Class 9th Full Atoms and Molecules Chapter Covered
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Course
Physics , Chemistry , Maths
Institution
Sixth Year / 12th Grade
I am selling my notes for Class 9th Atoms and Molecules . The Atoms and Molecules chapter in Class 9 Science covers the fundamental building blocks of matter - atoms and molecules. It explains that atoms are the smallest units of an element that exhibit its properties, while molecules are formed...
"Structure of an Atom | Part 1/1 | English | Class 9"
Introduction
● Topic: Structure of Atom
● Purpose: Understanding why atoms of different elements differ and whether atoms are
indivisible.
Basic Concept
● Atoms and Matter: Explanation that substances exist due to the atoms they are made
from.
Activity Demonstration
● Comb and Paper: Demonstration showing how a comb attracts small pieces of paper
after being rubbed on dry hair, introducing the concept of electrical charge.
Historical Discoveries
● 19th Century Discoveries:
○ Atoms are not indivisible; they contain subatomic particles.
○ Electrons: Discovered first, negatively charged.
○ Protons: Discovered by E. Goldstein in 1886, positively charged, with a mass
significantly greater than that of electrons.
Atomic Models
1. Thompson's Model:
○ Compared to a watermelon.
○ Structure: Positively charged sphere with embedded electrons (like seeds in a
watermelon).
○ Neutrality: Positive and negative charges cancel out, making the atom neutral.
○ Limitation: Could not explain results from other experiments.
2. Rutherford's Model:
○ Experiment: Alpha particles scattering experiment.
○ Observations:
■ Most alpha particles passed through gold foil.
■ Some were deflected at small angles.
■ A few rebounded.
○ Conclusions:
■ Atoms are mostly empty space.
■ Positive charge and mass are concentrated in a small nucleus.
, ■ Electrons revolve around the nucleus.
○ Limitation: Did not explain why electrons do not fall into the nucleus.
Bohr's Model
● Postulates:
○ Electrons revolve in certain discrete orbits without radiating energy.
○ These orbits are designated as K, L, M, N, etc.
Discovery of Neutrons
● J. Chadwick (1932):
○ Discovered neutrons, which are neutral and have a mass similar to protons.
○ Neutrons are found in the nucleus of all atoms except hydrogen.
Molecular Mass and Mole Concept Study Notes
Introduction
● Topic: Molecular Mass and Mole Concept
● Objective: Understanding molecular mass and the mole concept in chemistry.
Molecular Mass
● Definition: The molecular mass of a substance is the sum of the atomic masses of all
the atoms in a molecule of the substance.
● Unit: Expressed in atomic mass unit (u).
Example: Calculating Molecular Mass
● Water (H₂O):
○ Atomic mass of Hydrogen (H): 1u
○ Atomic mass of Oxygen (O): 16u
○ Calculation: 2(1u) + 1(16u) = 18u
● Sulfuric Acid (H₂SO₄):
○ Calculation: 2(1u) + 1(32u) + 4(16u) = 98u
Formula Unit Mass
● Definition: The formula unit mass of a substance is the sum of the atomic masses of all
its constituent ions.
● Example: Zinc Oxide (ZnO):
, ○ Zinc ion (Zn²⁺) and Oxide ion (O²⁻)
○ Calculation: Sum of masses of Zn and O ions
● Definition: A mole of a species (atom, molecule, ion, or particle) is the quantity in
number having a mass equal to its atomic or molecular mass in grams.
● Avogadro's Number: 6.022 × 10²³ (Avogadro constant, N₀)
Explanation and Examples
● Comparison: Like a dozen equals 12 objects, one mole equals 6.022 × 10²³ entities.
● Mass and Number Relationship: The mass of one mole of a substance is equal to its
relative atomic or molecular mass in grams.
Atomic and Molar Mass
● Atomic Mass: Mass of one atom of an element in atomic mass units (u).
● Molar Mass: Mass of one mole of atoms of an element in grams (g).
Example: Oxygen
● Atomic Mass of Oxygen: 16u
● Gram Atomic Mass: 16 grams
● Number of Atoms in 16 grams of Oxygen: 6.022 × 10²³ oxygen atoms
Practical Applications
● Chemists’ Need: Relating masses with numbers in grams for conducting reactions.
● Formula for Number of Moles:
○ Number of Moles (n) = Given Mass (m) / Molar Mass (M)
Solving Problems
Example 1: Number of Moles by Mass
● Given: 46 grams of Sodium (Na)
● Atomic Mass of Sodium: 23u
● Calculation:
○ Molar Mass of Sodium = 23 grams
○ Number of Moles = 46 grams / 23 grams = 2 moles
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