Problem 49 - A Complete Resource Book in Chemistry for JEE Main

Question

Lanthanoides are (a) 14 elements in the \(\mathrm{VI}\) period (atomic number \(=58\) to 71 ) that are filling the \(4 \mathrm{f}\)-sublevel (b) 14 elements in the VII period (atomic number = 50 to 71 ) that are filling the 4 f-sublevel (c) 14 elements in the VII period (atomic number = 90 to 103 ) that are filling the 5 f-sublevel (d) 14 elements in the VI period (atomic number \(=90\) to 103 ) that are filling the 4 f-sublevel

Step-by-Step Solution

Answer

The correct answer is (a): 14 elements in the VI period (atomic number 58 to 71) filling the 4f-sublevel.

Step 1: Understanding Lanthanoides

Lanthanoides, also known as lanthanides, refer to a series of elements with similar properties. These elements are located in the periodic table and are characterized by the filling of their 4f electron orbital.

Step 2: Check the Available Options

We need to determine which option correctly describes the lanthanoides. We have options with different periods and electron sublevel designations. Let's analyze each.

Step 3: Analyze Option (a)

Option (a) describes lanthanoides as 14 elements in the VI period (atomic numbers 58 to 71) that are filling the 4f-sublevel. This correctly matches the definition of lanthanoides, as they include elements from Cerium (Ce, 58) to Lutetium (Lu, 71), filling the 4f orbital.

Step 4: Analyze Option (b)

Option (b) describes 14 elements in the VII period with atomic numbers ranging from 50 to 71 filling the 4f-sublevel. However, the atomic number range is incorrect for lanthanoides, as it starts at 58, not 50.

Step 5: Analyze Option (c)

Option (c) describes 14 elements in the VII period (atomic number 90 to 103) filling the 5f-sublevel. This describes actinides, not lanthanoides, as actinides fill the 5f and start with Thorium (Th, 90).

Step 6: Analyze Option (d)

Option (d) describes 14 elements in the VI period (atomic number 90 to 103) filling the 4f-sublevel. This is incorrect since atomic numbers 90 to 103 refer to actinides and not lanthanoides.

Key Concepts

Periodic Table

The **Periodic Table** is a tabular arrangement of chemical elements based on their atomic number, electron configurations, and recurring chemical properties. Invented by Dmitri Mendeleev in 1869, it has become a cornerstone for organizing and understanding chemical behavior. Each element in the periodic table is placed into a specific group and period.

• **Groups**: Vertical columns that denote elements with similar chemical and physical properties.
• **Periods**: Horizontal rows that represent elements with the same number of atomic orbitals.

The lanthanoids are found in the sixth period and are of great significance due to their similar properties. They reside in the f-block of the periodic table. The arrangement helps to predict chemical reactions and understand the properties of various elements.

4f-sublevel

The **4f-sublevel** plays a crucial role when discussing the lanthanoides. In the electron configuration of an atom, the 4f-sublevel holds up to 14 electrons and is part of the f-block elements. These lanthanoide elements fill their 4f orbital after the 6s orbital during electron configuration.

• The 4f orbitals begin filling from Cerium (Ce, atomic number 58).
• They end with Lutetium (Lu, atomic number 71).

The gradual filling of this sublevel imparts unique properties to lanthanoide elements, such as being highly magnetic. Unlike the s, p, and d blocks, the 4f sublevel has complex electron interactions, which contribute to these special properties.

Atomic Number

The **Atomic Number** of an element is the number of protons in the nucleus of an atom and determines the identity of the element. Elements in the periodic table are arranged sequentially based on atomic number, reflecting their unique properties.

For the lanthanoides:

• They possess atomic numbers from 58 to 71.
• This range starts with Cerium (Ce) and ends with Lutetium (Lu).

These atomic numbers are crucial in distinguishing lanthanoides from other groups of elements. The precise knowledge of atomic numbers helps in locating the correct position of lanthanoides in the periodic table and understanding their electron filling sequence, especially their 4f-sublevel characteristics.