Dalton’s Atomic Theory
Learning Objective: State the postulates of Dalton’s atomic theory and understand its role in explaining the laws of chemical combination.
Quick Notes:
- Proposed by John Dalton in 1808.
- Dalton’s theory explained the laws of chemical combinations using the concept of atoms.
- Key ideas:
- All matter is made up of tiny, indivisible particles called atoms.
- Atoms of a given element are identical in mass and properties.
- Atoms of different elements are different in mass and properties.
- Atoms combine in simple whole-number ratios to form compounds.
- Atoms can’t be created or destroyed in chemical reactions — only rearranged.
- Dalton’s model laid the foundation for modern atomic theory, though some postulates were later modified.
Full Notes:
Who Was John Dalton?
John Dalton was an English scientist who, in 1808, proposed a scientific explanation for the laws of chemical combinations. His atomic theory was the first to describe matter in terms of atoms — indivisible building blocks.
Dalton’s Postulates (Key Statements)
- All matter is made up of indivisible atoms.
Atoms are the smallest units of matter and cannot be broken down by chemical means. - All atoms of a given element are identical in mass and properties.
Example: Every oxygen atom behaves the same as another oxygen atom. - Atoms of different elements have different masses and properties.
Example: A hydrogen atom is fundamentally different from a carbon or nitrogen atom. - Atoms combine in simple whole-number ratios to form compounds.
Example: H2O is formed by combining 2 hydrogen atoms and 1 oxygen atom (not fractions). - Atoms can neither be created nor destroyed in a chemical reaction.
Chemical reactions involve the rearrangement of atoms, not their destruction or creation.
Significance of Dalton’s Theory
Dalton’s theory:
- Gave a scientific explanation for the laws of conservation of mass and definite proportions.
- Introduced the idea that chemical reactions are just rearrangements of atoms.
- Formed the starting point for the development of the modern atomic model.
Limitations of Dalton’s Theory
Although groundbreaking at the time, some of Dalton’s postulates were later revised:
- Atoms are divisible – they contain protons, neutrons, and electrons.
- Atoms of the same element can differ in mass due to isotopes.
- Atoms can be converted to energy in nuclear reactions.
These limitations were resolved with the discovery of subatomic particles and advances in quantum theory.
Q: Which of Dalton’s postulates explain the Law of Multiple Proportions?
A: The postulate that “atoms combine in simple whole-number ratios to form compounds” supports the law of multiple proportions. It explains why elements can form more than one compound in simple ratios (e.g. CO and CO₂).
Don’t just memorise the list – understand which postulate explains which law. In NEET or CBSE exams, you may be asked to match laws with Dalton’s statements or identify outdated ideas.
Summary
- Dalton proposed atoms as indivisible building blocks of matter.
- His postulates explained the laws of chemical combination scientifically.
- Later discoveries of subatomic particles modified some of his ideas.
- The concept of atoms combining in whole-number ratios remains central to modern chemistry.