Problem 128 - A Complete Resource Book in Chemistry for JEE Main

Question

In the following questions two statements (Assertion) (A) and Reason (R) are given. Mark (a) If both \(\mathrm{A}\) and \(\mathrm{R}\) are correct and \(\mathrm{R}\) is the correct explanation of \(\mathrm{A}\). (b) If both \(\mathrm{A}\) and \(\mathrm{R}\) are correct but \(\mathrm{R}\) is not the correct expalnation of \(\mathrm{A}\). (c) A is true but \(\mathrm{R}\) is false. (d) A is false but \(R\) is true. (e) \(\mathrm{A}\) and \(\mathrm{R}\) both are false. Assertion: The \(4 \mathrm{f}\) and \(5 \mathrm{f}\) - inner transition series of elements are placed separately at the bottom of the periodic table. Reason: (i) This prevents the undue expansion of the periodic table, i.e., maintains its structure. (ii) This preserve the principle of classification by keeping elements with similar properties in a single column.

Step-by-Step Solution

Answer

The answer is (a): Both A and R are true, and R is the correct explanation of A.

Step 1: Identify Assertion and Reason Components

Assertion (A): The \(4f\) and \(5f\) inner transition series of elements are placed separately at the bottom of the periodic table. Reason (R): (i) This prevents the undue expansion of the periodic table, maintaining its structure. (ii) This preserves the principle of classification by keeping elements with similar properties in a single column.

Step 2: Analyze the Assertion

The Assertion is about the placement of \(4f\) (lanthanides) and \(5f\) (actinides) series in the periodic table. It is true that these series are placed separately at the bottom of the periodic table.

Step 3: Evaluate Reasoning (R) - Part (i)

Examine why placing these series at the bottom prevents undue expansion. Without placing them at the bottom, the periodic table would be very wide, disrupting its standard shape and size. Therefore, Reason (R) part (i) is true.

Step 4: Evaluate Reasoning (R) - Part (ii)

Check if placing \(4f\) and \(5f\) series separately preserves the classification principle. Indeed, it helps keep elements with similar properties together, aligning with the vertical column alignment of similar elements. Hence, Reason (R) part (ii) is also true.

Step 5: Determine the Relation Between A and R

Both the Assertion and each part of the Reason are correct. The placement of the inner transition series below the main table directly relates to preventing undue expansion and maintaining the classification principle, making (R) the correct explanation for (A).

Step 6: Select the Correct Option

Since both (A) and (R) are correct and (R) is the correct explanation of (A), the correct choice is (a).

Key Concepts

Inner Transition Series

The inner transition series is a fascinating section of the periodic table, often tucked away at the bottom to maintain its compactness. These series consist of two main groups of elements: the lanthanides and the actinides.
These elements are also called f-block elements because their outermost electrons are filling the f-orbitals. Each of these series plays a unique role in understanding chemical properties and behaviors.
Placing these series separately helps keep the periodic table manageable and allows for better classification. By positioning them at the bottom, we maintain structural harmony without altering the table's fundamental design.

Lanthanides

The lanthanides are a group of 15 metallic elements from cerium ( Ce ) to lutetium ( Lu ). They are named after lanthanum ( La ), and collectively, they inhabit the 4f block on the periodic table. Known for their 'rare earth' characteristic, they play a crucial role in modern technologies.

• Lanthanides are known for their similar properties, such as high melting points.
• They are typically shiny and when exposed to air, they tarnish, forming oxide layers.
• These elements are important for their magnetic, catalytic, and luminescent properties, making them significant in manufacturing electronics and lasers.

Actinides

Actinides, similar to the lanthanides, are a series of elements crucial to both scientific research and various applications. This series begins with actinium ( Ac ) and ends with lawrencium ( Lr ), making up the 5f block. What makes actinides stand out is their radioactivity.

• Many actinides are synthetic, and some are well-known for their use in nuclear reactors and weapons, such as uranium ( U ) and plutonium ( Pu ).
• These elements tend to be dense metals and exhibit multiple oxidation states, which affect their chemistry significantly.
• Their radioactive properties are studied for both potential energy solutions and understanding atomic behaviors better.

Classification of Elements

The classification of elements within the periodic table is crucial for understanding their relationships and properties. By grouping elements with similar properties, the periodic table becomes a tool for predicting element behavior.
Elements are arranged in increasing atomic number with rows called periods and columns called groups.
This arrangement allows elements with similar valence electron patterns to align vertically. This ensures elements in the same group exhibit comparable chemical and physical properties.
The inner transition series, although separate at the bottom, adheres to these classification principles, keeping elements with similar behaviors like lanthanides and actinides together.