Case Study:

In 1869, a Russian chemist, Dmitri Mendeleev, arranged known elements based on increasing atomic mass and observed periodic repetition in their properties. While constructing his table, he encountered several positions where no known element fit the pattern. Instead of forcing mismatched elements, he deliberately left gaps and predicted the existence and properties of unknown elements.

To understand the full journey from these early sketches to the high-tech charts we use today, explore the story of periodic tables made easy.

For example, Mendeleev predicted an element he called “eka-aluminium,” estimating its atomic mass (~68), density (~5.9 g/cm³), and oxide formula (E₂O₃). Years later, the element gallium was discovered, with properties remarkably close to his predictions: atomic mass 69.7 and density 5.91 g/cm³.

Scientists today find this accuracy surprising because Mendeleev did not know about atomic number or electronic configuration. Yet, his arrangement followed a hidden periodic trend in properties.

This raises a key scientific question: How could Mendeleev predict unknown elements so precisely, and why do these predictions still align with the modern periodic law, which is based on atomic number instead of atomic mass?

CASE-BASED QUESTIONS

Before you dive into these questions, if you need a quick refresher on the basics, check out these periodic table tricks and mnemonics or our guide on how to memorize the first 20 elements to sharpen your skills.

MCQ

Q1. Why did Mendeleev leave gaps in his periodic table?
A. To simplify the table
B. To adjust atomic masses
C. To accommodate undiscovered elements
D. To avoid classification errors

Q2. Which property was correctly predicted for eka-aluminium (gallium)?
A. Atomic number
B. Electronic configuration
C. Density and oxide formula
D. Melting point only

Assertion - Reason

Q3. Assertion (A): Mendeleev’s predictions were accurate because he followed periodic trends in properties.
Reason (R): Properties of elements are periodic functions of their atomic masses.

Options:
A. Both A and R are true, and R explains A
B. Both A and R are true, but R does not explain A
C. A is true, R is false
D. A is false, R is true

Application-Based Question

Q4. If a gap existed between elements of atomic masses 40 and 50 in Mendeleev’s table, what approach would he likely take? Explain briefly.

Q5. Modern periodic law is based on atomic number. Why do Mendeleev’s predictions still hold true despite being based on atomic mass?

Data/Logic-Based Question

Q6. Given predicted vs actual properties:

What does this close match indicate about periodic trends? Explain logically.

ANSWER KEY WITH EXPLANATION

A1. C - Mendeleev left gaps for undiscovered elements to maintain periodic trends instead of forcing incorrect placement.

A2. C - He accurately predicted physical and chemical properties like density and oxide formula using periodic patterns.

A3. A - Both statements are correct. Periodicity based on atomic mass helped him identify repeating trends, enabling predictions.

A4. He would leave a gap and predict properties based on neighboring elements, ensuring periodic trends remain consistent.

A5. Atomic mass and atomic number are generally correlated. Thus, periodic trends observed by Mendeleev still align with modern periodic law.

A6. The close match shows that periodic properties depend on underlying atomic structure trends, validating Mendeleev’s logic despite limited knowledge.

Ready to test your mastery? You can practice further with our Class 10 Chemistry worksheets. For exam-style preparation, try timing yourself with these unsolved practice papers or review the ideal answering techniques in our solved practice papers.

HOTS EXTENSION QUESTIONS

1. If Mendeleev had arranged elements strictly by atomic number instead of atomic mass, how might his table have differed? Would gaps still exist?

2. Can periodic trends be used today to predict properties of artificially synthesized elements? Justify with reasoning.