Dalton's Atomic Theory

Dalton's Atomic Theory was proposed by John Dalton in 1808. It is one of the most important theories in chemistry. The main postulates of Dalton's Atomic Theory are:

Main Postulates

1. All matter is made of very tiny indivisible particles called atoms.
2. Atoms of the same element are identical in all respects (mass, size, properties).
3. Atoms of different elements have different masses and properties.
4. Atoms cannot be created, destroyed, or divided in chemical reactions.
5. Atoms of different elements can combine in a fixed, simple whole-number ratio to form compounds.
6. Chemical reactions are rearrangement of atoms – atoms themselves remain unchanged.

Laws Explained by Dalton's Theory

• Law of Conservation of Mass (Lavoisier)
• Law of Definite Proportions (Proust)
• Law of Multiple Proportions (Dalton)

Limitations (Modern View)

• Atoms are divisible → protons, neutrons, electrons.
• Atoms of same element can have different masses → isotopes.
• Atoms of different elements can have the same mass → isobars.
• In nuclear reactions (fission or fusion), atoms can be created or destroyed where mass is converted into energy according to Einstein's equation E=mc².

Conclusion: Despite limitations, Dalton's Atomic Theory remains the foundation of chemistry and correctly explains most chemical phenomena.

🧠 Can you find law of multiple proportion and law of reciprocal proportion form Dalton's Atomic Theory Postulate?
Yes, both Law of Multiple Proportions and Law of Reciprocal Proportions can be directly derived from Dalton's Atomic Theory.
Dalton's relevant postulates:
1. Atoms of different elements have different masses.
2. Atoms combine in a fixed, simple whole-number ratio to form compounds.
3. A compound always contains the same elements combined in the same fixed ratio by mass.
1. Law of Multiple Proportions (derived from Postulate 2 & 3):
When two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in the ratio of small whole numbers.
Example: Carbon and oxygen form two compounds CO and CO₂.
In CO → 12 parts carbon combine with 16 parts oxygen → Ratio = 16 : 12 = 4 : 3
In CO₂ → 12 parts carbon combine with 32 parts oxygen → Ratio = 32 : 12 = 8 : 3
Masses of oxygen combining with fixed mass (12 g) of carbon = 16 g and 32 g
Ratio = 16 : 32 = 1 : 2 (simple whole-number ratio)
This is possible only because oxygen atoms combine with carbon in the ratio 1:1 in CO and 1:2 in CO₂ — exactly as per Dalton's postulate of simple whole-number combination.

2. Law of Reciprocal Proportions (derived from Postulate 1 & 2):
If two elements A and B combine separately with a fixed mass of a third element C, then the ratio in which A and B combine with each other is the same or a simple multiple of the ratio in which they combine with C.
Example: Hydrogen, oxygen and carbon form compounds as follows:
H₂ + O → H₂O → 2 g H combines with 16 g O → Ratio H : O = 2 : 16 = 1 : 8
C + O → CO → 12 g C combines with 16 g O → Ratio C : O = 12 : 16 = 3 : 4
Now H and C combine to form CH₄ → 12 g C combines with 4 g H → Ratio C : H = 12 : 4 = 3 : 1

Ratio in which H and C combine with each other (3 : 1) is the same as the ratio in which they combine with fixed mass of oxygen (3 : 4 and 1 : 8 → 3/4 : 1/8 = 3 : 1).

Conclusion: Both laws are natural consequences of Dalton's postulates that atoms have fixed masses and always combine in simple whole-number ratios. Thus, Dalton’s Atomic Theory successfully explains and predicts the Law of Multiple Proportions and Law of Reciprocal Proportions.

Test Your Knowledge

MCQs Asked in NEET, IIT-JEE, IIMS, AIPMT

NEET 2019

Q1: According to Dalton’s atomic theory, the smallest particle which is capable of independent existence is:

1. Atom
2. Molecule
3. Ion
4. Electron

Correct Answer: A

Dalton proposed that the atom is the smallest indivisible particle that can take part in chemical reactions and exist independently.

JEE Main 2021

Q2: Which of the following statements is NOT a postulate of Dalton’s atomic theory?

1. Atoms of the same element are identical in mass
2. Atoms combine in simple whole-number ratios
3. Atoms are indivisible particles
4. All isotopes of an element have the same mass

Correct Answer: D

The existence of isotopes was discovered later and contradicts Dalton’s postulate that all atoms of the same element are identical in mass.

NEET 2020

Q3: The law of conservation of mass was explained by:

1. Law of definite proportions
2. Dalton's atomic theory
3. Avogadro's law
4. Gay-Lussac's law

Correct Answer: B

Dalton explained the law of conservation of mass by stating that atoms are neither created nor destroyed in a chemical reaction.

NEET 2022

Q4: Which of the following is a limitation of Dalton’s atomic theory?

1. It failed to explain the existence of allotropes
2. It could not explain Gay-Lussac’s law
3. It explained the law of multiple proportions
4. It explained the law of conservation of mass

Correct Answer: A

Dalton’s theory assumed all atoms of an element are identical, so it could not explain why carbon exists as diamond and graphite (allotropes).

BSEB 2024

Q5: Which scientist gave the first scientific atomic theory?

1. J.J. Thomson
2. Ernest Rutherford
3. John Dalton
4. Democritus

Correct Answer: C

John Dalton proposed the first scientific atomic theory in 1808 based on experimental laws.

NEET 2021

Q6: The postulate of Dalton’s theory which is still considered valid is:

1. Atoms are indivisible
2. All atoms of an element have same mass
3. Atoms of different elements combine in simple whole-number ratio
4. Atoms of same element can have different masses

Correct Answer: C

Even after discovery of sub-atomic particles and isotopes, atoms combine in simple whole-number ratios to form compounds.