Class 9 Science Chapter 9 Atomic Foundations of Matter – Revision Notes

Quick revision notes for Class 9 Science Chapter 9 (Atomic Foundations of Matter). These Class 9 Science Chapter 9 notes are ideal for quick revision just before exams.

Laws of Chemical Combination

• Conservation of Mass: total mass of reactants = total mass of products.
• Constant Proportions: a compound always has elements in a fixed mass ratio.

Dalton’s Atomic Theory

• Matter = indivisible atoms; same element = identical atoms; different elements = different atoms; atoms combine in whole-number ratios; atoms not created/destroyed in reactions.

Bonding

• Ionic: electron transfer, charged ions, electrostatic attraction (e.g. NaCl).
• Covalent: electron sharing (e.g. H2O, CO2).

Chemical Formulae

• Criss-cross method: valency of one element = subscript of the other.
• Molecular mass: sum of atomic masses in a molecule.
• Formula unit mass: sum of atomic masses in simplest ratio (ionic compounds).

One-Line Summary

The laws of conservation of mass and constant proportions led to Dalton’s atomic theory, which explains how atoms combine (ionically or covalently) in fixed ratios determined by valency to form compounds with calculable molecular or formula unit mass.

Written by Satish

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