9th Class Science Notes in English chapter 3 Atoms and Molecules
CBSE Revision Notes for CBSE Class 09 Science Atoms and Molecules Atoms and Molecules: Particle nature, basic units : Atoms and molecules. Law of constant proportions. Atomic and molecular masses for class 09 Science. It includes all main concepts of the chapter in CBSE class 09 Science.
Class 9th Science chapter 3 Atoms and Molecules Notes in English.
✔ TOPICS IN THIS CHAPTER
🔹 Laws of Chemical Combination
▶ Law of Conservation of Mass
▶ Law of Constant Proportions
🔹 Dalton's Atomic Theory
▶ Modern day symbols of Elements
▶ Atomic Mass
▶ Atomic Mass Unit
▶ Atom existence
🔹 Chemical Formulae
▶ Characteristics of chemical formulae
▶ Rules for writing chemical formulae
🔹 Molecular Mass
▶ Formula Unit Mass
🔹 Molar Mass
🔹 Around 500 B.C., Indian philosopher Maharishi Kanad, postulated the theory if we go on dividing matter (padarth), we will obtain smallest particle beyond which further division can't be possible which is known as 'parmanu'.
🔹 Ancient Greek philosophers – Democritus and Leucippus called these particles atoms.
🔹 Antoine L. Lavoisier said the foundation of chemical sciences by establishing two important laws of chemical combination.
✔ Laws of Chemical Combination:-
🔹 This law established after the experiments by Lavoisier and Joseph L. Proust.
🔹 The process of combination of two or more elements to form new compounds is governed by certain laws called laws of chemical combination. These are:
1. Law of conservation of mass.
2. Law of constant proportions.
1. Law of Conservation of Mass (by Lavoisier in 1744):-
🔹 This law states that Mass can neither be created nor destroyed in a chemical reaction.
"During a chemical reaction, the total mass of reactants will be equal to the total mass of the products".
▶ Example:- A (Reactant) + B (Reactant) = AB (Product)
mass of A + mass of B = mass of AB
2. Law of constant proportions (by Proust in 1797):-
🔹This law states that in a chemical substance the elements are always present in definite proportions by mass.
▶ For Example:- 18 gm of H2O = 2 gm of hydrogen + 16 gm of oxygen
🔹mass of hydrogen/mass of oxygen = 2/16 = 1/8
(The ratio of hydrogen and oxygen in pure water is always 1: 8 by weight.)
- This law is also called law of definite proportions or law of constant proportions.
✔ Dalton’s Atomic Theory :-
🔹According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms.
▶ Six Postulates of Dalton's atomic theory:-
🔹 (I) All matter is made of very tiny particles called atoms.
🔹 (II) Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
🔹 (III) Atoms of a given element are identical in mass and chemical properties. (Law of conservation of mass)
🔹 (IV) Atoms of different elements have different masses and chemical properties.
🔹 (V) Atoms combine in the ratio of small whole numbers to form compounds. (Law of constant proportion)
🔹 (VI) The relative number and kinds of atoms are constant in a given compound.
✔ Atoms :-
🔹According to modern atomic theory, an atom is the smallest particle of an element which takes part in chemical reaction.
🔹Atoms are very small and hence can't be seen even through very powerful microscope.
🔹Atomic radius is measured in nanometres = 10⁻⁹ m
▶ Modern day symbols of Elements :-
🔹Dalton was the first scientist to use the symbols for elements.
🔹Berzilius suggested that the symbols of elements should be made from one or two letters of the name of the element.
🔹The name copper was taken from Cyprus, a place from where it was found for first time.
🔹Now, IUPAC (International Union of Pure and Applied Chemistry) approves names of elements.
🔹The first letter of a symbol is always written as a capital letter (uppercase) and the second letter as a small letter (lowercase). For example: hydrogen (H), aluminium (Al), cobalt (co).
🔹Some other symbols have been taken from the names of elements in Latin, German or Greek.
▶ For Example:- Fe from its Latin name ferrum, sodium is Na from natrium, potassium is K from kalium.