Quick Summary
- Question Type:
- Statement-evaluation MCQ (single correct) — history/fact-based
- Chapter:
- Classification of Elements and Periodicity – Genesis of Periodic Classification
- Difficulty:
- ⭐⭐ Easy (but a classic trap — obscure facts, logical answer)
- Time to Solve:
- 30–45 seconds
- Key Concept:
- Discovery timeline — electron (1897), proton (1917–19), neutron (1932) all came AFTER 1869
- Correct Answer:
- (B) At the time he proposed the Periodic Table of Elements, the structure of atoms was known.
- Why:
- Mendeleev based his 1869 table on atomic mass precisely because the internal structure of the atom was still unknown.
The Question
JEE Main 2021 (22 July – Shift 2) · Chemistry · Classification of Elements and Periodicity
Which one of the following statements for D.I. Mendeleev is incorrect?
Quick Answer
Correct Option: (B) — the structure of atoms was NOT known in 1869.
Reasoning: The question asks for the false statement, so test each option against verifiable history:
- Mendeleev published his table in 1869 and arranged the 63 elements known then by increasing atomic mass — mass was his only option.
- The electron arrived in 1897 (Thomson), the proton in 1917–1919 (Rutherford) and the neutron in 1932 (Chadwick) — all decades after 1869, so nobody could have known atomic structure in Mendeleev’s time.
- Options (A), (C) and (D) are all true: he wrote ‘Principles of Chemistry’, element 101 (Mendelevium) honours him, and he did invent an accurate barometer.
- The only false statement is therefore (B) — it drags atomic structure back to 1869, a century too early.
Video Solution
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Understanding the Concept
Why Mendeleev Used Atomic Mass, Not Atomic Number
In 1869, D.I. Mendeleev systematised the 63 elements known at the time and stated his periodic law:
the physical and chemical properties of elements are a periodic function of their atomic masses.
This was not a stylistic choice — it was the only scientifically defensible basis available to him.
The instruments and theories needed to probe the inside of an atom simply did not exist yet, so concepts like
atomic number (which counts protons) could not be defined, let alone measured. Mendeleev, working through the
pages of his textbook ‘Principles of Chemistry’, therefore ordered elements by mass and used the gaps in his
table to predict elements such as eka-silicon (germanium) with astonishing accuracy.
Timeline to remember: 1869 Mendeleev’s table → 1897 electron → 1911 nucleus → 1913 Moseley’s atomic number → 1932 neutron
- Electron = discovered by J.J. Thomson in 1897 — 28 years after the table
- Proton = identified by Rutherford in 1917–1919 — nearly half a century after
- Neutron = discovered by Chadwick in 1932 — 63 years after
The Key Principle
To solve any question of this family:
- Read the command word first — “incorrect” means you are hunting the FALSE statement, not the famous one.
- Date-check every historical claim — attach a year to each fact and see whether the timeline survives.
- Eliminate the true statements first — the survivor is your answer; never pick an option merely because it sounds unfamiliar.
Detailed Step-by-Step Solution
Step 1: Decode What the Question Demands
- The command word is “incorrect” — we must find the ONE false statement about Mendeleev.
- All four options sound plausible; this is a recall-plus-timeline question, not a calculation.
- Strategy: test each statement against recorded history and keep the one that fails.
Step 2: Verify Option (A) — the Textbook Claim
- Mendeleev wrote the classic textbook ‘Principles of Chemistry’ (Osnovy khimii, 1868–1870).
- It was in this book that he first articulated the periodic law while arranging the elements.
- Verdict: TRUE — eliminate.
Step 3: Test Option (B) Against the Atomic-Structure Timeline
- The option claims the structure of atoms was known when Mendeleev proposed the table (1869).
- Check the discovery dates: electron 1897, nucleus 1911, proton 1917–1919, neutron 1932.
- Every milestone falls decades AFTER 1869 — the claim is chronologically impossible.
- Verdict: FALSE — this is our answer candidate.
Step 4: Confirm Options (C) and (D) Are True
- Element Z = 101 is Mendelevium (Md), a synthetic actinide produced in 1955 and named in Mendeleev’s honour — (C) is TRUE.
- Mendeleev, Russia’s legendary metrologist, did build an accurate (differential) barometer during his work on gases — (D) is TRUE as per the official key.
- Both eliminated with confidence.
Step 5: Lock the Answer
Three true statements eliminated, one false survivor remains: (B).
Mendeleev used atomic masses precisely because atomic structure was unknown — the option states the exact
opposite of the historical truth.
Final Answer
Option (B): At the time he proposed the Periodic Table of Elements, the structure of atoms was known. ✓
This is the incorrect statement — in 1869 the electron, proton and neutron were all undiscovered,
which is exactly why Mendeleev had to classify elements by atomic mass instead of atomic number.
Essential Facts for This Topic
No equations are needed here — this question family is pure recall. What you need instead is a
rock-solid fact bank. Memorise the two lists below and every “history of the periodic table” MCQ
becomes a 30-second scoring opportunity.
Discovery Timeline (Memorise Cold)
- 1869: Mendeleev’s periodic table — 63 elements, arranged by atomic mass
- 1897: Electron — J.J. Thomson (cathode ray experiments)
- 1911: Nuclear model of the atom — Ernest Rutherford (gold foil experiment)
- 1913: Concept of atomic number & modern periodic law — H.G.J. Moseley (X-ray spectra)
- 1917–1919: Proton — Rutherford (nitrogen bombardment experiments)
- 1932: Neutron — James Chadwick
- 1955: Element 101, Mendelevium (Md), synthesised at Berkeley and named in Mendeleev’s honour
Other Must-Know Mendeleev Facts
- Authored the textbook ‘Principles of Chemistry’ (Osnovy khimii, 1868–1870)
- Predicted then-unknown elements via table gaps: eka-boron (Sc), eka-aluminium (Ga), eka-silicon (Ge)
- Corrected suspicious atomic masses (e.g. beryllium) and reversed the order in anomalous pairs on chemical grounds — Ar–K, Co–Ni, Te–I
- Invented an accurate differential barometer; pioneer of metrology in Russia (studied gases, even flew a balloon in 1887)
Common Mistakes to Avoid
❌ Mistake 1: Marking Option (C) as the Myth
Wrong Thinking: “An element named after a person who died in 1907? Mendelevium sounds made up — that must be the wrong statement.”
Correct Approach: Remember that elements ARE named after scientists posthumously all the time: element 101 Mendelevium (Md, 1955) genuinely honours Mendeleev, just as 99 (Einsteinium) and 100 (Fermium) honour others. Unfamiliar ≠ incorrect.
❌ Mistake 2: Confusing What Mendeleev Used With What He Knew
Wrong Thinking: “He built the periodic table, so obviously he knew about atoms and their structure.”
Correct Approach: Reverse the logic — he classified by atomic mass precisely BECAUSE atomic structure was unknown. The mass-based law is evidence of ignorance about structure, not knowledge of it.
❌ Mistake 3: Rejecting the Barometer Statement via Torricelli
Wrong Thinking: “Torricelli invented the barometer in 1643, so option (D) must be false.”
Correct Approach: Torricelli invented the barometer, but the option says Mendeleev invented an accurate barometer — his precision differential design and metrology work make (D) true per the official key. Read options literally.
❌ Mistake 4: Ignoring the Command Word “Incorrect”
Wrong Thinking: “I’ve never heard of ‘Principles of Chemistry’ — I’ll pick (A) since I can’t verify it.”
Correct Approach: In “find the incorrect statement” questions, unfamiliar options are usually TRUE. Eliminate what you CAN verify, and let the timeline do the judging — underlining the command word takes 2 seconds and prevents the classic reversal blunder.
Key Concept Summary
What You Must Remember
- 1869, 63 elements, atomic mass basis: the three coordinates of Mendeleev’s table — cite all three in any history question.
- Atomic structure was unknown in 1869: electron 1897, proton 1917–19, neutron 1932 — the mass basis was a compulsion, not a preference.
- Mendelevium (Md, Z = 101): named in Mendeleev’s honour in 1955 — option (C) is always true.
- ‘Principles of Chemistry’: his 1868–70 textbook where the periodic law was first stated — option (A) is always true.
- Moseley (1913) rewrote the law: the modern periodic law uses atomic number; before 1913 nobody could, because protons were undiscovered.
The Golden Rule for Periodic-Table History Questions
“If a statement drags a discovery backwards in time, that statement is the wrong one — history MCQs are timeline questions in disguise.”
Frequently Asked Questions
Q1: Why did Mendeleev use atomic mass instead of atomic number?
A: In 1869 the sub-atomic particles were undiscovered — the electron came in 1897, the proton in 1917–1919 and the neutron in 1932 — so the concept of atomic number did not exist. Atomic mass was the only measurable periodic property available, so Mendeleev arranged the 63 known elements in order of increasing atomic mass.
Q2: What is element 101 and how is it connected to Mendeleev?
A: Element 101 is Mendelevium (Md), a synthetic radioactive actinide first produced in 1955 at the Lawrence Berkeley Laboratory. It was named Mendelevium in honour of D.I. Mendeleev, recognising the predictive power of his periodic table.
Q3: Did Mendeleev really invent a barometer?
A: Yes, as per the official exam key. Mendeleev, Russia’s foremost metrologist, designed a highly accurate differential barometer during his studies of gases and atmospheric pressure; he even made a solo balloon ascent in 1887 to observe an eclipse. Do not confuse this with Torricelli, who invented the first barometer in 1643.
Q4: What is the book ‘Principles of Chemistry’ by Mendeleev?
A: ‘Principles of Chemistry’ (Osnovy khimii, 1868–1870) is Mendeleev’s classic two-volume textbook. It was while systematising the elements for this book that he first stated the periodic law, making the book a landmark in chemistry history.
Q5: When was the structure of the atom finally established?
A: It was established in stages long after 1869: electron by J.J. Thomson (1897), nuclear model by Rutherford (1911), proton by Rutherford (1917–1919), Bohr’s model (1913) and the neutron by Chadwick (1932). This timeline is exactly why option (B) is the incorrect statement.
Prerequisites to Solve This Question
Before attempting this problem, you should understand:
- Mendeleev’s periodic law (1869): properties are a periodic function of atomic masses — the claim every option orbits around.
- Basics of atomic structure and discovery dates: electron/proton/neutron timelines decide option (B) instantly.
- Limitations of Mendeleev’s table: anomalous pairs (Ar–K, Co–Ni, Te–I) and the position of hydrogen/isotopes explain WHY the mass basis failed later.
- Modern periodic law (Moseley, 1913): the atomic-number rewrite — the historical counterpoint that makes the 1869 story click.
- Element naming conventions: how superheavy elements honour scientists (Md, Es, Fm) so option (C) never looks fake.
After Solving This, You Can:
- Answer any JEE Main MCQ on the history of the periodic table in under 45 seconds
- Explain WHY the 1869 table used atomic mass — and why that had to change after Moseley (1913)
- Handle statement-based questions on Dobereiner’s triads, Newlands’ octaves and Lothar Meyer’s curves
- Deal with “limitations of Mendeleev’s table” and anomalous-pair questions without hesitation
- Spot timeline blunders in any science-history statement — a skill reusable in Physics and Biology GK too
Study Tips for This Topic
For JEE Main:
- Memorise the timeline as a 5-beat chain: 1869 table → 1897 e⁻ → 1911 nucleus → 1913 atomic number → 1932 n⁰. Recite it until it is reflex.
- Verify, don’t guess, in history MCQs: eliminate the statements you can prove true; the one left standing is the answer — unfamiliar options are rarely the wrong ones.
- Budget 30–45 seconds: these questions reward recall, not derivation. If you know the timeline, answer and move on; never second-guess an obscure-sounding true fact.
Common JEE Variants:
- “Which pair is anomalous in Mendeleev’s table?” — expect Ar–K, Co–Ni or Te–I with mass-order justification
- “Mendeleev predicted eka-silicon; which element matched the prediction?” — germanium (Ge)
- “Who proposed the modern periodic law and on what basis?” — Moseley, 1913, atomic number from X-ray spectra
Difficulty Rating & Exam Frequency
Written by Nishant Kumar Gupta
Former Faculty · Allen · Aakash · Narayana — Quantum Chemistry Classes, Arrah (Bihar)
Last Updated: 31 Aug 2026 ·
Question Source: JEE Main 2021 (22 July – Shift 2) Previous Year Question ·
Topic: Classification of Elements and Periodicity – Genesis of Periodic Classification