ANAND CLASSES Study Material and Notes to learn Dalton’s Atomic Theory with detailed explanations of its postulates, limitations, and significance. Perfect for Class 11, NEET, and JEE aspirants. Includes Q&A format, examples, and exam-focused insights.
🌟 Dalton’s Atomic Theory – Q&A
🔹 Q1. What is the historical background behind the concept of atoms?
A:
The concept that matter is composed of tiny indivisible particles dates back to ancient times. Around 460–370 BC, Democritus, a Greek philosopher, proposed that all matter is made up of tiny, indestructible units called “a-tomio”, meaning indivisible.
However, this was a philosophical idea with no experimental evidence. The concept remained dormant until the 19th century, when John Dalton, an English scientist, revived and refined it through experimental observations and gave it a scientific basis.
🔹 Q2. Who was John Dalton, and what was his contribution to atomic theory?
A:
John Dalton (1766–1844) was an English chemist, physicist, and meteorologist. In 1808, he published a book titled “A New System of Chemical Philosophy”, in which he proposed the first modern atomic theory based on scientific evidence. This theory laid the foundation for modern chemistry and explained many laws of chemical combination.
🔹 Q3. What are the main postulates of Dalton’s Atomic Theory? Explain each in detail.
A:
Dalton proposed five fundamental postulates:
1. Matter is made up of tiny, indivisible particles called atoms.
- Atoms are the smallest units of matter and cannot be created, divided, or destroyed in a chemical reaction.
- This explained the conservation of mass in chemical reactions.
2. All atoms of a given element are identical in mass and properties.
- For example, every atom of hydrogen has the same mass and same chemical behavior.
- However, this idea was later modified due to the discovery of isotopes (atoms of the same element with different masses).
3. Atoms of different elements have different masses and properties.
- Atoms of oxygen differ from atoms of nitrogen in both mass and behavior.
4. Compounds are formed when atoms of different elements combine in a fixed ratio.
- A compound like water (H₂O) is formed by combining 2 atoms of hydrogen and 1 atom of oxygen.
- This explains the Law of Definite Proportions.
5. Chemical reactions involve the rearrangement of atoms.
- Atoms recombine during reactions but are not created or destroyed.
- Example: 2H2+O2→2H2O2H_2 + O_2 \rightarrow 2H_2O Here, atoms rearrange to form water, but no new atoms are formed or lost.
🔹 Q4. How did Dalton’s theory explain the laws of chemical combination?
A:
Dalton’s theory provided a logical explanation for the following laws:
✔️ Law of Conservation of Mass
Atoms are neither created nor destroyed during chemical reactions.
✔️ Law of Definite Proportions
Atoms combine in fixed ratios to form compounds.
✔️ Law of Multiple Proportions
When two elements form more than one compound, the ratios of the masses of one element that combine with a fixed mass of the other are simple whole numbers.
🧪 Example:
Carbon and oxygen form:
- CO (carbon monoxide): 12 g of C + 16 g of O
- CO₂ (carbon dioxide): 12 g of C + 32 g of O
The ratio of oxygen is 16:32 = 1:2
🔹 Q5. What are the major limitations of Dalton’s Atomic Theory?
A:
Although revolutionary, Dalton’s theory had several flaws:
- ❌ Atoms are indivisible → Disproved by the discovery of subatomic particles:
- Electron (J.J. Thomson)
- Proton (Goldstein)
- Neutron (Chadwick)
- ❌ All atoms of an element are identical → Disproved by the discovery of isotopes
- Example:
- Hydrogen has 3 isotopes: Protium (¹H), Deuterium (²H), Tritium (³H)
- Example:
- ❌ No explanation of bonding or structure
- Dalton couldn’t explain how or why atoms combine (later explained by electronic theory of bonding)
- ❌ Failed to explain Gay-Lussac’s Law of Gaseous Volumes
- Couldn’t explain why gases combine in simple volume ratios (e.g., 2H₂ + O₂ → 2H₂O)
🔹 Q6. How is Dalton’s Atomic Theory relevant today?
A:
Despite its limitations, Dalton’s theory:
- Laid the groundwork for modern atomic structure.
- Introduced the concept of atoms, which is still valid.
- Helped develop the mole concept, stoichiometry, and chemical equations.
- Initiated a scientific approach to understanding matter.
📘 Quick Revision Points
| Feature | Dalton’s Postulate |
|---|---|
| Nature of atoms | Indivisible, indestructible |
| Atoms of the same element | Identical in mass and properties |
| Atoms of different elements | Different in mass and properties |
| Compound formation | Fixed whole-number ratio |
| Chemical reaction | Rearrangement of atoms only |
✅ MCQs on Dalton’s Atomic Theory (with Answers & Explanations)
Q1. According to Dalton’s Atomic Theory, which of the following statements is correct?
A) Atoms are created during chemical reactions
B) Atoms of different elements have the same mass
C) Atoms of the same element are identical in all respects
D) Atoms can be divided into smaller particles
Answer: ✅ C) Atoms of the same element are identical in all respects
Explanation:
Dalton stated that all atoms of a given element are identical in mass and properties, though this was later disproved due to isotopes. At his time, this was a foundational concept.
Q2. Which law of chemical combination can be explained by the statement: “Atoms combine in simple whole-number ratios”?
A) Law of Conservation of Mass
B) Law of Definite Proportions
C) Law of Multiple Proportions
D) Avogadro’s Law
Answer: ✅ B) Law of Definite Proportions
Explanation:
Dalton’s theory supports the Law of Definite Proportions, which states that a chemical compound contains its component elements in fixed ratio by mass.
Q3. Which of the following discoveries challenged Dalton’s idea that atoms are indivisible?
A) Discovery of Isotopes
B) Discovery of Molecules
C) Discovery of Subatomic Particles
D) Discovery of Atomic Mass
Answer: ✅ C) Discovery of Subatomic Particles
Explanation:
Dalton assumed that atoms were indivisible. The later discovery of electrons, protons, and neutrons proved that atoms have internal structure and can be divided.
Q4. Which statement about Dalton’s Atomic Theory is now known to be incorrect?
A) Atoms of different elements have different masses
B) Atoms combine in simple whole-number ratios to form compounds
C) Atoms can neither be created nor destroyed in a chemical reaction
D) All atoms of a given element have identical mass and properties
Answer: ✅ D) All atoms of a given element have identical mass and properties
Explanation:
This was disproved by the discovery of isotopes, which are atoms of the same element with different masses (e.g., Protium, Deuterium, Tritium in Hydrogen).
Q5. Dalton’s Atomic Theory could not explain which of the following?
A) Law of Conservation of Mass
B) Law of Definite Proportion
C) Law of Gaseous Volumes (Gay-Lussac’s Law)
D) Law of Multiple Proportions
Answer: ✅ C) Law of Gaseous Volumes (Gay-Lussac’s Law)
Explanation:
Dalton’s theory failed to explain why gases combine in simple volume ratios, like 2 volumes of hydrogen + 1 volume of oxygen = 2 volumes of water vapor. This was later explained by Avogadro’s Hypothesis.
💡 Do You Know?
- ⚛️ The word “atom” comes from Greek “atomos”, meaning indivisible.
- ⚛️ Isotopes and subatomic particles proved that atoms are not indivisible.
- ⚛️ Dalton also contributed to color blindness research, which is sometimes called Daltonism.
📝 Exam Corner
Important for:
- CBSE Class 11 Chemistry – Chapter 1: Some Basic Concepts of Chemistry
- JEE Main/Advanced – Conceptual & MCQ Questions
- NEET – Understanding the structure of matter and basic laws
