Dalton's Atomic Theory — JEE Main Chemistry MCQs with Solutions
Free JEE Main Chemistry Dalton's Atomic Theory MCQs with step-by-step solutions (7 questions). Part of Some Basic Concepts of Chemistry. Practise online on Prepizo — no login needed.
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Questions with solutions
Q1 — Dalton's Atomic Theory · medium · theory
The existence of isotopes (for example ¹²C, ¹³C and ¹⁴C) shows that which postulate of Dalton’s atomic theory is not strictly correct?
A. Atoms are indivisible
B. All atoms of a given element are identical in mass ✓ Correct
C. Atoms are neither created nor destroyed in a reaction
D. Atoms combine in simple whole-number ratios
Solution: Isotopes are atoms of the same element with different masses (e.g. ¹²C, ¹³C and ¹⁴C), which contradicts Dalton’s postulate that all atoms of a given element are identical in mass. The other three postulates remain broadly valid for ordinary chemical behaviour.
Q2 — Dalton's Atomic Theory · hard · theory
The species ⁴⁰Ar, ⁴⁰K and ⁴⁰Ca have practically the same mass but belong to different elements. This observation is inconsistent with Dalton’s assumption that:
A. compounds are formed in a fixed ratio
B. atoms of different elements differ in mass ✓ Correct
C. atoms of the same element are identical
D. atoms are indivisible
Solution: Atoms of different elements having the same mass are called isobars (here all have mass number 40). Their existence contradicts Dalton’s postulate that atoms of different elements must differ in mass. The other postulates are not challenged by isobars.
Q3 — Dalton's Atomic Theory · hard · theory
The discovery of electrons, protons and neutrons directly overturned which postulate of Dalton’s atomic theory?
A. Atoms are indivisible ✓ Correct
B. Atoms are rearranged in chemical reactions
C. Atoms combine in a fixed ratio to form compounds
D. Atoms of different elements differ in mass
Solution: Dalton’s first postulate held that atoms are the ultimate, indivisible units of matter. The discovery of subatomic particles (electrons, protons, neutrons) showed that atoms are in fact divisible, overturning this postulate while leaving the atom’s role as the unit of chemical combination intact.
Q4 — Dalton's Atomic Theory · hard · theory
Which statement correctly distinguishes Dalton’s view of gaseous elements from Avogadro’s and explains why Dalton could not account for combining gas volumes?
A. Dalton believed atoms of the same kind could not combine, so molecules such as H₂ and O₂ did not exist, whereas Avogadro allowed gases to be polyatomic ✓ Correct
B. Avogadro denied the existence of atoms, unlike Dalton
C. Dalton proposed diatomic molecules while Avogadro insisted every gas is monatomic
D. Both held that gaseous elements are always monatomic
Solution: Dalton (and others of his time) believed atoms of the same kind could not combine, so diatomic molecules like H₂ and O₂ did not exist. Avogadro instead proposed that gaseous elements could be polyatomic; treating H₂ and O₂ as diatomic makes the 2:1:2 volume relationship of hydrogen, oxygen and water vapour understandable — something Dalton’s picture could not do.
Q5 — Dalton's Atomic Theory · medium · theory
Besides failing to explain the law of gaseous volumes, another important limitation of Dalton’s atomic theory is that it:
A. denied that a compound has a fixed composition
B. was unable to account for the law of definite proportions
C. contradicted the law of conservation of mass
D. could not explain why atoms combine, i.e. the nature of the chemical bond ✓ Correct
Solution: The NCERT text notes that Dalton’s theory "could not provide the reason for combining of atoms" — that is, it offered no explanation for the nature of chemical bonding, which was answered later by other scientists. It did successfully explain conservation of mass and definite/multiple proportions, so those are not limitations.
Q6 — Dalton's Atomic Theory · medium · theory
Even today, in an ordinary (non-nuclear) chemical reaction, which part of Dalton’s atomic theory is still accepted as valid?
A. Atoms of an element are all identical in mass
B. Atoms of different elements always differ in mass
C. Atoms are neither created nor destroyed but are only rearranged ✓ Correct
D. Atoms are the smallest indivisible particles of matter
Solution: The postulate that atoms are conserved and merely rearranged during a chemical reaction still holds for ordinary chemistry and underlies the balancing of equations. The other three postulates were later modified by the discovery of isotopes (different masses for one element), subatomic particles (atoms are divisible) and isobars (equal masses for different elements).
Q7 — Dalton's Atomic Theory · hard · numerical
Two oxides of nitrogen contain 63.6% and 46.7% nitrogen by mass respectively. For a fixed mass of nitrogen, the ratio of the mass of oxygen in the first oxide to that in the second (illustrating the law of multiple proportions) is:
A. 3:4
B. 2:1
C. 1:2 ✓ Correct
D. 2:3
Solution: Oxygen combined per unit nitrogen = (%O)/(%N). First oxide: 36.4 ÷ 63.6 = 0.572; second oxide: 53.3 ÷ 46.7 = 1.14. The ratio 0.572:1.14 = 1:2 (the oxides are N₂O and NO). The nitrogen mass must be fixed first — comparing the oxygen percentages directly (36.4:53.3 ≈ 2:3) is the common mistake.