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

Question

Assertion: The 4 f and 5 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

Both reasons correctly explain the placement of the 4f and 5f elements separately.

Step 1: Understand the Assertion

The exercise asks you to determine why the 4f and 5f inner transition series are placed separately at the bottom of the periodic table. The assertion points to their specific placement at the bottom.

Step 2: Identify Reason (i)

First, examine Reason (i), which states that placing these elements separately prevents the undue expansion of the periodic table. This maintains the periodic table's structure by avoiding the need to stretch the table to accommodate these series in-matrix, which would disrupt the order.

Step 3: Identify Reason (ii)

Examine Reason (ii), which claims that this placement helps preserve the principle of classification, whereby elements with similar properties are kept in a single column. The 4f and 5f series elements share similar properties with one another, hence they are best kept together.

Step 4: Determine the Correctness of Reasons

Both reasons given correctly explain why the 4f and 5f series are placed separately. The positioning prevents the expansion of the periodic table while maintaining classification principles. Therefore, both parts of the reason support the assertion.

Key Concepts

4f and 5f Inner Transition Series

The 4f and 5f inner transition series are special groups of elements in the periodic table. These elements are commonly known as the lanthanides and actinides. They are called "inner transition metals" because they form a transition series within the transition metals themselves.

• Lanthanides (4f series): These elements include the 14 elements from Cerium (Ce) to Lutetium (Lu). They are mainly comparable in their electron configurations, with electrons filling the 4f orbital. Lanthanides have similar properties, such as high magnetic susceptibility and reactivity with oxygen.
• Actinides (5f series): These elements cover the 14 elements from Thorium (Th) to Lawrencium (Lr). Like lanthanides, they fill the 5f orbital in their atomic structure. Many actinides are radioactive, with Uranium and Plutonium being well known for their applications in nuclear energy.

These groups are placed separate from the main body of the periodic table. This helps maintain the table's regularity and prevents an overly wide layout.

Periodic Table Structure

The periodic table is a systematic arrangement of chemical elements, ordered by increasing atomic number. Its layout reveals patterns in element properties, enabling easy understanding and study. Keeping the table structured and organized is crucial. One reason the 4f and 5f series are placed separately is to prevent an overly expanded table that would disrupt readability. Instead of placing these elements within the table's main body, where they would complicate the visual and structural consistency, they are positioned in their own distinct rows at the bottom. This design preserves the columns, or "groups," where elements exhibiting similar chemical behavior are aligned. The orderly presentation enhances learning and scientific communication among chemists.

Element Classification

Element classification within the periodic table follows distinct principles, organizing elements into groups with similar properties.

• By Similar Properties: Elements are grouped by similar chemical behaviors. For instance, elements in the same column usually share reactivity trends or physical characteristics, making it easier for scientists to predict behaviors.
• By Electron Configuration: This aspect examines the distribution of electrons across atomic orbitals. The 4f and 5f series demonstrate this as they fill inner "f" orbitals, influencing their specific classification.

These principles ensure that elements with comparable properties, like the lanthanides and actinides, are classified together, even if placed separately from the main body. This methodical organization lets us observe new trends, innovate in material science, and understand elemental potential.