Atoms and Molecules

🔹 I. Laws of Chemical Combination

1. Law of Conservation of Mass (Lavoisier)
   • Mass can neither be created nor destroyed in a chemical reaction.
   • Example: In the reaction of barium chloride + sodium sulphate → barium sulphate + sodium chloride, total mass of reactants = total mass of products.
2. Law of Constant Proportions (Proust)
   • A chemical compound always contains the same elements in the same proportion by mass.
   • Example: Water (H₂O) always has 2 parts hydrogen and 16 parts oxygen by mass.

🔹 II. Dalton’s Atomic Theory

🔹 III. What is an Atom?

• Atom: Smallest particle of an element that takes part in chemical reaction.
• Represented by symbols (IUPAC system).

🔸 Atomic Symbols (By Berzelius)

🔹 IV. Molecule

• Molecule: A group of two or more atoms chemically bonded together.
• Can be of:
  • Elements (e.g., O₂, N₂)
  • Compounds (e.g., H₂O, CO₂)

🔸 Types:

• Monoatomic: He, Ne (noble gases)
• Diatomic: O₂, H₂, N₂
• Triatomic & Polyatomic: O₃, P₄, S₈

🔹 V. Atomicity

• Number of atoms in a molecule.

🔹 VI. Ions

• Ion: An atom or group of atoms with a charge.
  • Cation (+ve): Na⁺, Ca²⁺
  • Anion (–ve): Cl⁻, SO₄²⁻

🔸 Chemical Formula:

• Formed using valency.
• Criss-cross method used.
• Example: Na⁺ + Cl⁻ → NaCl

🔹 VII. Molecular Mass and Formula Unit Mass

• Molecular Mass: Sum of atomic masses of all atoms in a molecule.
  • E.g., H₂O = 2(1) + 16 = 18 u
• Formula Unit Mass: Used for ionic compounds.
  • E.g., NaCl = 23 + 35.5 = 58.5 u

🔹 VIII. Mole Concept

• 1 mole = 6.022 × 10²³ particles (Avogadro’s number)
• Used to count atoms, molecules, ions

🔸 Molar Mass:

• Mass of 1 mole of a substance in grams.
  • E.g., H₂O = 18 g/mol

🔸 Important Formulas:

Number of moles (n)=Given mass (g)Molar mass (g/mol)\text{Number of moles (n)} = \frac{\text{Given mass (g)}}{\text{Molar mass (g/mol)}}Number of moles (n)=Molar mass (g/mol)Given mass (g)​ Number of particles=Moles×6.022×1023\text{Number of particles} = \text{Moles} \times 6.022 \times 10^{23}Number of particles=Moles×6.022×1023 Mass=Moles×Molar mass\text{Mass} = \text{Moles} \times \text{Molar mass}Mass=Moles×Molar mass

🔹 IX. Writing Chemical Formulae

🔸 Steps:

1. Write symbol of element/ion.
2. Write valency.
3. Criss-cross valencies.
4. Simplify (if required).

🔸 Examples:

• Na (1) + Cl (1) → NaCl
• Ca (2) + Cl (1) → CaCl₂
• Al (3) + O (2) → Al₂O₃

🔹 X. Examples of Common Compounds and Valencies

🔹 XI. NCERT Textbook Important Questions

Q1: Define 1 mole.

• 1 mole is the amount of substance containing 6.022 × 10²³ particles.

Q2: Calculate the number of atoms in 12 g of carbon.

• Atomic mass = 12 g/mol → Moles = 12/12 = 1 mol
• Atoms = 1 × 6.022 × 10²³ = 6.022 × 10²³ atoms

Q3: Write molecular formula of:

• Calcium oxide → CaO
• Magnesium chloride → MgCl₂
• Aluminium sulphate → Al₂(SO₄)₃

🔹 XII. Mind Map: Atoms and Molecules

markdownCopyEdit            ATOMS & MOLECULES
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    |               |                 |
Laws of Combo   Dalton's Theory    Mole Concept
    |                                   |
Chemical Formulae                 Avogadro’s Number
    |                                   |
Valency & Ions                 Mass, Mole, Particles