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💡 One theory to explain both laws
Why is mass conserved, and why are proportions fixed? Dalton answered both with a single bold idea — atoms simply rearrange; they are never made or destroyed.
What are the main postulates of Dalton’s atomic theory?
Dalton's Postulates
Matter is made of tiny atoms Atoms are indivisible in reactions Atoms of one element are identical Atoms combine in whole-number ratios
- All matter is made of tiny particles called atoms .
- Atoms are indivisible — not created or destroyed in a reaction.
- Atoms of a given element are identical in mass and properties.
- Atoms of different elements differ in mass and properties.
- Atoms combine in simple whole-number ratios to form compounds.
👤 Meet a Scientist — John Dalton
- Born in England; moved to Manchester in 1793 to teach mathematics, physics and chemistry.
- Presented his atomic theory in 1808 , a turning point in the study of matter.
A postulate is a basic assumption accepted as true without formal proof, from which further ideas are developed. Dalton’s postulates explain why substances combine in fixed proportions and why mass is conserved — atoms merely rearrange.
⏸▶ Pause and Ponder
- 7. Assertion (A): 2 g of hydrogen combines with 16 g of oxygen to form 18 g of water. Reason (R): By Dalton’s theory, atoms combine in a simple whole-number ratio by mass. Answer: (i) Both A and R are true and R is the correct explanation of A.
✅ Test Yourself
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In which year did Dalton present his atomic theory?
Show Answer
In 1808. -
What is a postulate?
Show Answer
A fundamental assumption accepted as true without formal proof. -
According to Dalton, in what ratio do atoms combine?
Show Answer
In the ratio of simple whole numbers. -
Are atoms of a given element identical, per Dalton?
Show Answer
Yes — identical in mass and in chemical properties.
Important Definitions
- Postulate — a basic assumption accepted as true without formal proof.
- Dalton’s Atomic Theory — a set of postulates explaining conservation of mass and fixed proportions using indivisible atoms.
