Problem 142 - A Complete Resource Book in Chemistry for JEE Main

Question

A reduction in atomic sizes with increase in atomic number is a characteristic of element of \(\quad\) [2003] (a) high atomic masses (b) d-block (c) f-block (d) radioactive series

Step-by-Step Solution

Answer

The characteristic is associated with f-block elements.

Step 1: Understanding the Question

We are given a statement about the reduction in atomic sizes with an increase in atomic number, and we need to identify which block or series of elements this characteristic applies to the most.

Step 2: Recalling Periodic Trends

In the periodic table, as we move across a period from left to right, the atomic size usually decreases. This is because the effective nuclear charge increases, pulling the electrons closer to the nucleus.

Step 3: Analyzing the Options

Let's consider each option: (a) High atomic masses do not necessarily relate to atomic size reduction. (b) D-block elements experience a decrease in atomic size across a period due to increasing nuclear charge. (c) F-block elements (lanthanides and actinides) also show decrease in size along the series, particularly known as lanthanide contraction. (d) Radioactive elements do not specifically show a size reduction pattern related to atomic number increases.

Step 4: Identifying the Correct Option

Both D-block and especially F-block elements exhibit notable size reduction phenomena due to increases in effective nuclear charge. Between the two, the f-block's lanthanide contraction is a classic example of significant size reduction due to increased atomic number.

Key Concepts

f-block

The f-block of the periodic table comprises two series of elements: the lanthanides and the actinides.
These elements are positioned separately at the bottom of the periodic table, often to keep the table more manageable in size. The f-block is known for its involvement in specific trends like lanthanide contraction.
The f-block elements have electrons filling the "f" orbitals. This makes them unique compared to other elements in the periodic table.
These elements are typically known for complex electron configurations:

• Lanthanides (from Lanthanum to Lutetium)
• Actinides (from Actinium to Lawrencium)

Lanthanides, in particular, are well-studied for their atomic size reduction as atomic number increases, highlighting the concept of lanthanide contraction.

atomic size reduction

Atomic size refers to how big an atom is, usually in terms of the radius from the nucleus to the outer shell of electrons. In the periodic table, atomic size generally decreases across a period from left to right. This trend also holds in certain groups or series of elements.
Atomic size reduction, particularly in the f-block, owes its notorious reputation to the lanthanide contraction. As more protons are added to a nucleus, pulling more electrons, the atomic size effectively decreases even as the atomic number increases.
This behavior is caused by increased effective nuclear charge, but we'll get to that soon! Important to note:

• Size decreases across a period
• Size reduction prominent in f-block (lanthanides)
• Caused by increasing protons attracting electrons more strongly

effective nuclear charge

The effective nuclear charge ( Z_{ ext{eff}} ) is the net positive charge experienced by electrons in an atom. It impacts how tightly the electrons are held by the nucleus. The idea is simple: the inner electrons shield outer electrons from the full effect of the nucleus, reducing the effective charge felt by the outermost electrons. In the f-block, especially in the lanthanide series, as you move from one element to the next, the number of protons increases.
Despite this increase, the shielding effect of inner electrons does not compensate fully for the added protons, leading to a stronger pull on the outer electrons.
Increased effective nuclear charge causes:

• Tighter electron cloud
• Reduction in atomic size
• Higher attraction of electrons towards nucleus

periodic trends

Periodic trends are predictable patterns seen in the periodic table as one moves across periods or down groups.
They help us predict the behavior and properties of elements and involve properties like atomic size, ionization energy, and electron affinity.
When it comes to atomic size, the periodic trend across a period is generally a reduction, due to the increasing effective nuclear charge. For f-block elements, the periodic trend reveals a detailed aspect of this behavior with lanthanide contraction.
Understanding these trends, students can better anticipate chemical reactions and the nature of elements based on their position on the table. Key periodic trends in the context of atomic size include:

• Decrease in atomic size across a period due to increased Z_eff
• Lanthanide contraction showing a subtle yet significant size decrease
• Predicting behavior of individuals elements based on position