Problem 75 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following orders is incorrect? (a) \(\mathrm{NH}_{3}<\mathrm{PH}_{3}<\mathrm{AsH}_{3} \longrightarrow\) acidic nature. (b) \(\mathrm{Li}<\mathrm{Be}<\mathrm{B}<\mathrm{C} \longrightarrow\) first ionization energy (c) \(\mathrm{A} 1_{2} \mathrm{O}_{3}>\mathrm{MgO}<\mathrm{Na}_{2} \mathrm{O}<\mathrm{K}_{2} \mathrm{O} \longrightarrow\) basic nature. (d) \(\mathrm{Li}^{+}<\mathrm{Na}^{+}<\mathrm{K}^{+}<\mathrm{Cs}^{+} \longrightarrow\) ionic radius

Step-by-Step Solution

Answer

Option (c) is incorrect. The correct order for basic nature is \(\text{K}_2\text{O} > \text{Na}_2\text{O} > \text{MgO} > \text{Al}_2\text{O}_3\).

Step 1: Analyze Option (a)

This order describes the acidic nature of hydrides like \(\text{NH}_3, \text{PH}_3, \text{AsH}_3\). Generally, hydrides become more acidic as we move down the group in the periodic table. Thus, \(\text{NH}_3 < \text{PH}_3 < \text{AsH}_3\) is a correct order for increasing acidic nature.

Step 2: Analyze Option (b)

This order describes the first ionization energy of the elements \(\text{Li}, \text{Be}, \text{B}, \text{C}\). Ionization energy generally increases across a period because of increasing nuclear charge. Thus, \(\text{Li} < \text{Be} < \text{B} < \text{C}\) represents a correct increasing order of first ionization energy for these elements.

Step 3: Analyze Option (c)

This order refers to the basic nature of compounds \(\text{Al}_2\text{O}_3, \text{MgO}, \text{Na}_2\text{O}, \text{K}_2\text{O}\). Here, the correct order must reflect increasing basicity, which increases down a group and from left to right (due to increasing metallic character). The correct decreasing basicity order should be: \(\text{K}_2\text{O} > \text{Na}_2\text{O} > \text{MgO} > \text{Al}_2\text{O}_3\). Thus, this order given is incorrect.

Step 4: Analyze Option (d)

This order describes the ionic radii of \(\text{Li}^+, \text{Na}^+, \text{K}^+, \text{Cs}^+\). Ionic radius generally increases as we move down a group because of the addition of electron shells. Hence, \(\text{Li}^+ < \text{Na}^+ < \text{K}^+ < \text{Cs}^+\) is a correct order for increasing ionic radius.

Key Concepts

Acidic and Basic Nature

The acidic and basic nature of elements and their compounds are critical periodic trends that help in predicting chemical behavior. Acidity increases as we move down a group due to the increase in atomic size, which leads to weaker H-X bonds in hydrides like in ammonia (\(\text{NH}_3\)). For example, ammonia is less acidic compared to phosphine (\(\text{PH}_3\)), with arsine (\(\text{AsH}_3\)) being the most acidic among the three.

• Acidity Increases: Moving down a group results in more acidic hydrides.
• Basicity Influences: Basicity is influenced by electronic structure and metallic character.

When we consider basic nature in compounds like oxides, it is primarily influenced by their metallic character. For instance, as the metallic character increases, as seen from \(\text{Al}_2\text{O}_3\) to \(\text{K}_2\text{O}\), the basicity increases in the given order: \(\text{K}_2\text{O} > \text{Na}_2\text{O} > \text{MgO} > \text{Al}_2\text{O}_3\). Here, \(\text{K}_2\text{O}\) is the most basic.

Ionization Energy

Ionization energy is the energy required to remove an electron from a gaseous atom. It is a significant periodic trend that increases across a period and decreases down a group. Across a period, nuclear charge increases, leading to a greater pull on the electrons which makes them harder to remove. Consequently, ionization energy rises.

• Across a Period: Increases due to greater nuclear charge.
• Down a Group: Decreases because of added electron shells that reduce the effective nuclear charge on the outermost electrons.

For example, in the sequence \(\text{Li}, \text{Be}, \text{B}, \text{C}\), ionization energy increases. Carbon (\(\text{C}\)) has the highest first ionization energy in this sequence, while lithium (\(\text{Li}\)) has the lowest. This trend is crucial for understanding element reactivity and helps predict how likely an element is to participate in chemical reactions.

Ionic Radius

The concept of ionic radius is closely related to atomic size, but it specifically refers to the size of an ion in a crystal lattice. As a key periodic trend, ionic radius generally increases as we move down a group due to additional electron shells being added for each successive element.

• Down a Group: Increases due to added electron shells.

Taking alkali metal ions like \(\text{Li}^+, \text{Na}^+, \text{K}^+, \text{Cs}^+\) as an example, \(\text{Li}^+\) is the smallest and \(\text{Cs}^+\) is the largest among them. Moving down the group, each element has an additional electron shell which increases the ion's size despite them maintaining the same charge. This trend is essential for understanding ionic interactions and lattice energy in crystals.

Periodic Properties of Elements

The periodic properties of elements encompass trends observable across the periodic table, such as electronegativity, atomic radius, ionization energy, and electron affinity. These properties help chemists understand and predict the chemical behavior of different elements.

• Across a Period: Properties like electronegativity and ionization energy generally increase, while atomic size decreases.
• Down a Group: Electronegativity and ionization energy decrease, while atomic size and ionic radius increase.

Such trends are attributed to changes in nuclear charge, electron shielding, and the overall atomic structure. Recognizing these patterns allows us to make informed predictions regarding element reactivity, bonding capabilities, and their role in chemical reactions. Understanding periodic properties is key to mastering the chemistry of elements and anticipating their behavior in various chemical contexts.