Class 9 Science Chapter 9, Atomic Foundations of Matter, builds on the atom chapter: atoms gain, lose or share electrons to reach an octet, and the compounds they form behave very differently from their elements. Hydrogen burns and oxygen supports burning, yet water does neither. The chapter covers the laws of chemical combination, Dalton’s theory, covalent and ionic bonds, chemical formulae and molecular mass.
What Class 9 Science Chapter 9 Covers
9.1 to 9.3 Laws and Dalton’s Theory cover the law of conservation of mass, the law of constant proportions and Dalton’s atomic theory.
9.4 How Atoms Combine? explains covalent bonds (sharing electrons) and ionic bonds (transferring electrons).
9.5 and 9.6 Formulae and Properties show how to write formulae of covalent and ionic compounds and compare their properties.
9.7 and 9.8 Mass calculate the molecular mass of covalent compounds and the formula unit mass of ionic compounds.
The scientists behind the laws: Antoine Lavoisier, called the Father of Modern Chemistry, proposed the Law of Conservation of Mass in 1789: matter is neither gained nor lost in a reaction. Joseph Proust then showed, by studying copper carbonate, that the elements in a compound always combine in a fixed ratio by mass (the Law of Constant Proportions). The chapter also mentions hingula, known in English as cinnabar, a red mineral pigment used in ancient India for painting.
Exercises in Chapter 9
- Revise, Reflect, Refine problems on formulae, bonding and molecular mass.
Read Class 9 Science Chapter 9 Online
More Chapters
- Previous: Chapter 8: Journey Inside the Atom
- Next: Chapter 10: Sound Waves: Characteristics and Applications
- Hindi medium: Class 9 Science Chapter 9 PDF in Hindi (Anveshan)
- All Science chapters and the other Class 9 books: Class 9 NCERT books 2026-27
FAQ
What is the difference between molecular mass and formula unit mass?
Molecular mass is used for covalent compounds made of molecules; formula unit mass is used for ionic compounds, which have no separate molecules.
Is Dalton’s atomic theory in Class 9 Science?
Yes, in Chapter 9, section 9.3.
Who proposed the Law of Conservation of Mass?
Antoine Lavoisier, in 1789.