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Chapter-9-Atomic-Foundations-of-Matter .pdf/Prepared by K Sandeep Swamy (M.Sc, B.Ed) For ONLINE Classes (ACADEMICS,IITJEE & NEET ) Contact 9491878325 Subscribe samyans Eduhub youtube channel

Prepared by K Sandeep Swamy (M.Sc, B.Ed) For ONLINE Classes (ACADEMIC,IITJEE & NEET ) Contact 9491878325 Subscribe samyans Eduhub youtube channelGrade 9 · Curiosity Chapter 9: Atomic Foundations of Matter In this chapter, we explore the fundamental laws that govern how matter behaves during physical and chemical changes — and discover how atoms combine to form the substances all around us. Prepared by K Sandeep Swamy (M.Sc, B.Ed) For ONLINE Classes (ACADEMIC,IITJEE & NEET ) Contact 9491878325 Subscribe samyans Eduhub youtube channel Think It Over Before we dive in, consider these thought-provoking questions. They connect to key ideas you'll explore throughout this chapter. Water from Different Sources Water can be obtained from rivers, taps, rain, and wells. Are all these samples of water chemically identical — do they have the same composition? O vs. O₂ Oxygen is sometimes written as O and sometimes as O₂. What is the difference between a single atom and a molecule? Salt vs. Sugar in Water Dissolved salt in water conducts electricity, but sugar does not — even though both dissolve. Why does this happen? Connecting to What You Already Know In Chapter 8, you explored the structure of the atom — electrons, protons, and neutrons — and learned that atoms achieve stability by completing their valence shell (an octet). You also discovered that when elements combine into a compound, the compound's properties are entirely different from those of the original elements. Hydrogen gas is combustible and oxygen supports combustion — yet when they combine, they form water, which extinguishes fire. The properties of a compound are completely new! One striking observation, however, is that the mass of water formed always equals the sum of the masses of hydrogen and oxygen that combined. This raises a powerful question: is mass always conserved during changes? Activity 9.1 Investigating a Physical Change What You Do Place a clean, dry 100 mL beaker on a digital balance and tare to zero. Pour ~50 mL of water into the beaker. Add a spatula of common salt to the water. Record the mass before and after swirling until dissolved. What You Observe The mass of the salt-water solution equals the sum of the masses of the water and salt taken separately. Mass does not change during this physical change. You can repeat this by weighing a piece of paper before and after tearing it — the mass stays the same! Activity 9.2 Investigating a Chemical Change When baking soda is added to vinegar, carbon dioxide gas is produced. The reaction is: Vinegar + Baking soda → Carbon dioxide + Other substances Set-up 1 — Open System Baking soda is poured directly from a balloon into vinegar placed in a flask on a balance. The initial and final readings do not match — the CO₂ gas escapes into the air, reducing the recorded mass. Set-up 2 — Closed System The balloon is sealed to the flask mouth before the reaction. When the baking soda is tipped in, the

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GRADE 9 · CURIOSITY Chapter 9: Atomic Foundations of Matter In this chapter, we explore the fundamental laws that govern how matter behaves during physical and chemical changes — and discover how atoms combine to form the substances all around us. Prepared by K Sandeep Swamy (M.Sc, B.Ed) For ONLINE Classes (ACADEMIC,IITJEE & NEET ) Contact 9491878325 Subscribe samyans Eduhub youtube channel
Think It Over 🤔 Before we dive in, consider these thought-provoking questions. They connect to key ideas you'll explore throughout this chapter. Water from Different Sources Water can be obtained from rivers, taps, rain, and wells. Are all these samples of water chemically identical — do they have the same composition? O vs. O₂ Oxygen is sometimes written as O and sometimes as O₂. What is the difference between a single atom and a molecule? Salt vs. Sugar in Water Dissolved salt in water conducts electricity, but sugar does not — even though both dissolve. Why does this happen?
Connecting to What You Already Know In Chapter 8, you explored the structure of the atom — electrons, protons, and neutrons — and learned that atoms achieve stability by completing their valence shell (an octet). You also discovered that when elements combine into a compound, the compound's properties are entirely different from those of the original elements. Hydrogen gas is combustible and oxygen supports combustion — yet when they combine, they form water, which extinguishes fire. The properties of a compound are completely new! One striking observation, however, is that the mass of water formed always equals the sum of the masses of hydrogen and oxygen that combined. This raises a powerful question: is mass always conserved during changes?
ACTIVITY 9.1 Investigating a Physical Change What You Do Place a clean, dry 100 mL beaker on a digital balance and tare to zero. 1. Pour ~50 mL of water into the beaker. 2. Add a spatula of common salt to the water. 3. Record the mass before and after swirling until dissolved. 4. What You Observe The mass of the salt-water solution equals the sum of the masses of the water and salt taken separately. Mass does not change during this physical change. You can repeat this by weighing a piece of paper before and after tearing it — the mass stays the same!
ACTIVITY 9.2 Investigating a Chemical Change When baking soda is added to vinegar, carbon dioxide gas is produced. The reaction is: Vinegar + Baking soda →Carbon dioxide + Other substances Set-up 1 — Open System Baking soda is poured directly from a balloon into vinegar placed in a flask on a balance. The initial and final readings do not match — the CO₂ gas escapes into the air, reducing the recorded mass. Set-up 2 — Closed System The balloon is sealed to the flask mouth before the reaction. When the baking soda is tipped in, the CO₂ inflates the balloon — it stays in the system. Now the initial and final readings match perfectly. Key lesson: Always keep the system closed when testing conservation of mass. If gas escapes, the reading will appear to decrease — but mass is still conserved!
SECTION 9.1 The Law of Conservation of Mass The Law Matter can neither be created nor destroyed in a chemical reaction. The total mass of reactants always equals the total mass of products. Proposed By Antoine Lavoisier in 1789 — the Father of Modern Chemistry — who stated: "In every operation, an equal quantity of matter exists both before and after." Applies To Every chemical reaction, without exception — whether producing a gas, a precipitate, or a solution.
Antoine Lavoisier Father of Modern Chemistry Meet the Scientist Antoine Lavoisier transformed chemistry from a collection of observations into a rigorous science. Through careful, quantitative experiments, he demonstrated that mass is always conserved — even when substances appear to disappear (like burning wood or dissolving metals in acid). His insistence on precise measurement set the standard for modern experimental science. Activity 9.3 further verifies this law: when sodium sulfate and barium chloride solutions are mixed, a white precipitate of barium sulfate forms — yet the total mass before and after mixing remains the same (≈ 20.217 g vs. 20.218 g, within experimental error).
Worked Examples: Conservation of Mass Example 9.1 — Calcium Carbonate + HCl Reactants: 4.0 g CaCO₃ + 2.92 g HCl = 6.92 g total Products: 1.76 g CO₂ + 0.72 g H₂O + 4.44 g CaCl₂ = 6.92 g total ✓Law of Conservation of Mass is obeyed. Example 9.2 — Carbon + Oxygen 12 g of carbon + 32 g of oxygen →44 g of carbon dioxide. How much CO₂ from 2.4 g of carbon? ​ × 12 44 2.4 = 8.8 g of CO ​ 2 Answer: 8.8 g of carbon dioxide is produced. Pause and Ponder: A student burns 10 g of ethanol in an open beaker. No residue remains. Does this violate the Law of Conservation of Mass? (Hint: think about what happens to the gases!)
SECTION 9.2 The Law of Constant Proportions Soon after Lavoisier, the French chemist Joseph Louis Proust made another landmark discovery: in any pure compound, the elements are always present in the same fixed ratio by mass, regardless of the source of the compound. Water — Always 1:8 Whether water comes from a river, a borewell, or the ocean, purified water always contains hydrogen and oxygen in a mass ratio of 1:8. Decomposing 9 g of water always yields 1 g of H and 8 g of O. Cinnabar — Always 86.22 : 13.78 The red mineral cinnabar (mercury sulfide) always contains mercury and sulfur in the same mass percentages — no matter where it is found or how it is made. Also Known As This principle is called the Law of Definite Proportions or Proust's Law. It holds for all pure compounds, but not for mixtures.
Joseph Louis Proust French Chemist, 175431826 Meet the Scientist Proust's careful experiments with copper carbonate showed that — no matter how the compound was prepared or where it was found — it always contained copper, carbon, and oxygen in the same proportions by mass. This was a revolutionary idea: the composition of a pure compound is a fixed property of that compound. Threads of Curiosity: Cinnabar, known as hingula in ancient India, was used as a red pigment. Civilizations around the world discovered independently that heating it yields mercury and sulfur in the ratio 86.22% : 13.78% 4 a perfect demonstration of Proust's Law across cultures and centuries.
SECTION 9.3 Dalton's Atomic Theory John Dalton Born in England, Dalton presented his atomic theory in 1808 — a turning point in the study of matter. 01 All matter is made of tiny atoms Atoms are the smallest particles that participate in chemical reactions. 02 Atoms are indivisible They cannot be created or destroyed in a chemical reaction — only rearranged. 03 Atoms of the same element are identical All atoms of a given element share the same mass and chemical properties. 04 Atoms combine in whole-number ratios Compounds form when atoms combine in simple, fixed whole-number ratios — explaining the Law of Constant Proportions.
SECTION 9.4 How Atoms Combine Atoms combine to form molecules — electrically neutral entities consisting of more than one atom that can exist independently and show all the properties of that substance. Atoms bond to achieve a stable valence shell (8 electrons, or 2 for the K-shell). Sharing Electrons Atoms share some or all of their valence electrons with another atom. This forms a covalent bond. The energy of the system decreases, making the arrangement stable. Transferring Electrons One atom donates one or more valence electrons to another. The donor becomes a positive ion (cation) and the acceptor becomes a negative ion (anion). This forms an ionic bond.
SECTION 9.4.1 · COVALENT BONDS Bonding by Sharing: The Covalent Bond When two atoms each contribute one or more electrons to a shared pair, they form a covalent bond. The shared electrons attract both nuclei, holding the atoms together. Single Bond: H₂ and Cl₂ Each hydrogen atom has 1 electron and needs 1 more. Two H atoms share one pair →H–H. Similarly, each Cl has 7 valence electrons; sharing one pair gives Cl–Cl. Double Bond: O₂ Oxygen has 6 valence electrons and needs 2 more. Two O atoms share two pairs of electrons →O=O (a double bond).
Covalent Compounds: HCl and H₂O Atoms of different elements can also share electrons to form covalent compounds. The key is that each atom contributes electrons to complete both of their valence shells. Hydrogen Chloride (HCl) H needs 1 electron; Cl needs 1 electron. They share one pair →H– Cl. This is a single covalent bond. Water (H₂O) O needs 2 electrons; each H needs 1. Two H atoms each share one electron with O →H₂O with two single covalent bonds. Together We Are Strong: Just as atoms share electrons to form stable covalent bonds, sharing and caring with people around us builds strong, stable communities.
Naming Covalent Compounds Covalent compounds are named using a prefix system to indicate the number of atoms of each element. The first element keeps its name; the second ends in -ide. The prefix mono- is usually omitted for the first element. Formula Prefix(es) Name Note CO mono- Carbon monoxide Not "monooxide" CO¢ di- Carbon dioxide Not "monocarbon dioxide" CS¢ di- Carbon disulfide Two S atoms PCl£ tri- Phosphorus trichloride Three Cl atoms SF₆ hexa- Sulfur hexafluoride Six F atoms N¢O¤ di-, tetr- Dinitrogen tetroxide Not "tetraoxide" H¢O — Water Common name used NH£ — Ammonia Common name used When H is the first element, no prefix is added before hydrogen — e.g., H₂S is "hydrogen sulfide," not "dihydrogen sulfide."
SECTION 9.4.2 · IONIC BONDS Bonding by Transfer: The Ionic Bond Forming Na⁺ (Sodium Cation) Sodium (atomic number 11) has 1 valence electron. It loses this electron to become Na⁺ — 11 protons, 10 electrons, net charge +1. Forming Cl⁻ (Chloride Anion) Chlorine has 7 valence electrons. It gains one electron to become Cl⁻ — completing its octet with net charge −1. The oppositely charged Na⁺ and Cl⁻ ions attract each other. This electrostatic force of attraction is called an ionic bond. The result: sodium chloride (NaCl), common table salt.
The Crystal Structure of NaCl Ionic compounds don't exist as single pairs of ions — they form giant three-dimensional crystal lattices in which each ion is surrounded by oppositely charged ions. (a) NaCl crystals — the visible result of a perfectly ordered ionic lattice. (b) Space-filling model: each Na⁺ (grey) is surrounded by 6 Cl⁻ (green), and each Cl⁻ by 6 Na⁺. (c) Crystal lattice diagram — shows the repeating cubic pattern of ions as points in space. You will explore crystal structures in greater depth in higher grades. For now, remember that the strong forces throughout the lattice explain why ionic compounds have high melting and boiling points.
Naming Ionic Compounds & Common Ions In naming ionic compounds, write the cation name first, then the anion name. Simple anion names end in -ide. Polyatomic ion names generally do not end in -ide. Common Monoatomic Ions Ion Formula Valency Sodium Na⁺ 1 Potassium K⁺ 1 Calcium Ca²⁺ 2 Magnesium Mg²⁺ 2 Aluminium Al³⁺ 3 Iron (Ferrous) Fe²⁺ 2 Iron (Ferric) Fe³⁺ 3 Chloride Cl⁻ 1 Oxide O²⁻ 2 Sulfide S²⁻ 2 Common Polyatomic Ions Ion Formula Valency Hydroxide OH⁻ 1 Nitrate NO£⁻ 1 Hydrogencarbonate HCO£⁻ 1 Carbonate CO£²⁻ 2 Sulfate SO¤²⁻ 2 Ammonium NH¤⁺ 1 Polyatomic ions are groups of atoms that carry a charge together — they act as a single unit in forming compounds.
SECTION 9.5 Writing Chemical Formulae There's a quick and reliable method: write the symbols of the elements, note their valencies (or ion charges), then criss-cross the numbers as subscripts. Covalent: Hydrogen Sulfide H (valency 1) + S (valency 2) →criss-cross →H₂S Ionic: Calcium Chloride Ca²⁺ + Cl⁻ →criss-cross charges →CaCl₂ (2 Cl⁻ per Ca²⁺) Ionic: Aluminium Oxide Al³⁺ + O²⁻ →criss-cross →Al₂O₃ Ionic: Magnesium Oxide Mg²⁺ + O²⁻ →same valency →simplify Mg₂O₂ →MgO Polyatomic: Magnesium Hydroxide Mg²⁺ + OH⁻ →criss-cross →Mg(OH)₂. Brackets needed for multiple polyatomic ions! Polyatomic: Aluminium Sulfate Al³⁺ + SO₄²⁻ →criss-cross →Al₂(SO₄)₃
SECTION 9.6 Properties of Ionic vs. Covalent Compounds Why does sugar not conduct electricity in water? Sugar dissolves but does not split into ions — it remains as neutral molecules. Without charged particles to carry current, no electricity flows.
Activity 9.4: Testing Solubility & Conductivity Fig. 9.15 — Connect a 9V battery to carbon electrodes through a light bulb. Insert electrodes into the solution. If the bulb glows, the solution conducts electricity. Observation Table Compound Water Kerosene Petrol Solid cond uctivi ty Solut ion cond uctivi ty Camphor Insoluble Soluble Soluble No No NaCl Soluble Insoluble Insoluble No Yes CuSO₄ Soluble Insoluble Insoluble No Yes Sugar Soluble Insoluble Insoluble No No Naphthalene Insoluble Soluble Soluble No No Safety First: Use a low-voltage battery only. Petrol and kerosene are flammable — keep them away from sparks and flames.
SECTIONS 9.7 & 9.8 Molecular Mass & Formula Unit Mass Once you know a compound's formula, you can calculate its mass by adding the atomic masses of all the atoms present. Molecular Mass (Covalent Compounds) The sum of atomic masses of all atoms in one molecule. H₂O: (1×2) + (16×1) = 18 u CO₂: (12×1) + (16×2) = 44 u Note: Ionic compounds do not form molecules — they form crystal lattices. Formula Unit Mass (Ionic Compounds) A formula unit is the simplest whole-number ratio of ions in an ionic compound. Its mass is the sum of atomic masses of all atoms in the formula unit. Na¢O: (23×2) + (16×1) = 62 u Ca(NO£)¢: 40 + [14 + (16×3)]×2 = 164 u Try it! Find the formula unit mass of KCl (K=39, Cl=35.5 u) and Mg(OH)₂ (Mg=24, O=16, H=1 u).
Bridging Science and Society Atomic Energy: From Theory to Reality When atomic nuclei split (fission) or combine (fusion), enormous amounts of energy are released. In nuclear power plants, this thermal energy produces steam, which drives turbines to generate electricity — a cleaner alternative to fossil fuels. Atomic energy also powers breakthroughs in medicine, space exploration, and scientific research. In India, Raja Ramanna — often called the Father of the Indian Nuclear Programme — made landmark contributions to developing nuclear technology for peaceful and developmental purposes. Raja Ramanna Indian nuclear physicist and champion of peaceful atomic energy
At a Glance: Chapter 9 Summary Conservation of Mass Definite Proportions Dalton's Atomic Theory Covalent Bonding Ionic Bonding Law of Conservation of Mass: Total mass of reactants = total mass of products in every chemical reaction. Law of Definite Proportions: A pure compound always contains elements in the same fixed ratio by mass, regardless of source. Covalent Bond: Formed by sharing electrons; produces molecules with low melting points, often soluble in non-polar solvents. Ionic Bond: Formed by electron transfer; produces crystal lattices with high melting points, conduct electricity when dissolved in water. Molecular Mass (covalent) and Formula Unit Mass (ionic) are calculated by summing the atomic masses of all atoms in the formula.
The Journey Beyond 🚀 Keep exploring! Here are some ways to extend your learning beyond the classroom: Water Composition Experiment Design and perform an experiment showing that water always contains H and O in the same ratio, regardless of its source (tap, rain, river). Atoms vs. Ions Bar Graph Compare atoms and ions of three elements. Use bar graphs to show the number of electrons before and after ion formation. Ion Card Game Create a card game with cation and anion cards. Match them to form valid ionic compounds — a fun way to practice writing chemical formulae! Build-a-Molecule Simulation Explore the PhET simulation to build molecules interactively online and visualize how atoms combine. The Quest Continues... Are there any chemical changes that do not obey the Law of Conservation of Mass? Investigate and find out!