Which one of the following is not explained by the Dalton's atomic theory?

Correct — C, Presence of different kind of sub-atomic particles in an atom. Dalton's theory of 1808 rests on the atom being the ultimate, indivisible particle of matter — it cannot be created, destroyed or divided. That postulate is what gives the theory its power over the laws of chemical combination, and it is also precisely what rules out any account of sub-atomic particles. If the atom cannot be divided, it can have no parts, so electrons, protons and neutrons are not merely absent from the theory but incompatible with it. The other three options are the laws Dalton's postulates were designed to explain, and each follows directly. Mass is conserved because a reaction only rearranges atoms and no atom is created or destroyed. Composition is constant because a given compound is made of the same elements combined in the same fixed whole-number ratio of atoms. Multiple proportions follow because when two elements form more than one compound, the atoms combine in small whole-number ratios — one carbon to one oxygen, or one to two — so the masses of oxygen combining with a fixed mass of carbon stand in a simple ratio. The theory's failure came later, with Thomson's discovery of the electron in 1897, which showed the atom had structure after all.

• (a)Law of conservation of mass — Explained. A chemical reaction only rearranges atoms, and since atoms are neither created nor destroyed, the total mass cannot change.
• (b)Law of constant composition — Explained. A compound consists of the same elements in the same fixed ratio of atoms, so its composition by mass is always the same whatever its source.
• (d)Law of multiple proportion — Explained, and it is the law Dalton's theory accounts for most convincingly. Atoms combining in small whole-number ratios is exactly what produces the simple mass ratios observed.

Dalton's postulates are that matter consists of indivisible atoms; that atoms of a given element are identical in mass and properties while atoms of different elements differ; that compounds are formed by atoms of different elements combining in fixed, simple whole-number ratios; and that chemical reactions merely rearrange atoms, creating and destroying none. From these the laws of conservation of mass, constant composition and multiple proportions all follow. Later work undermined two of the postulates: isotopes showed that atoms of one element need not have equal mass, and the discovery of the electron showed the atom is divisible.

The structure of this item is worth recognising because it recurs across the sciences: three options belong to one category and the fourth to another. Here, three are laws of chemical combination — the things Dalton set out to explain — and one is a statement about the internal structure of the atom, which belongs to a later era of physics entirely. Spotting the odd category is faster and safer than assessing each law separately. It is also worth being clear about what 'not explained' means: the theory does not simply omit sub-atomic particles, it forbids them, because indivisibility is a postulate rather than an oversight. Two questions later in this paper the same pattern of successes-and-limits appears for Rutherford's model, which explains the nucleus but not the atom's stability. Every model in the sequence Dalton, Thomson, Rutherford, Bohr is defined as much by what it fails at as by what it accounts for.

• Dalton proposed his atomic theory in 1808, with the indivisible atom as its central postulate.
• It explains the law of conservation of mass, the law of constant composition and the law of multiple proportions.
• Because the atom is postulated to be indivisible, the theory cannot admit sub-atomic particles at all.
• J. J. Thomson's discovery of the electron in 1897 showed the atom is divisible, overturning that postulate.
• The discovery of isotopes overturned another postulate — that all atoms of an element have identical mass.
• The law of multiple proportions is the one Dalton's theory accounts for most convincingly, since whole-number atomic ratios produce simple mass ratios directly.

• Assuming the theory merely fails to mention sub-atomic particles, when indivisibility positively rules them out.
• Trying to find a flaw in one of the three laws instead of noticing that the fourth option is a different kind of statement altogether.
• Confusing the law of constant composition with the law of multiple proportions.

As a limitation-of-the-theory item like this one, or from the other side — 'which law is explained by Dalton's atomic theory'. The postulate overturned by isotopes is asked separately.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which postulate of Dalton's atomic theory was contradicted by the discovery of isotopes?

  • (a)Atoms are indivisible
  • (b)All atoms of a given element have identical mass
  • (c)Atoms combine in whole-number ratios
  • (d)Atoms are neither created nor destroyed in a reaction

  Answer(b) All atoms of a given element have identical mass — isotopes of the same element differ in mass number.

• practice — not a real PYQ

  Carbon forms both CO and CO₂. This is best illustrated by which law?

  • (a)Law of conservation of mass
  • (b)Law of constant composition
  • (c)Law of multiple proportions
  • (d)Avogadro's law

  Answer(c) Law of multiple proportions — the masses of oxygen combining with a fixed mass of carbon are in the simple ratio 1 : 2.