CBSE Class 9 Chemistry Atoms & Molecules Notes

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CBSE Class 9 Chemistry Atoms & Molecules Notes - Simple to learn and understand concepts for CBSE students by the best teachers in Delhi. 

Chapter notes

Class: IX

Chapter Name: Atoms and molecules

Top concepts

1. Laws of chemical combination

    Law of conservation of mass: Mass can neither be created nor destroyed in a chemical reaction

    Law of constant proportion or Law of definite proportion: In a chemical substance the elements are always present in definite proportions by mass

2. Postulates of Dalton’s atomic theory

   All matter is made of very tiny particles called atoms

   Atoms are indivisible particles, which 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

3. Some Limitations of Dalton’s atomic theory  According to Dalton’s atomic theory atoms of a given element are identical in mass and chemical properties. Atoms of different elements have different masses and chemical properties. This is wrong because atoms of some elements vary in their mass and density. Such atoms of the same element having different masses are called isotopes. For example, chlorine has two isotopes having mass numbers 35 a.m.u and 37 a.m.u.  According to Dalton’s atomic theory atoms of different elements have different masses and chemical properties. This has been proved wrong in certain cases like argon and calcium atoms, which have the same atomic mass of 40. Such atoms of different elements that have the same atomic mass are called isobar.  According to Dalton’s atomic theory atoms combine in the ratio of small whole numbers to form compounds. This is not seen in complex  organic compounds like sugar C12H22O11. Ratio of C, H and O is 12:22:11 which is not simple.

4. Atoms are building block of all matter

5. Atomic radius is measured in nanometres (1 m = 109 nm)

6. Individual atoms can be observed using Scanning Tunnelling Microscopes

7. Each element has a name and a unique symbol

8. Rules for assigning symbols for atoms of various elements a. The abbreviation used to represent an element


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