ANAND CLASSES Study Material and Notes to learn about the origin of atomic theory, detailed explanation of Dalton’s atomic theory, its postulates, and limitations. Ideal for Class 11 Chemistry, JEE, and NEET preparation.
📘 The Concept and Evolution of the Atom
🔹 1. What was the earliest idea about the structure of matter?
Answer:
The earliest concept about the structure of matter was proposed around 400 B.C. by Indian and Greek philosophers. They suggested that if we keep dividing matter into smaller and smaller pieces, a stage will come when we reach the smallest indivisible particle, which they called an atom.
- Indian philosopher Maharshi Kanad named this particle ‘Paramanu’.
- Greek philosopher Democritus used the term ‘atomos’, meaning indivisible.
🔹 2. What is the meaning of the word ‘atom’?
Answer:
The word ‘atom’ is derived from the Greek word ‘a-tomio’:
- ‘a’ means not.
- ‘tomio’ means cuttable or divisible.
Hence, ‘atomos’ means “uncuttable” or “indivisible”.
It reflects the ancient belief that atoms are the smallest particles of matter that cannot be further divided.
🔹 3. Were the early ideas about atoms based on scientific evidence?
Answer:
No. The early atomic ideas were philosophical and not based on experimental evidence. They were speculative in nature. Due to the lack of scientific tools and methods, these ideas could not be tested or verified experimentally at that time.
🔹 4. Who gave the first scientific theory of the atom and when?
Answer:
The first scientific atomic theory was proposed by John Dalton, a British school teacher and scientist, in the year 1808. His theory laid the foundation for modern chemistry and gave a scientific explanation of matter in terms of atoms.
🔹 5. What are the main postulates of Dalton’s Atomic Theory?
Answer:
Dalton’s Atomic Theory consists of the following main postulates:
- All matter is made up of tiny indivisible particles called atoms.
- Atoms of the same element are identical in mass and properties.
- Atoms of different elements are different in mass and properties.
- Atoms cannot be created or destroyed in a chemical reaction.
- Atoms combine in simple whole-number ratios to form compounds.
- In a chemical reaction, atoms are rearranged, but not created or destroyed.
🔹 6. How did Dalton’s Atomic Theory support the Laws of Chemical Combination?
Answer:
Dalton’s Atomic Theory provided explanations for the following laws:
- Law of Conservation of Mass: Since atoms are indestructible, mass remains conserved in a chemical reaction.
- Law of Constant Proportion: Compounds are made up of atoms in fixed ratios, so their composition remains constant.
- Law of Multiple Proportions: When elements combine in more than one ratio, the different compounds formed have simple ratio relationships.
🔹 7. What were the limitations of Dalton’s Atomic Theory?
Answer:
Dalton’s Atomic Theory had several limitations:
- It could not explain the existence of subatomic particles like electrons, protons, and neutrons.
- It assumed all atoms of an element are identical, but later isotopes were discovered (atoms of the same element with different masses).
- It did not explain the nature of bonding between atoms.
- It failed to explain the electrical nature of matter, such as how rubbing glass or ebonite with silk generates electric charges.
🔹 8. How was Dalton’s view of the atom later modified?
Answer:
Scientific experiments conducted during the late 19th and early 20th centuries led to the discovery that:
- Atoms are not indivisible.
- They are composed of sub-atomic particles like:
- Electrons (discovered by J.J. Thomson)
- Protons (discovered by Goldstein and named by Rutherford)
- Neutrons (discovered by James Chadwick)
This marked a shift from Dalton’s indivisible atom to a complex structure of the atom.
🧪 MCQs with Explanations: Dalton’s Atomic Theory & Origin of Atom
Q1. Who was the first to propose the idea of indivisible particles called atoms?
A. Aristotle
B. John Dalton
C. Democritus
D. Rutherford
✅ Correct Answer: C. Democritus
🧠 Explanation:
Democritus, a Greek philosopher, proposed that all matter is composed of tiny indivisible particles called “atomos”. This idea was purely philosophical and not based on experimental evidence.
Q2. The term “atom” is derived from the Greek word ‘atomos’. What does it mean?
A. Divisible
B. Invisible
C. Uncuttable or Indivisible
D. Particle
✅ Correct Answer: C. Uncuttable or Indivisible
🧠 Explanation:
The word ‘atom’ comes from the Greek word ‘atomos’ where ‘a’ means “not” and ‘tomos’ means “cuttable”. Hence, it literally means ‘indivisible’ or ‘uncuttable’.
Q3. Dalton’s atomic theory was proposed in which year?
A. 1789
B. 1808
C. 1897
D. 1911
✅ Correct Answer: B. 1808
🧠 Explanation:
John Dalton proposed his atomic theory in 1808, laying the foundation for modern chemistry. It gave a scientific explanation of the laws of chemical combination.
Q4. According to Dalton’s atomic theory, atoms:
A. Can be created and destroyed in chemical reactions
B. Are different for each compound
C. Combine in any ratio to form compounds
D. Combine in simple whole number ratios
✅ Correct Answer: D. Combine in simple whole number ratios
🧠 Explanation:
Dalton stated that atoms of different elements combine in simple whole number ratios to form compounds, e.g., H₂O is formed by a 2:1 ratio of hydrogen to oxygen.
Q5. Which of the following was a limitation of Dalton’s atomic theory?
A. It explained the laws of chemical combination
B. It proposed atoms are indivisible
C. It accounted for the electrical nature of matter
D. It considered all atoms of an element to be identical
✅ Correct Answer: C. It accounted for the electrical nature of matter
🧠 Explanation:
Dalton’s theory did not explain how materials get electrically charged or the nature of subatomic particles. This was a major limitation, especially after discoveries in electricity and magnetism.
Q6. Which law is not explained by Dalton’s Atomic Theory?
A. Law of Conservation of Mass
B. Law of Constant Proportion
C. Law of Multiple Proportions
D. Law of Electrolysis
✅ Correct Answer: D. Law of Electrolysis
🧠 Explanation:
Dalton’s atomic theory could explain basic chemical laws but failed to explain phenomena related to electricity, like electrolysis, which involves the movement of charged particles (ions or electrons).
Q7. The discovery of which particle proved that atoms are divisible?
A. Proton
B. Neutron
C. Electron
D. All of the above
✅ Correct Answer: D. All of the above
🧠 Explanation:
The discoveries of:
- Electron (by J.J. Thomson, 1897),
- Proton (by Goldstein, 1886; named by Rutherford),
- Neutron (by James Chadwick, 1932)
proved that atoms consist of smaller subatomic particles, thus atoms are divisible.
🧠 Do You Know?
- Dalton’s Atomic Theory is still foundational in chemistry, even though it was later modified.
- The modern atomic theory is based on quantum mechanics and particle physics, which explain atomic behavior in much greater detail.
🧪 Worksheet – Dalton’s Atomic Theory & Origin of the Atom
📚 Subject: Chemistry
🎯 Class: 11 | JEE | NEET
📝 Topic: Early Atomic Models & Dalton’s Atomic Theory
👨🏫 Name: _____________________ 📅 Date: _______________
🔢 Total Marks: 14
Section A: Multiple Choice Questions (1 mark each)
Instructions: Choose the correct option and write it in the space provided.
Q1. Who was the first to propose the idea of indivisible particles called atoms?
A. Aristotle B. John Dalton C. Democritus D. Rutherford
Answer: _____________
Q2. The term “atom” is derived from the Greek word ‘atomos’. What does it mean?
A. Divisible B. Invisible C. Uncuttable D. Particle
Answer: _____________
Q3. Dalton’s atomic theory was proposed in which year?
A. 1789 B. 1808 C. 1897 D. 1911
Answer: _____________
Q4. According to Dalton’s atomic theory, atoms:
A. Can be created and destroyed in chemical reactions
B. Are different for each compound
C. Combine in any ratio to form compounds
D. Combine in simple whole number ratios
Answer: _____________
Q5. Which of the following was a limitation of Dalton’s atomic theory?
A. It explained the laws of chemical combination
B. It proposed atoms are indivisible
C. It accounted for the electrical nature of matter
D. It considered all atoms of an element to be identical
Answer: _____________
Q6. Which law is not explained by Dalton’s Atomic Theory?
A. Law of Conservation of Mass
B. Law of Constant Proportion
C. Law of Multiple Proportions
D. Law of Electrolysis
Answer: _____________
Q7. The discovery of which particle proved that atoms are divisible?
A. Proton B. Neutron C. Electron D. All of the above
Answer: _____________
Section B: Short Answer Question (2 marks)
Q8. State any two limitations of Dalton’s atomic theory.
