Atoms and Molecules

• Introduction

  • Definition of Matter
  • Historical background (ancient Indian and Greek philosophers)
    • Maharishi Kanad (Parmanu)
    • Democritus and Leucippus (Atom)

• Laws of Chemical Combination

  • Law of Conservation of Mass
    • Matter can neither be created nor destroyed in a chemical reaction
    • Antoine Lavoisier
  • Law of Constant Proportions (Law of Definite Proportions)
    • Elements are present in a definite proportion by mass in a chemical substance
    • Joseph Proust

• Dalton's Atomic Theory

  • Postulates
    • All matter is made of tiny particles called atoms
    • Atoms are indivisible and cannot be created or destroyed
    • Atoms of a given element are identical in mass and chemical properties
    • Atoms of different elements have different masses and properties
    • Atoms combine in fixed whole-number ratios to form compounds
    • Relative number and kinds of atoms are constant in a given compound

• Atom

  • Definition
    • Building blocks of all matter
  • Size
    • Extremely small, measured in nanometers ($10^{-9}$ m)
    • Example: Radius of hydrogen atom is $10^{-10}$ m
  • Symbols of Atoms
    • Representation of elements by letters (Berzelius system)
    • Examples: Hydrogen (H), Carbon (C), Oxygen (O), Sodium (Na)
  • Atomic Mass
    • Mass of an atom relative to carbon-12 isotope ($^{12}\text{C}$) standard
    • Defined as 1 atomic mass unit (u)

• Molecule

  • Definition
    • Group of two or more atoms chemically bonded together
  • Molecules of Elements
    • Composed of the same type of atoms
    • Atomicity: Number of atoms in a molecule
      • Monoatomic (e.g., Argon, Helium)
      • Diatomic (e.g., Oxygen, Nitrogen)
      • Polyatomic (e.g., Phosphorus, Sulphur)
  • Molecules of Compounds
    • Composed of atoms of different elements joined in fixed proportions
    • Examples: Water ($\text{H}_2\text{O}$), Carbon Dioxide ($\text{CO}_2$)

• Ions

  • Definition
    • Charged particles formed by gain or loss of electrons
  • Types
    • Cations: Positively charged ions (e.g., $\text{Na}^+$, $\text{Ca}^{2+}$)
    • Anions: Negatively charged ions (e.g., $\text{Cl}^-$, $\text{O}^{2-}$))
  • Polyatomic Ions
    • Group of atoms carrying a charge (e.g., Ammonium $\text{NH}_4^+$, Hydroxide $\text{OH}^-$)

• Chemical Formulae

  • Rules for Writing
    • Write symbols of constituent elements and their charges/valencies
    • Cross-over the valencies of combining atoms
  • Valency
    • Combining capacity of an element
  • Examples
    • Hydrogen chloride ($\text{HCl}$)
    • Water ($\text{H}_2\text{O}$)
    • Aluminum oxide ($\text{Al}_2\text{O}_3$)

• Molecular Mass and Mole Concept

  • Molecular Mass
    • Sum of atomic masses of all atoms in a molecule
    • Expressed in atomic mass units (u)
  • Formula Unit Mass
    • Sum of atomic masses for ionic compounds (e.g., $\text{NaCl}$)
  • The Mole Concept
    • Counting unit in chemistry representing $6.022 \times 10^{23}$ particles (Avogadro's Number / Constant)
    • Molar Mass: Mass of 1 mole of a substance in grams ($\text{g/mol}$)
    • Conversion Formulas
      • Number of moles = $\frac{\text{Given Mass}}{\text{Molar Mass}}$
      • Number of moles = $\frac{\text{Given Number of Particles}}{\text{Avogadro Number}}$