Dalton’s atomic theory successfully explained i. Law of conservation of mass. ii. Law of constant composition. iii. Law of radioactivity. iv. Law of multiple proportion.
i, ii and iv
Dalton’s atomic theory (1808) accounts for the three laws of chemical combination — conservation of mass, constant composition and multiple proportions — but has nothing to say about radioactivity. The correct combination is therefore i, ii and iv, option 3.
The postulates first. Dalton proposed that matter consists of indivisible atoms; that all atoms of a given element are identical in mass and properties; that atoms of different elements differ; that compounds form when atoms combine in fixed, simple whole-number ratios; and that atoms are neither created nor destroyed in a chemical reaction. Every law the theory explains follows from one of these.
| Law | Which postulate explains it |
|---|---|
| i. Conservation of mass | Atoms are only rearranged in a reaction, never created or destroyed, so total mass is unchanged |
| ii. Constant composition | A compound always contains the same atoms in the same fixed ratio, so its composition by mass is fixed |
| iii. Radioactivity | Not explained — it requires the atom to have a divisible nucleus |
| iv. Multiple proportion | Atoms combine in small whole numbers, so the masses of one element combining with a fixed mass of another are in simple ratios |
Why radioactivity is the odd one out. Dalton’s central claim is that the atom is indivisible. Radioactivity, discovered by Henri Becquerel in 1896 and explained by Rutherford and Soddy as spontaneous nuclear disintegration, is the direct contradiction of that claim: one element transmutes into another, and mass is lost as energy. It is a nuclear phenomenon, not a chemical one, and it was one of the observations that forced Dalton’s model to be replaced. Chronology alone settles it — the theory of 1808 could not explain a phenomenon discovered almost ninety years later.
An illustration of the law of multiple proportions. Carbon forms both CO and CO2. Twelve grams of carbon combine with 16 g of oxygen in the first and 32 g in the second, a ratio of 1 : 2 — exactly the simple whole-number ratio Dalton’s atoms predict.
Hence, the answer is i, ii and iv.
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