Class 8 Science Chapter 9 Matter

Class 8 Science Chapter 9 Matter is one of the most fundamental and scoring chapters in your science syllabus. In this chapter, you will explore the structure of atoms, electronic configuration, the first 20 elements, the modern periodic table, classification of elements, and different types of chemical reactions. From learning the criss-cross method for molecular formulas to understanding how elements are arranged in groups and periods, this complete guide to Class 8 Science Chapter 9 Matter will help you master every concept with clarity and confidence.

 Introduction to Matter

• Matter: Anything that has mass and occupies space.
• Types of Matter:
  • Solid: definite shape & volume.
  • Liquid: definite volume, no definite shape.
  • Gas: no definite shape or volume.

Exam Tip: Always link matter → mass + occupies space.

 Atom

• Definition: The smallest unit of matter that cannot be further divided chemically.
• Structure of Atom:
  • Protons (p⁺): Positive charge, mass ≈ 1 amu, located in nucleus.
  • Neutrons (n⁰): Neutral, mass ≈ 1 amu, located in nucleus.
  • Electrons (e⁻): Negative charge, negligible mass, revolve around nucleus in shells/orbits.

 Symbol

• Every element is represented by a symbol (usually first or first two letters).
• Atomic number (Z) → No. of protons = No. of electrons (in neutral atom).
• Mass number (A) → Protons + Neutrons.

Table: First 20 Elements (symbols, protons, neutrons, electrons)
(for exam: must memorize till Calcium, Z = 20)

Element	Symbol	Protons (Z)	Neutrons (approx.)	Electrons
Hydrogen	H	1	0	1
Helium	He	2	2	2
Lithium	Li	3	4	3
Beryllium	Be	4	5	4
Boron	B	5	6	5
Carbon	C	6	6	6
Nitrogen	N	7	7	7
Oxygen	O	8	8	8
Fluorine	F	9	10	9
Neon	Ne	10	10	10
Sodium	Na	11	12	11
Magnesium	Mg	12	12	12
Aluminium	Al	13	14	13
Silicon	Si	14	14	14
Phosphorus	P	15	16	15
Sulphur	S	16	16	16
Chlorine	Cl	17	18	17
Argon	Ar	18	22	18
Potassium	K	19	20	19
Calcium	Ca	20	20	20

Electronic Configuration and 2n² Rule

• Definition: Arrangement of electrons in different shells around the nucleus.
• 2n² Rule: Maximum no. of electrons in a shell = 2n² (n = shell number).
  • K shell (n=1) → 2(1)² = 2 electrons
  • L shell (n=2) → 8 electrons
  • M shell (n=3) → 18 electrons
  • N shell (n=4) → 32 electrons

Example: Oxygen (Z=8) → 2, 6.

Element	Atomic No.	Shell-wise	Subshell-wise  (to be memorized):
H	1	1	1s¹
He	2	2	1s²
Li	3	2,1	1s² 2s¹
Be	4	2,2	1s² 2s²
B	5	2,3	1s² 2s² 2p¹
C	6	2,4	1s² 2s² 2p²
N	7	2,5	1s² 2s² 2p³
O	8	2,6	1s² 2s² 2p⁴
F	9	2,7	1s² 2s² 2p⁵
Ne	10	2,8	1s² 2s² 2p⁶
Na	11	2,8,1	1s² 2s² 2p⁶ 3s¹
Mg	12	2,8,2	1s² 2s² 2p⁶ 3s²
Al	13	2,8,3	1s² 2s² 2p⁶ 3s² 3p¹
Si	14	2,8,4	1s² 2s² 2p⁶ 3s² 3p²
P	15	2,8,5	1s² 2s² 2p⁶ 3s² 3p³
S	16	2,8,6	1s² 2s² 2p⁶ 3s² 3p⁴
Cl	17	2,8,7	1s² 2s² 2p⁶ 3s² 3p⁵
Ar	18	2,8,8	1s² 2s² 2p⁶ 3s² 3p⁶
K	19	2,8,8,1	1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹
Ca	20	2,8,8,2	1s² 2s² 2p⁶ 3s² 3p⁶ 4s²

 Valence Shell, Valence Electrons, Valency

• Valence shell: Outermost electron shell of an atom.
• Valence electrons: Electrons present in valence shell.
• Valency: Combining capacity of an atom, equal to number of electrons lost, gained, or shared.

Examples:

• Na (2,8,1) → Valence shell = 3rd, valence electron = 1, valency = 1.
• O (2,6) → Valence shell = 2nd, valence electrons = 6, valency = 2.

 Molecules

• Definition: The smallest particle of a substance that retains its properties, made up of two or more atoms chemically combined.
• Examples: O₂, H₂O, CO₂.

 Molecular Formula

• Definition: Symbolic representation showing types and numbers of atoms in a molecule.
• Uses: Identifies composition, ratio, and molecular structure.

Criss-cross Method

Steps:

1. Write symbols with valency below each.
2. Cross valency numbers to opposite atoms.
3. Write formula with numbers as subscripts.

Example:

• Mg²⁺, Cl⁻ → Criss-cross → MgCl₂.

 Atomic Weight of Elements

• Definition: Sum of protons + neutrons in an atom.
• Formula:
  Atomic weight (A) = No. of protons + No. of neutrons.

 Molecular Weight

• Definition: Sum of atomic weights of all atoms in a molecule.

• Examples:
  • H₂O → (2 × 1) + (1 × 16) = 18.
  • CaCO₃ → 40 + 12 + (16 × 3) = 100.
  • Al₂(SO₄)₃ → (2×27) + (3×32) + (12×16) = 342.

Interesting Facts

• Hydrogen is the lightest and most abundant element in the universe.
• Helium was first detected in the sun, not on Earth.
• Carbon forms more compounds than all other elements combined.

Mnemonics

• First 20 Elements: Hi He Likes Beautiful Boys Caring Nice Old Funny Naughty Naughty Mangoes. All Silly People Should Call A Kind Cat.
  (H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca)

Summary / Quick Revision

• Matter: anything with mass + space.
• Atom = protons + neutrons (nucleus) + electrons (shells).
• First 20 elements → memorize protons, electrons, neutrons, configs.
• 2n² Rule → max electrons per shell.
• Valence shell/electrons/valency = determines reactivity.
• Molecule = 2+ atoms chemically combined.
• Molecular formula via Criss-cross method.
• Atomic weight = p + n.
• Molecular weight = sum of atomic weights in a molecule.

Unit 9: Matter (Part 2)

Learning Objectives

• Classify elements into metals, non-metals, metalloids, and noble gases.
• Recall the modern periodic law and features of the modern periodic table.
• Understand corrections made over Mendeleev’s periodic table.
• Identify positions of different element types in the modern table.
• Learn about periods and groups and their related properties.

 Classification of Elements

1. Metals
   • Good conductors of heat & electricity.
   • Malleable (can be hammered into sheets).
   • Ductile (can be drawn into wires).
   • Lustrous (shiny).
   • Example: Fe, Cu, Al.
2. Non-Metals
   • Poor conductors of heat & electricity.
   • Brittle, not malleable.
   • Usually gases or soft solids.
   • Example: O, N, S.
3. Metalloids
   • Show properties of both metals and non-metals.
   • Example: B, Si, As.
4. Noble Gases
   • Chemically inert (non-reactive).
   • Complete octet (except He, with 2 electrons).
   • Example: He, Ne, Ar.

Exam Tip: Metals → left, Non-metals → right, Metalloids → staircase, Noble gases → extreme right.

Modern Periodic Law

• Statement: “The properties of elements are the periodic functions of their atomic numbers.”

Modern Periodic Table

• Definition: A tabular arrangement of elements in order of increasing atomic number, showing periodic repetition of properties.

Corrections of Defects of Mendeleev’s Periodic Table

1. Atomic Number: Based on atomic number, not atomic mass → removed anomalies.
2. Position of Hydrogen: Placed with alkali metals but also resembles halogens.
3. Noble Gases: Introduced in a separate group (Group 18).
4. Transition Metals: Placed in middle section (d-block).
5. Lanthanides & Actinides: Placed separately below main table (f-block).
6. Isotopes: Same atomic number → placed in same position.
7. Correction of Periodic Law: Now periodicity depends on atomic number, not mass.
8. Periodicity: Repetition of properties explained by repeating electronic configuration.
9. Position of Alkali Metals & Coinage Metals: Group 1 (alkali), Group 11 (coinage).

Position of Elements in Modern Periodic Table

1. Hydrogen: Group 1, Period 1. Shows dual nature (alkali & halogen-like).
2. Metals: Left & center (s-block and d-block).
3. Non-Metals: Right side (p-block).
4. Metalloids: Along diagonal staircase (B, Si, As, Te).

Lanthanides & Actinides

• Lanthanides (Z = 57–71): Rare earth metals, similar properties, used in electronics.
• Actinides (Z = 89–103): Mostly radioactive, include Uranium & Thorium.

Periods in Modern Periodic Table

• Periods = Horizontal rows (7 total).
• Groups = Vertical columns (18 total).

Properties Across Periods & Groups

1. Atomic Size/Radius:
   • Across a period → decreases (more nuclear charge).
   • Down a group → increases (more shells).
2. Valence Electrons:
   • Across a period → increases gradually (1 → 8).
   • Down a group → remains same.
3. Valency:
   • Across a period → first increases (1–4), then decreases (4–0).
   • Down a group → remains same.
4. Metallic Character:
   • Across a period → decreases (Na → Cl).
   • Down a group → increases (Li → Cs).
5. Non-Metallic Character:
   • Across a period → increases.
   • Down a group → decreases.

Interesting Facts

• Noble gases were discovered after Mendeleev’s table, yet perfectly fit the modern one.
• Hydrogen is unique: behaves like both alkali metals and halogens.
• Actinides are mostly synthetic and radioactive (except a few like U, Th).

Mnemonics

• Periods vs Groups: “Periods go flat, Groups go tall.” (horizontal vs vertical).
• First 3 Periods’ Valency Trend: “1,2,3,4,3,2,1,0.”

Summary / Quick Revision

• Classification: Metals, Non-metals, Metalloids, Noble gases.
• Modern Periodic Law: Properties depend on atomic number.
• Corrections: Isotopes, noble gases, hydrogen position, transition metals, lanthanides & actinides.
• Periods: 7 rows; Groups: 18 columns.
• Across a period → smaller size, more valence electrons, less metallic.
• Down a group → larger size, same valence electrons, more metallic.

Chemical Reaction – Class 8 Science Chapter 9 Matter

• Definition:
  A chemical reaction is a process in which one or more substances (reactants) are converted into new substances (products) with different properties.
• Examples of Chemical Reactions:
  1. Burning of magnesium: Mg + O₂ → MgO
  2. Rusting of iron: Fe + O₂ + H₂O → Fe₂O₃·xH₂O
  3. Respiration: Glucose + O₂ → CO₂ + H₂O + Energy
  4. Neutralization: HCl + NaOH → NaCl + H₂O
• Reactants:
  Substances that undergo change in a reaction.
  Example: In 2H₂ + O₂ → 2H₂O, H₂ and O₂ are reactants.
• Products:
  Substances formed after the reaction.
  Example: In above, H₂O is the product.

Representation of Chemical Reactions

1. Word Equation:
   • Written in words.
   • Example: Hydrogen + Oxygen → Water
2. Formula Equation:
   • Written with chemical symbols.
   • Example: 2H₂ + O₂ → 2H₂O

 Chemical Equation

• Definition:
  A chemical equation is the symbolic representation of a chemical reaction using formulas of reactants and products.
• Example:
  Zn + H₂SO₄ → ZnSO₄ + H₂

 Balanced Chemical Equation

• Definition:
  A chemical equation in which the number of atoms of each element is equal on both sides is called a balanced chemical equation.
• Why Balance?
  • To obey the Law of Conservation of Mass (mass is neither created nor destroyed).
• Example:
  Unbalanced: H₂ + O₂ → H₂O
  Balanced: 2H₂ + O₂ → 2H₂O

Types of Chemical Reactions

a. Combination (Addition / Synthesis) Reaction

• Definition: Two or more reactants combine to form a single product.
• Examples:
  1. C + O₂ → CO₂
  2. 2Mg + O₂ → 2MgO
  3. N₂ + 3H₂ → 2NH₃
  4. CaO + H₂O → Ca(OH)₂

Exam Tip: Look for → “many reactants form one product.”

b. Decomposition (Dissociation) Reaction

• Definition: A compound breaks down into two or more simpler substances.
• Examples:
  1. 2HgO → 2Hg + O₂
  2. CaCO₃ → CaO + CO₂
  3. 2H₂O → 2H₂ + O₂
  4. 2KClO₃ → 2KCl + 3O₂

Exam Tip: Look for → “one reactant splits into many products.”

c. Displacement (Replacement) Reaction

i. Single Displacement Reaction

• Definition: A more reactive element displaces a less reactive element from its compound.
• Examples:
  1. Zn + H₂SO₄ → ZnSO₄ + H₂
  2. Fe + CuSO₄ → FeSO₄ + Cu
  3. 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu
  4. Cl₂ + 2KBr → 2KCl + Br₂

ii. Double Displacement Reaction

• Definition: Exchange of ions between two compounds forming two new compounds.
• Examples:
  1. NaCl + AgNO₃ → NaNO₃ + AgCl
  2. HCl + NaOH → NaCl + H₂O
  3. BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl
  4. CaCl₂ + Na₂CO₃ → CaCO₃ + 2NaCl

Exam Tip: Look for → “ion exchange producing precipitate, gas, or water.”

Interesting Facts

• Fireflies produce light through a chemical reaction called bioluminescence.
• Decomposition of KClO₃ was historically used to supply oxygen in laboratories.

Mnemonics

• Reaction Types: “Come Down Slowly, Dear” → Combination, Decomposition, Single displacement, Double displacement.

Summary / Quick Revision

• Chemical Reaction: Process of changing reactants → products.
• Reactants & Products: Reactants undergo reaction; products are formed.
• Chemical Equation: Word equation & formula equation.
• Balanced Equation: Equal atoms on both sides, law of conservation of mass.
• Types:
  • Combination → many → one.
  • Decomposition → one → many.
  • Displacement:
    • Single (metal replaces another).
    • Double (ion exchange).

This completes the full revision of Class 8 Science Chapter 9 Matter.