Periodic Table Atomic Radius Question JEE Main 2026

Quick Summary

Question Type: Atomic radius and periodic trend comparison
Chapter: Periodic Table – Atomic Size Trends
Difficulty: ⭐⭐ Easy
Time to Solve: 2-3 minutes
Key Formula: Atomic radius decreases across a period from left to right
Correct Answer: (C) K & Br
Why: In period 4, potassium (K) is the leftmost element and has the largest atomic radius, while bromine (Br) lies near the right end and has the smallest atomic radius among the given choices.

The Question

JEE Main 2026 (28th January Morning Shift) – Periodic Table

In period 4 of the periodic table, the elements with highest and lowest atomic radii are respectively.

(A) Rb & Br
(B) Na & Cl
(C) K & Br
(D) K & Se

Quick Answer

Correct Option: (C) K & Br

Reasoning: Across a period, atomic radius generally decreases from left to right because effective nuclear charge increases while electrons are added to the same shell.

• In period 4, the leftmost element is K, so it has the highest atomic radius
• Among the given options from period 4, Br has the lowest atomic radius

Therefore, the correct pair is K and Br.

Video Solution

If you want the full explanation in a clear step-by-step teaching format, watch the video solution below:

Watch Full Video Solution on YouTube

Understanding the Concept

Why Atomic Radius Changes Across a Period

Atomic radius depends on the pull of the nucleus on the outermost electrons. Across a period, the number of protons increases, but electrons are added to the same principal shell. This increases effective nuclear charge and pulls electrons closer to the nucleus.

Atomic radius decreases from left to right across a period

So to solve this question, we just identify the leftmost and the smallest relevant element in period 4.

The Key Principle

1. Locate the correct period
2. Recall the left-to-right atomic size trend
3. Pick the largest and smallest elements from that period

Detailed Step-by-Step Solution

Step 1: Identify Period 4 Elements

• Period 4 starts from K and goes across to Kr
• Among the options given, K, Se and Br belong to period 4
• Rb belongs to period 5, while Na and Cl belong to period 3

Step 2: Apply Atomic Radius Trend

Download Handwritten Solution

• Atomic radius is maximum on the left side of a period
• Atomic radius decreases as we move to the right
• So in period 4, K has the highest atomic radius
• Among the listed period 4 non-metals, Br has the lowest atomic radius

Step 3: Eliminate Incorrect Options

• (A) Rb is not in period 4
• (B) Na and Cl are not in period 4
• (D) Se is not the smallest; Br is smaller than Se

Step 4: Final Choice

The correct pair is:

K & Br

So the correct option is (C).

Final Answer

Option (C): K & Br ✓

In period 4, K has the highest atomic radius and Br has the lowest atomic radius among the given choices.

Essential Formulas for This Topic

Primary Rules

1. Atomic Radius Trend Across a Period:
   • Decreases from left to right
   • Effective nuclear charge increases
   • Electrons are added in the same shell
2. Atomic Radius Trend Down a Group:
   • Increases down the group
   • New shells are added
   • Size becomes larger
3. Period 4 Logic:
   • K is at the extreme left of period 4
   • Br lies near the right side
   • So K is largest and Br is smallest among listed period 4 choices

Important Constants

• Atomic size depends on nuclear pull and shielding
• Higher effective nuclear charge means smaller atomic radius
• K belongs to period 4 and group 1
• Br belongs to period 4 and group 17

Common Mistakes to Avoid

❌ Mistake 1: Choosing Rb as the Largest

Wrong Thinking: “Rb is larger than K, so it must be correct.”

Correct Approach: The question specifically asks for period 4. Rb is in period 5, so it is not valid here.

❌ Mistake 2: Ignoring the Period Restriction

Wrong Approach: Comparing all atoms without checking whether they belong to period 4

Correct Approach: First identify only the elements belonging to period 4, then apply the radius trend.

❌ Mistake 3: Choosing Se Instead of Br

Common Error:

• Stopping before moving fully to the right of period 4
• Forgetting that Br lies to the right of Se

Correct Approach: Since atomic radius decreases across the period, Br is smaller than Se.

❌ Mistake 4: Mixing Group Trend with Period Trend

Wrong Thinking: “Largest and smallest should be found only by looking at metal vs non-metal.”

Correct Understanding:

• For this question, period trend is the key
• In one period, size decreases from left to right
• So K is largest and Br is smallest among valid choices

Key Concept Summary

What You Must Remember

1. Atomic radius decreases across a period: Due to increasing effective nuclear charge
2. Atomic radius increases down a group: Due to addition of new shells
3. K is the largest in period 4: It is on the extreme left
4. Br is the smallest among the given period 4 options: It lies further right than Se
5. Always check the period first: This avoids choosing wrong elements like Rb or Na

The Golden Rule for Atomic Size Questions

“First identify the correct period or group, then apply the periodic trend of atomic radius.”

Frequently Asked Questions

Q1: Why is potassium the largest in period 4?

A: Because potassium is the leftmost element in period 4 and atomic radius decreases across the period from left to right.

Q2: Why is bromine smaller than selenium?

A: Bromine lies to the right of selenium in period 4, so it experiences greater effective nuclear charge and has a smaller atomic radius.

Q3: Why is rubidium not the answer?

A: Rubidium is larger than potassium, but it belongs to period 5, not period 4.

Q4: Why are sodium and chlorine incorrect options?

A: Both sodium and chlorine belong to period 3, not period 4.

Q5: What is the final answer to this question?

A: The correct pair is K and Br.

Prerequisites to Solve This Question

Before attempting this problem, you should understand:

1. Periodic table layout: Periods and groups
2. Atomic radius trend: Across period and down group
3. Effective nuclear charge: Its effect on atomic size
4. Basic element positions: Period 3, 4 and 5 elements
5. Comparison logic: Restricting to the required set before comparing

After Solving This, You Can:

✅ Compare atomic radii across a period confidently
✅ Avoid confusion between period and group trends
✅ Eliminate incorrect options quickly in periodic table questions
✅ Solve direct atomic size MCQs faster
✅ Strengthen your understanding of effective nuclear charge
✅ Apply the same logic to ionic radius and periodic trend questions

Study Tips for This Topic

For JEE Main:

1. Memorize the major periodic trends: Atomic size, ionization enthalpy, electronegativity, metallic character
2. Always check the period/group first: Many wrong options exploit this mistake
3. Practice elimination method: It works very well in direct periodic table MCQs
4. Link trend with reason: Effective nuclear charge explains many questions

Common JEE Variants:

• Highest or lowest atomic radius
• Correct order of atomic size
• Period trend and group trend comparisons
• Atomic vs ionic size questions

Difficulty Rating & Exam Frequency

Difficulty Level: ⭐⭐ (2/5) – Easy
JEE Main Frequency: High – Periodic trend questions are common
JEE Advanced Frequency: Medium – Usually combined with deeper reasoning
Topic Importance: Very High – Atomic size is a core trend in the periodic table

Written by Nishant Kumar
Chemistry Educator with 10+ Years of Experience Teaching JEE Aspirants
Founder – PadhoLikhoJEE

Last Updated: March 2026
Question Source: JEE Main 2026 PYQ
Topic: Periodic Table – Atomic Size Trends
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