Chemistry · Some Basic Concepts Of Chemistry · NEET
From NCERT: (1) Matter is made of indivisible atoms. (2) All atoms of a given element are identical, including identical mass; atoms of different elements differ in mass. (3) Compounds form when atoms of different elements combine in a fixed ratio. (4) Chemical reactions only rearrange atoms; atoms are neither created nor destroyed. Learn these four lines word for word for NEET.
Three laws of chemical combination: Law of Conservation of Mass (atoms are not created or destroyed), Law of Definite (Constant) Proportions (fixed ratio of atoms), and Law of Multiple Proportions (whole-number atom ratios). All three deal with MASS, and Dalton's mass-based postulates explain them well.
It could NOT explain the Law of Gaseous Volumes (Gay-Lussac's law) — why gases combine in simple whole-number VOLUME ratios. It also could not explain WHY atoms join together to form compounds. These gaps were later filled by Avogadro's law (equal volumes of gases at the same temperature and pressure have equal numbers of molecules) and by the discovery of subatomic particles.
Dalton believed atoms of the same element could not join with each other, so he thought gases like hydrogen and oxygen were single atoms. But gases like H2 and O2 are diatomic (two atoms joined). Because Dalton rejected this idea, he could not explain the whole-number volume ratios Gay-Lussac observed. Avogadro fixed it by treating gas particles as molecules.
No. Two postulates are now known to be wrong. Atoms are NOT indivisible — they contain electrons, protons and neutrons (Structure of Atom chapter). Also, not all atoms of an element have the same mass, because of isotopes (like carbon-12 and carbon-14). But the mass-conservation and fixed-ratio ideas are still used every day in chemistry.
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.
John Dalton proposed it in 1808 in his book 'A New System of Chemical Philosophy'. The idea of indivisible atoms is older (Democritus, and Acharya Kanda in ancient India), but Dalton gave it a firm scientific base.
Dalton's atomic theory could not explain the law of gaseous volumes (Gay-Lussac's law). It could explain conservation of mass, definite proportions and multiple proportions.
Avogadro. His law states that equal volumes of all gases, at the same temperature and pressure, contain equal numbers of molecules. Treating gases as diatomic molecules (like H2 and O2) made the volume ratios simple whole numbers.
First, atoms are not indivisible — they contain electrons, protons and neutrons. Second, all atoms of one element are not identical in mass, because of isotopes. This matters for the next topic, average atomic mass.
