Chemistry · Some Basic Concepts Of Chemistry · NEET
It explained the three laws of chemical combination that deal with mass: the Law of Conservation of Mass, the Law of Constant (Definite) Proportions, and the Law of Multiple Proportions. All three are about how MASS combines, and Dalton's idea of fixed-mass atoms combining in fixed ratios explains them nicely. This is why NEET options like 'conservation of mass' or 'constant proportions' are the WRONG answers when asked what Dalton could NOT explain.
Dalton could not explain the Law of Gaseous Volumes (Gay-Lussac's law). This law says gases react in simple whole-number volume ratios (for example, 1 volume H2 + 1 volume Cl2 gives 2 volumes HCl). Dalton thought each element was single atoms, so he could not explain how 2 volumes of product form. This was solved later by Avogadro, who said equal volumes of gases at the same temperature and pressure have equal numbers of molecules, and that gases like H2 and O2 are diatomic. In NEET 2025 the correct answer to this exact question was 'Law of gaseous volumes'.
Dalton believed atoms of the same element could not combine with each other, so hydrogen and oxygen were single atoms in his model. With single atoms you cannot get 2 volumes of water vapour from 2 volumes of hydrogen plus 1 volume of oxygen. You need diatomic molecules (H2, O2). Avogadro's idea of polyatomic/diatomic molecules fixed this. So the gap was not in the mass laws but in the VOLUME (gas) law.
No. Dalton's second postulate said 'all atoms of a given element have identical mass.' But isotopes are atoms of the SAME element with DIFFERENT masses (like Cl-35 and Cl-37). Isotopes were discovered later, so this postulate of Dalton was proven wrong. This is one important limitation you should remember for NEET.
Four main gaps: (1) the law of gaseous volumes, (2) WHY atoms combine (the reason for bonding came later from electronic structure), (3) isotopes and isobars — atoms of the same element having different masses, and (4) that the atom is divisible. Dalton called the atom 'indivisible', but later electrons, protons and neutrons were discovered, so the atom is NOT the smallest particle.
No. Dalton said the atom is the ultimate, indivisible particle. By the end of the 19th century scientists proved the atom is made of sub-atomic particles: electrons, protons and neutrons. So this postulate is outdated. NEET may test this as a 'limitation' of Dalton's theory.
Dalton's atomic theory could NOT explain which of the following?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The Law of Gaseous Volumes (Gay-Lussac's law). This is the most commonly tested answer, and it was the correct choice in NEET 2025.
Avogadro. He proposed that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules, and that gases can be diatomic (H2, O2). This explained the simple volume ratios.
No. It only said atoms combine in fixed ratios, but not WHY. The reason comes from electronic structure (valence electrons and bonding), which was discovered much later.
The postulate that 'all atoms of a given element have identical mass.' Isotopes are atoms of the same element with different masses, so this postulate is not fully true.
No. The atom is divisible into electrons, protons and neutrons. Dalton's 'indivisible atom' idea was later proven wrong.
