Lecture on Atoms and Molecules 🧪

Introduction

• Teacher: Sania Nair ✨
• Chapter: Atoms and Molecules
• Goal: Make concepts of atoms and molecules clear & tackle chemistry fears!

Historical Perspectives on Atoms & Molecules

Ancient Indian Philosophers

• Maharsi Kanad: Around 500 BCE, suggested the existence of indivisible particles called paramanu.
• Pakudha Katyayana: Stated that paramanus exist in combined forms creating various forms of matter.

Greek Philosophers

• Democritus and Leucippus: Proposed that matter is composed of indivisible particles called atoms.

Laws of Chemical Combination

Law of Conservation of Mass

• Given by Lavoisier in 1774.
• States: Mass is neither created nor destroyed in a chemical reaction.
  • Example: H₂ (Hydrogen) + O₂ (Oxygen) = H₂O (Water)
  • Reactants: Substances that react.
  • Products: New substances formed.
• Key Point: Total mass of reactants = Total mass of products.

Law of Constant Proportions

• Given by Proust in 1779.
• States: A chemical compound always contains the same elements in the same proportions by mass, regardless of the source.
  • Example: Water (H₂O) always consists of Hydrogen and Oxygen in a 1:8 ratio by mass.

Dalton's Atomic Theory

• Became a foundation of modern chemistry.
• Postulates:
  • Matter is made up of atoms.
  • Atoms are indivisible and indestructible.
  • Atoms of an element are identical in mass and properties.
  • Atoms of different elements have different masses and properties.
  • Compounds are formed by a combination of atoms in simple whole-number ratios.
• Drawbacks:
  • Atoms are divisible (subatomic particles: electrons, protons, neutrons).
  • Isotopes (same element, different masses).
  • Isobars (different elements, same mass).

Atoms

• Atoms are the building blocks of matter.
• Size and Shape:
  • Very small; cannot be seen even with powerful microscopes.
  • Spherical in shape.
  • Sizes measured in nanometers (1 nm = 10⁻⁹ meters).

Symbols of Elements

• Dalton's Symbols: Early, inconvenient symbols for elements.
• Modern Symbols (by Berzelius):
  • One or two letters from the element's name.
  • Example: Hydrogen (H), Helium (He), Sodium (Na).
  • Rules: Capitalize the first letter, second letter (if any) is lowercase.
  • Some symbols derived from Latin names (e.g., Sodium from 'Natrium' -> Na).

First 20 Elements

• Memorize using mnemonics:
  • H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca

Atomic Mass of Elements

• Concept: Relative mass measured against Carbon-12.
• Units: Atomic mass unit (amu or unified mass 'u').
• Example: 1 amu = 1/12 mass of a Carbon-12 atom.

Molecules

Definition

• Group of two or more atoms bonded together by attractive forces.

Types of Molecules

• Molecules of Elements: Same type of atoms combine.
  • Example: O₂, P₄, S₈
• Molecules of Compounds: Different types of atoms combine.
  • Example: H₂O, CO₂, NH₃

Atomicity

• Number of atoms in a molecule.
  • Monoatomic: One atom (He, Ne)
  • Diatomic: Two atoms (H₂, O₂)
  • Triatomic: Three atoms (O₃)
  • Tetraatomic: Four atoms (P₄)
  • Polyatomic: More than four atoms (S₈)

Ions

Definition

• Electrically charged species formed by gain or loss of electrons.

Types

• Cations: Positively charged (loss of electrons).
  • Example: Na⁺, Mg²⁺
• Anions: Negatively charged (gain of electrons).
  • Example: Cl⁻, OH⁻
• Simple Ions: Single atom ions (Na⁺).
• Polyatomic Ions: Ions formed from groups of atoms (OH⁻, CO₃²⁻).

Valency

• Combining capacity of an element (electrons lost, gained, or shared to achieve stability).
• Examples: Na (1), Mg (2), Cl (1), O (2)

Chemical Formula

• Represents the composition of molecules and compounds.
• Method:
  • Cross-multiply valencies (e.g., Na⁺ and Cl⁻ -> NaCl).

Mole Concept

• Definition: One mole = 6.022 × 10²³ particles (Avogadro's number).
• Molar Mass: Mass of one mole of a substance (GAM for elements, GMM for compounds).
• Formulas:
  • Number of moles: Given mass / Molar mass
  • Number of particles: Given number of particles / Avogadro’s number

Exercises

• Examples and practice problems on calculating moles, masses, and atoms.

Conclusion

• Revision of key points.
• Encouragement to practice and master the concepts.

Tips:

• Understand the laws and theories.
• Practice writing chemical formulas.
• Memorize first 20 elements and their symbols.
• Grasp the mole concept through examples.

Let's make chemistry easy and fun! 🌟