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

The solubility of hydroxides increases with atomic number.

Step 1: Understand the Question

The question asks us to identify which property of alkaline earth metals increases as their atomic number increases. Alkaline earth metals are in Group 2 of the periodic table.

Step 2: Analyze Each Property

We need to consider each option: (a) ionization energy, (b) solubility of their hydroxides, (c) solubility of their sulphates, and (d) electronegativity, and determine how each behaves with increasing atomic number.

Step 3: Ionization Energy

Ionization energy generally decreases as atomic number increases for alkaline earth metals due to the addition of electron shells increasing the distance from the nucleus.

Step 4: Solubility of Hydroxides

The solubility of hydroxides of alkaline earth metals increases down the group. For example, beryllium hydroxide is almost insoluble, while barium hydroxide is very soluble.

Step 5: Solubility of Sulphates

The solubility of sulphates generally decreases as one moves down the group, opposite to the hydroxides.

Step 6: Electronegativity

Electronegativity tends to decrease as atomic number increases in alkaline earth metals because the electron cloud is further from the nucleus.

Step 7: Conclusion

Comparing the behaviors, only the solubility of hydroxides increases as atomic number increases for alkaline earth metals.

Key Concepts

Ionization Energy

Ionization energy is the energy needed to remove an electron from an atom in its gaseous state. As you move down the group of alkaline earth metals in the periodic table, the ionization energy tends to decrease. This is because with each increase in atomic number, a new electron shell is added.
This increased distance between the outermost electrons and the nucleus reduces the nuclear attraction exerted on these electrons.

• As a result, less energy is required to remove these outer electrons.
• Therefore, ionization energy decreases down the group, not increases.

Alkaline earth metals exhibit this property, as seen in the decreasing ionization energy from beryllium to barium.

Solubility of Hydroxides

The solubility of hydroxides among alkaline earth metals increases as we move down the group. This means hydroxides of heavier alkaline earth metals dissolve more readily in water.
For example, beryllium hydroxide is almost insoluble, while barium hydroxide is highly soluble.

• As the ionic size increases down the group, lattice energy decreases, making it easier for water molecules to disrupt and dissolve the ionic bonds in the hydroxides.
• Certain practical implications include the use of soluble hydroxides in products like antacids.

This solubility pattern is the property that increases with atomic number in alkaline earth metals, distinguishing it from other properties.

Electronegativity

Electronegativity is the tendency of an atom to attract electrons towards itself within a chemical bond. In the case of alkaline earth metals, electronegativity decreases with increasing atomic number.
This trend is due to the increasing atomic size as you move down the group.

• Each additional electron shell increases the atomic radius.
• The outer electrons are farther from the nucleus, lessening the nucleus's pull on them.

As a result, the ability of the atom to attract additional electrons diminishes, making electronegativity decrease, not increase, with greater atomic number.

Atomic Number

The atomic number of an element is the number of protons in the nucleus of its atoms. It is a fundamental property that determines the element's position in the periodic table.
In alkaline earth metals, the atomic number increases as you move from one element to the next down the group.

• As the atomic number increases, the number of electron shells increases, causing size expansion.
• This directly impacts other properties such as ionization energy, electronegativity, and solubility of compounds.

Understanding how atomic number relates to these properties helps predict the behavior of these metals and understand trends in the periodic table.