CBSE Notes Class 9 Science Chapter 9 Atomic foundations of Matter Notes
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Chapter 9, “Atomic Foundations of Matter”, covers atoms, molecules, atomicity, chemical formulae, and laws of chemical combinations. Students can learn how matter is formed, how atoms combine, and how chemical reactions are represented using symbols and formulas.
1.0Download CBSE Notes in Class 9 Atomic foundations of Matter Notes
Download CBSE Class 9 Science Chapter 9: Atomic Foundations of Matter Notes to understand the basic concepts of atoms, molecules, and chemical combinations.
2.0Important Concepts of Class 9 Science Chapter 9 Atomic foundations of Matter
Ionic Compounds
- Ionic compounds: formed by transfer of electrons from one atom to another, resulting in oppositely charged ions.
- Metals lose electrons → form positive ions (cations).
- Non-metals gain electrons → form negative ions (anions).
- Oppositely charged ions attract each other via strong electrostatic forces called ionic bonds.
- Examples: NaCl, MgCl₂
Ionic (Electrovalent) Bond
- The electrostatic force of attraction holding oppositely charged ions together.
- Formed by complete transfer of electrons between two atoms so each attains a stable octet (nearest noble gas configuration).
- Electrovalency: the number of electrons lost or gained by an atom.
Formation of MgCl₂ (Example)
- Mg has 2 valence electrons → loses both → Mg²⁺ (2, 8) — attains Ne configuration.
- Each Cl needs 1 electron → gains 1 → Cl⁻ (2, 8, 8) — attains Ar configuration.
- Mg transfers 1 electron to each of 2 Cl atoms → MgCl₂.
Common Ionic Compounds (Formation Pattern)
Properties of Ionic Compounds
- Naming rule: name of cation + name of anion (e.g., Al₂(NO₃)₃ = Aluminium nitrate).
Covalent Compounds
- Formed when two non-metal atoms share electron pairs to attain stable electronic configuration (instead of transferring).
- Examples: H₂, O₂, N₂, HCl, CO₂
Covalent Bond
- A bond formed by the mutual sharing of electrons between two atoms.
Necessary Conditions for Covalent Bond Formation
- Formed between non-metal atoms (or non-metal + metalloid).
- Atoms have similar electronegativity.
- Combining atoms need 1, 2, or 3 electrons to complete octet (hydrogen needs 1 to complete duplet).
- Combining atoms need 1, 2, 3, or 4 electrons to complete octet.
- Both atoms contribute equal number of electrons to the shared pair.
- After sharing, each atom attains stable configuration like the nearest noble gas.
Classification of Covalent Bonds
Formation of Some Covalent Compounds
Properties of Covalent Compounds
Note: Diamond and Graphite (covalent network solids) have very high melting points and hardness due to large interconnected networks.
Ionic vs Covalent — Quick Comparison
3.0Detailed CBSE Notes Class 9 Science Atomic foundations of Matter
Other Study Materials for Class 9 Science
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4.0Why CBSE Notes for Class 9 Science Chapter 9 Atomic foundations of Matter
- Cover the laws of conservation of mass and constant proportions with simple explanations and examples.
- Explain Dalton's Atomic Theory and its role in understanding the structure of matter.
- Clarify the differences between atoms, molecules, elements, compounds, and ions.
- Simplify the formation of cations and anions and their significance in chemical bonding.
- Explain ionic and covalent compounds, including how they are formed and their basic characteristics.
- Provide easy methods to write chemical formulae using valency and ionic charges.
- Include shortcuts for calculating molecular mass and formula unit mass with solved examples.
- Summarise the relationship between atoms, molecules, and chemical formulae to strengthen conceptual understanding.