in water.

Thus, the correct answer is (C) Solubility of their hydroxide in water, as this property increases with the atomic number of the alkaline earth metals.