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) \(\mathrm{A}\) 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 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

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

Step 1: Understanding the Assertion

The assertion states that the elements belonging to the 4f and 5f inner transition series, also known as the lanthanides and actinides, are placed separately at the bottom of the periodic table.

Step 2: Evaluating the Reason (Part i)

Part (i) of the reason explains that placing these series separately prevents the undue expansion of the periodic table, maintaining its compactness and structure. This is a true statement because including all elements in a single layout would result in a very wide periodic table.

Step 3: Evaluating the Reason (Part ii)

Part (ii) of the reason maintains that this organization helps in preserving the classification principle by grouping elements with similar chemical properties into single columns. This is also true as it keeps the periodic table organized, aligning elements with similar electron configurations and properties.

Step 4: Linking Assertion and Reason

Both the assertion and the reason (parts i and ii) are true. Placing the lanthanides and actinides at the bottom prevents the expansion of the periodic table (reason i) and maintains grouping with similar properties (reason ii). Hence, the reason provided is relevant and explains why the assertion is practiced.

Step 5: Determining the Correct Option

Since both the assertion (A) and reason (R) are correct and R provides the correct explanation for A, option (a) is the correct choice.

Key Concepts

Inner Transition Series

The Inner Transition Series refers to a specific group of elements on the periodic table. These elements are known as lanthanides and actinides. They include elements where the filling of f-orbitals takes place. Usually, these series are placed below the main body of the periodic table, forming their own two separate rows.
Placing these series separately helps maintain a cleaner and more organized layout of the periodic table. Such an arrangement avoids making the table excessively wide.

• Lanthanides: Elements from element 57 (Lanthanum) to element 71 (Lutetium).
• Actinides: Elements from element 89 (Actinium) to element 103 (Lawrencium).

The elements in the inner transition series showcase unique properties, such as high magnetic susceptibility and varied oxidation states. This unique positioning aids in maintaining the periodic classification system's integrity.

Lanthanides

Lanthanides consist of elements from Lanthanum (La) to Lutetium (Lu), occupying the 4f block on the periodic table. These elements are often referred to as rare earth elements, but are not particularly scarce in the Earth's crust.
Lanthanides are known for their bright silvery appearance and are usually known for their similar chemical properties, which include:

• Having high melting and boiling points.
• Being highly reactive, especially with water.
• Exhibiting strong magnetic and luminescent properties.

They are extensively used in various industries and technologies, such as in the manufacture of strong permanent magnets, phosphors for color television tubes and LED screens, and in the petroleum refining industry.

Actinides

The actinides are the elements spanning from Actinium (Ac) to Lawrencium (Lr) in the periodic table, sitting in the 5f block. These elements are primarily noted for their radioactive properties. The most well-known actinide is uranium, which is critical in nuclear power generation and weapons.
Actinides are characterized by their:

• Radioactivity: Most are highly radioactive with a wide range of half-lives.
• High density and metallic nature.
• Multiple oxidation states, which makes them chemically reactive.

The differing and intense radioactivity of these elements makes them both fascinating and challenging to study. Their use extends to various applications, including energy generation and medical treatments, however, their radioactive nature requires careful handling.

Classification of Elements

The classification of elements in the periodic table is designed to group elements with similar properties together. This allows for easier study and comparison of these elements based on trends. The elements are classified into different blocks based on their electron configurations: s-block, p-block, d-block, and f-block.
Each block indicates a specific type of orbital that the elements' electrons are filling. For instance:

• S-block includes elements with electrons in the outer s-orbital.
• P-block includes those with electrons filling p-orbitals.
• D-block, often referred to as transition metals, have d-orbitals being filled.
• F-block, the inner transition series like lanthanides and actinides, have f-orbitals.

The periodic table's structure helps in predicting the chemical behavior of the elements and understanding the periodic trends such as atomic size, ionization energy, and electronegativity.