⚖ Class 9 Science – Atomic Foundations of Matter Original Visual Revision Notes
Aligned with the Class 9 Science syllabus. Independently created study material. Not affiliated with or endorsed by NCERT.
💡 Think about this first! Imagine you are making a sandwich. No matter where in the world you make it, a sandwich with 30 g of bread and 20 g of filling always gives you a 50 g sandwich — matter never disappears! Chemistry works exactly the same way. And just like a recipe uses ingredients in fixed amounts (two slices of bread for every one slice of cheese), atoms combine in fixed, predictable ratios. This lesson is about those beautiful rules that make chemistry reliable and logical.
⚖️ Topic 1: Law of Conservation of Mass
Lavoisier gave this law in 1789.
Mass of reactants equals mass of products.
Matter cannot be created in a reaction.
Matter cannot be destroyed in a reaction.
This law holds true in every reaction.
📊 Topic 2: Law of Constant Proportions
Proust gave the law of constant proportions.
Elements combine in a fixed mass ratio.
Carbon dioxide always has carbon and oxygen in ratio 3:8 by mass.
This ratio stays same in every sample.
The law applies to any pure compound.
🧠 Topic 3: Dalton's Atomic Theory
All matter is made of tiny atoms.
Atoms cannot be divided in a reaction.
Atoms of one element are all identical.
Atoms of different elements are always different.
Atoms join in simple whole number ratios.
🔭 Topic 4: Molecules of Elements
A molecule is the smallest part of matter.
Some elements form molecules with two atoms.
Hydrogen gas exists as H₂ molecules.
Oxygen and chlorine also form diatomic molecules.
Some elements like helium exist as single atoms.
🧬 Topic 5: Molecules of Compounds
Compounds form when different atoms join together.
Water molecule has two hydrogen one oxygen atom.
Hydrogen chloride has one hydrogen one chlorine atom.
Carbon dioxide has one carbon two oxygen atoms.
Ammonia has one nitrogen and three hydrogen atoms.
🤝 Topic 6: Covalent Bonds — Sharing Electrons
Covalent bonds form when atoms share electrons.
Sharing one pair makes a single bond.
Sharing two pairs makes a double bond.
Shared electrons are pulled by both nuclei.
Covalent bonds form mostly between non-metal atoms.
⚡ Topic 7: Ionic Bonds — Transferring Electrons
Ionic bonds form when electrons transfer fully.
A metal atom loses electrons to form a cation.
A non-metal atom gains electrons to form an anion.
Cation means a positively charged particle.
Opposite charges attract and hold ions together.
✏️ Topic 8: Writing Chemical Formulae
Formula shows symbols and numbers of atoms.
Valency tells how many bonds an atom forms.
Criss-cross method swaps valency numbers as subscripts.
Polyatomic ions act as one single unit.
Common polyatomic ions include hydroxide and carbonate.
🔍 Topic 9: Properties of Ionic and Covalent Compounds
Ionic compounds have high melting and boiling points.
Ionic compounds conduct electricity when dissolved in water.
Ionic compounds are usually soluble in water.
Covalent compounds have low melting and boiling points.
Covalent compounds usually do not conduct electricity.
📏 Topic 10: Molecular Mass and Formula Unit Mass
Molecular mass is the sum of atomic masses.
It is used mostly for covalent compounds.
Formula unit mass is used for ionic compounds.
It is the sum of masses in one unit.
Both masses are measured in unified mass unit u.
🎨 Section 2 — Easy Illustrations
Fig 1: A balance scale shows mass is conserved in a reaction
Fig 2: Chlorine atoms bond by sharing one pair of electrons
Fig 3: Sodium loses an electron and chlorine gains it to form NaCl
Fig 4: The criss-cross method gives the formula Al₂O₃
Fig 5: Ionic compounds and covalent compounds differ in properties
🔬 Section 3 — Worked Examples
💡 Example 1: Applying Conservation of Mass
6 g of carbon burns completely with 16 g of oxygen. Find the mass of carbon dioxide formed.
1
Total mass of reactants = 6 + 16 g.
2
The law of conservation of mass applies here.
3
Mass of product must equal mass of reactants.
Mass of Reactants = Mass of Products
Answer: 22 g of carbon dioxide is formed.
💡 Example 2: Finding Molecular Mass
Find the molecular mass of sulfur dioxide (SO₂). Atomic mass: S = 32u, O = 16u.
1
Molecular mass is the sum of atomic masses.
2
SO₂ has one sulfur atom and two oxygen atoms.
3
Mass = 32 + 2 × 16 = 32 + 32.
Molecular Mass = Sum of Atomic Masses
Answer: Molecular mass of SO₂ is 64 u.
💡 Example 3: Finding Formula Unit Mass
Find the formula unit mass of calcium chloride (CaCl₂). Ca = 40u, Cl = 35.5u.
1
CaCl₂ has one calcium atom and two chlorine atoms.
2
Mass = 40 + 2 × 35.5 = 40 + 71.
3
Add both parts to get the total mass.
Formula Unit Mass = Sum of Masses in Unit
Answer: Formula unit mass of CaCl₂ is 111 u.
📋 Section 4 — Cheat Sheet
Term
Meaning
Conservation of Mass
Mass of reactants equals mass of products.
Constant Proportions
Elements combine in a fixed mass ratio.
Atom
Smallest particle of an element.
Molecule
Group of atoms bonded together.
Diatomic Molecule
Molecule made of exactly two atoms.
Covalent Bond
Bond formed by sharing electrons.
Ionic Bond
Bond formed by transferring electrons.
Cation
Positive ion formed by losing electrons.
Anion
Negative ion formed by gaining electrons.
Valency
Combining power of an atom.
Polyatomic Ion
Group of atoms acting as one ion.
Chemical Formula
Symbols showing atoms present in a compound.
Molecular Mass
Sum of atomic masses in a molecule.
Formula Unit Mass
Sum of atomic masses in an ionic unit.
Unified Mass Unit (u)
Standard unit used for atomic mass.
🏠 Section 5 — Take-Home Handout
Part A: Fill in the Blanks
1. Mass of reactants always equals mass of _______.
2. Water always has hydrogen and oxygen in a fixed _______.
3. A covalent bond forms when atoms _______ electrons.
4. An ionic bond forms when electrons are _______.
5. A positively charged ion is called a _______.
Part B: True or False
1. Atoms can be divided during a chemical reaction.
2. Oxygen gas exists as a diatomic molecule.
3. Ionic compounds usually conduct electricity in solution.
4. Covalent compounds have very high melting points.
5. The criss-cross method helps write chemical formulae.
Part C: Solve These
1. 4 g of hydrogen reacts with 32 g of oxygen. Find the mass of water formed.
2. Find the molecular mass of carbon dioxide (CO₂). C = 12u, O = 16u.
3. Find the formula unit mass of magnesium chloride (MgCl₂). Mg = 24u, Cl = 35.5u.
Part D: Match the Columns
Column A
Column B
Your Answer
1. Cation
A. Negatively charged ion
2. Anion
B. Bond formed by sharing electrons
3. Covalent Bond
C. Positively charged ion
4. Ionic Bond
D. Fixed ratio of elements by mass
5. Constant Proportions
E. Bond formed by transferring electrons
Part E: Draw and Label
Draw two atoms sharing one electron pair. Label the shared pair and both atoms.
Water always has hydrogen and oxygen in a fixed ratio (mass ratio / proportion).
A covalent bond forms when atoms share electrons.
An ionic bond forms when electrons are transferred (given away / gained).
A positively charged ion is called a cation.
Part B: True or False — Answers
False. Dalton’s atomic theory states that atoms cannot be divided in a chemical reaction.
True. Oxygen exists as O&sub2;, a molecule made of two oxygen atoms bonded together.
True. When ionic compounds dissolve, ions move freely in solution and carry electrical current.
False. Covalent compounds have low melting and boiling points because the bonds between molecules are weak.
True. In the criss-cross method, the charge on each ion is used as the subscript for the other ion.
Part C: Solve These — Step-by-Step Solutions
Q1. 4 g of hydrogen reacts with 32 g of oxygen. Find the mass of water formed.
1
By law of conservation of mass: mass of products = mass of reactants.
2
Total mass of reactants = 4 + 32 = 36 g.
Answer: 36 g of water is formed.
Q2. Find the molecular mass of carbon dioxide (CO&sub2;). C = 12 u, O = 16 u.
1
CO&sub2; has 1 carbon atom and 2 oxygen atoms.
2
Molecular mass = 12 + 2 × 16 = 12 + 32.
3
Molecular mass = 44 u.
Answer: Molecular mass of CO&sub2; = 44 u.
Q3. Find the formula unit mass of magnesium chloride (MgCl&sub2;). Mg = 24 u, Cl = 35.5 u.
1
MgCl&sub2; has 1 magnesium atom and 2 chlorine atoms.
2
Formula unit mass = 24 + 2 × 35.5 = 24 + 71.
3
Formula unit mass = 95 u.
Answer: Formula unit mass of MgCl&sub2; = 95 u.
Part D: Match the Columns — Answers
Column A
Match
Column B
1. Cation
→ C
Positively charged ion
2. Anion
→ A
Negatively charged ion
3. Covalent Bond
→ B
Bond formed by sharing electrons
4. Ionic Bond
→ E
Bond formed by transferring electrons
5. Constant Proportions
→ D
Fixed ratio of elements by mass
🔄 Section 6 — Revision Flashcards
Light side = Question. Dark side = Answer.
Who gave the law of conservation of mass?
Antoine Lavoisier, in 1789.
Who gave the law of constant proportions?
Joseph Proust.
What did Dalton say about atoms?
Atoms cannot be divided in a reaction.
What is a diatomic molecule?
A molecule made of two atoms.
What is a covalent bond?
A bond formed by sharing electrons.
What is an ionic bond?
A bond formed by transferring electrons.
What method is used to write formulae?
The criss-cross method.
What is molecular mass?
The sum of atomic masses in a molecule.
🧠 Section 7 — Mind Map
Fig 6: Mind map of atomic foundations of matter
⚠ This is independently created study material aligned with the Class 9 Science syllabus. Not affiliated with or endorsed by NCERT or any government body.