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

Atomic Foundations of Matter sets out the laws of chemical combination and Dalton’s atomic theory before turning to how ionic and covalent bonds form, and this page walks through each idea in order.

Last Updated: September 23, 2026

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.

Frequently Asked Questions

What are isotopes, and why do they have the same chemical properties?
Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons; since chemical properties depend on the number of electrons and protons, isotopes of an element behave identically in chemical reactions.

How is the atomic number different from the mass number of an atom?
The atomic number is the number of protons in the nucleus of an atom and defines the element, while the mass number is the total number of protons and neutrons combined, representing the total mass of the atomic nucleus.

Written by Satish

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