Dalton's atomic theory

Dalton’s Atomic Theory Class 11th Chemistry Unit -I

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Dalton's atomic theory
Dalton’s Atomic Theory | JKBOSE Class 11 Chemistry

Dalton’s Atomic Theory

Unit-I: Some Basic Concepts of Chemistry | JKBOSE Class 11

Historical Development

John Dalton (1766-1844) proposed his atomic theory in 1808, revolutionizing chemistry by providing a theoretical foundation for:

  • Law of Conservation of Mass (Lavoisier, 1789)
  • Law of Definite Proportions (Proust, 1799)
  • Law of Multiple Proportions (Dalton, 1803)
Exam Note: Always mention these three laws when asked about Dalton’s inspiration (2 mark question).

Five Postulates (Detailed)

1. Indivisible Atoms:

All matter is composed of extremely small, indivisible particles called atoms.

Example: Water (H₂O) breaks down into hydrogen and oxygen atoms, but these atoms cannot be further divided chemically.
2. Element Identity:

All atoms of a given element are identical in mass, size, and chemical properties.

Example: All oxygen atoms have same properties, different from nitrogen atoms.
3. Compound Formation:

Compounds form when atoms of different elements combine in fixed, simple whole-number ratios.

Example: Carbon dioxide (CO₂) always has 1 carbon atom bonded to 2 oxygen atoms.
4. Chemical Reactions:

Atoms rearrange during chemical reactions but are neither created nor destroyed.

Example: 2H₂ + O₂ → 2H₂O shows atoms regrouping (conservation of mass).
5. Distinct Properties:

Compounds have properties different from their constituent elements.

Example: NaCl (table salt) is edible, while sodium (Na) reacts violently with water.

Limitations (Modern Discoveries)

1. Subatomic Particles:

Atoms can be divided into protons, neutrons, and electrons (discovered 1897-1932).

Exam Tip: Mention J.J. Thomson’s cathode ray experiment (1897) for electrons.
2. Isotopes:

Atoms of same element can have different masses (e.g., ¹²C and ¹⁴C).

3. Nuclear Reactions:

Atoms can be converted to energy in nuclear processes (E=mc²).

4. Allotropes:

Same element exists in different forms (e.g., diamond vs graphite).

5. Isobars:

Different elements can have same mass number (e.g., ⁴⁰Ar and ⁴⁰Ca).

Expected Exam Questions

5-Mark Question: “Explain Dalton’s atomic theory and its limitations.”

Structure:
1. Introduction (1 mark)
2. All 5 postulates with examples (3 marks)
3. Any 3 limitations (1 mark)

3-Mark Question: “How does Dalton’s theory explain the law of definite proportions?”

Key Points:
– Fixed atom ratios in compounds
– Example (H₂O always 1:8 mass ratio)
– Contrast with mixtures

Next Concept: Atomic Structure

Discover subatomic particles (protons, neutrons, electrons) and modern atomic models

Continue to Atomic Structure →

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