Ace Chemistry
Struggling to keep up with Chemistry? This Comprehensive Class 9 Chemistry Notes Package is your perfect study companion! Covering the critical lessons Structure of Atoms — these notes provide clear, concise explanations to help you grasp even the toughest concepts.
What sets...
CHEMISTRY
CHAPTER - 03
ATOMS AND MOLECULES
Laws of Chemical Combination:
1. Law of conservation of mass (Antoine Lavoisier)
The law of conservation of mass states that mass can neither be created nor be destroyed in a chemical reaction.
The law of conservation of mass means that in a chemical reaction, the total mass of products is equal to the total
mass of reactants. There is no change in the mass during a chemical reaction.
Suppose a chemical reaction takes place between substances A and B and if the products formed are C and D as:
If ‘a’ g of A and ‘b’ g of B react to produce ‘c’ g if C and ‘d’ g of D. Then according to law of conservation of
mass, we have
a + b = c + d.
2. Law of constant proportions (Joseph L. Proust.)
“In a chemical substance, the elements are always present in definite proportions by their mass”. The chemical
composition of pure substance is not dependent on the source from which it is obtained.
3. Dalton’s atomic theory
● All matter is made of very tiny particles called atoms, which participate in chemical reactions.
● 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.
4. Atom: Atoms are defined as “the basic building blocks of matter”.
Atomic radius is measured in nanometre.
1nm = 10-9m
5. Dalton’s atomic symbols:
Dalton was the first scientist to use the symbols for elements in a very specific sense.
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