Case Study:

In the late 19th century, chemists faced confusion while organizing elements systematically. Dmitri Mendeleev arranged elements based on increasing atomic mass and grouped them according to similar chemical properties. However, certain anomalies were observed. For instance, iodine (atomic mass ≈ 127) was placed after tellurium (atomic mass ≈ 128) despite having a lower mass, simply because its chemical properties matched elements in the same group.

If you want to dive deeper into the historical drama and the 'why' behind these shifts, check out our full guide on Mendeleev to Modern: The Story of Periodic Tables Made Easy.

Later, scientists discovered that atomic number (number of protons) is a more fundamental property than atomic mass. When elements were rearranged according to increasing atomic number, these anomalies were resolved. For example, iodine (atomic number 53) correctly follows tellurium (atomic number 52), aligning both in proper groups without disturbing chemical similarity.

Additionally, isotopes - atoms of the same element with different masses - could not be explained in Mendeleev’s table, as they would occupy different positions. However, since isotopes share the same atomic number, they occupy a single position in the modern periodic table.

This shift led to the formulation of the Modern Periodic Law, fundamentally changing the logic of classification and improving prediction accuracy.

CASE-BASED QUESTIONS

MCQ

Q1. Why was iodine placed after tellurium in Mendeleev’s table despite having a lower atomic mass?
A. Due to experimental error
B. Based on atomic number
C. Based on similar chemical properties
D. Due to isotope presence

Q2. Which property resolved the anomalies of Mendeleev’s periodic table?
A. Atomic mass
B. Atomic size
C. Atomic number
D. Valency

Assertion - Reason

Q3. Assertion (A): Isotopes occupy the same position in the modern periodic table.
Reason (R): Isotopes have the same atomic number but different atomic masses.

A. Both A and R are true, and R is correct explanation of A
B. Both A and R are true, but R is not correct explanation of A
C. A is true, R is false
D. A is false, R is true

Application-Based Question

Q4. An element X has atomic mass 39 and atomic number 19, while element Y has atomic mass 40 and atomic number 20.
Predict their order in:
(a) Mendeleev’s table
(b) Modern periodic table

Application-Based Question

Q5. A chemist finds two elements with similar properties but different atomic masses. How would modern periodic classification handle their placement more accurately?

Data/Logic-Based Question

Q6. Consider the following data:

(a) Identify which element would be placed first in Mendeleev’s table
(b) Identify correct order in modern periodic table
(c) Explain the discrepancy

Struggling to keep these elements straight? We’ve simplified the process with these Periodic Table Tricks and Mnemonics and a dedicated guide on how to memorize the first 20 elements for CBSE Class 10.

ANSWER KEY WITH EXPLANATION

A1. C - Mendeleev prioritized chemical properties over atomic mass, so iodine was placed correctly with similar elements.

A2. C -  Atomic number determines element identity; arranging by it resolves inconsistencies.

A3. A - Isotopes have same atomic number, so they occupy one position; this explains their placement.

A4. (a) Mendeleev: X before Y (based on mass)
(b) Modern: X before Y (based on atomic number)
Both align here, but modern table uses atomic number as the correct basis.

A5. Modern classification uses atomic number, ensuring elements with similar properties align correctly regardless of mass differences.

Q6.  (a) Mendeleev: A before B (lower mass)
(b) Modern: B before A (lower atomic number)
(c) Discrepancy arises because atomic mass does not reflect true elemental identity; atomic number does.

Ready to test your knowledge under exam conditions? Download our Chemistry Class 10 Worksheets or sharpen your skills with these Solved Practice Papers. For a real challenge, try timing yourself with these Unsolved Practice Papers.

HOTS EXTENSION QUESTIONS

1. If a new element is discovered with atomic number 119, predict its placement and properties based on modern periodic trends. Justify your reasoning.

2. Suppose classification was still based on atomic mass. Predict two major scientific problems that would arise in modern chemistry.