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Chapter 3 - Atoms and Molecules
Clean typed notes from the handwritten pages
1. Early ideas about atoms
Indian philosopher: Maharishi Kanad said that the smallest particle is called Parmanu.
An object can be broken into smaller and smaller pieces. At one stage, the smallest part is called an atom.
Ancient Greek philosophers: Democritus and Leucippus changed the name Parmanu to atom.
2. Laws of chemical combination
Antoine Lavoisier gave two laws of chemical combination:
- Law of conservation of mass
- Law of constant proportions
Law of conservation of mass: During a chemical reaction, the total mass of reactants is equal to the total
mass of products.
Mass can neither be created nor destroyed in a chemical reaction.
Equation: Total mass of reactants = Total mass of products.
Law of constant proportions: By Joseph Proust. The elements in a pure chemical compound are always
present in the same proportion by mass, regardless of how the compound is prepared.
In a compound, atoms combine in a fixed proportion.
Example of water:
Sample Mass of hydrogen Mass of oxygen Ratio (H:O)
18 g of water 2 g 16 g 1:8
36 g of water 4 g 32 g 1:8
In water, the mass ratio of hydrogen to oxygen is always 1:8.
3. Dalton's atomic theory
According to Dalton's atomic theory, all matter, whether an element, a compound, or a mixture, is
composed of very small particles called atoms.
Postulates of Dalton's atomic theory:
- All matter is made of very tiny particles called atoms.
- Atoms are indivisible particles and cannot be created or destroyed in a chemical reaction.
- Atoms of a given element are identical in mass and chemical properties.
- Atoms of different elements have different masses and chemical properties.
- Atoms combine in the ratio of small whole numbers to form compounds.
- The relative number and kinds of atoms are constant in a given compound.
Drawbacks of Dalton's atomic theory:
- It did not explain subatomic particles such as electrons, protons, and neutrons.
- It did not explain isotopes, because atoms of the same element can have different masses.
- It did not explain isobars, because atoms of different elements can have the same mass number.
