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Matter, Atoms, and Chemical Reactions

States of Matter and Properties​

1. Three States of Matter​

Solid (ठोस):

  • Definite shape and volume
  • Particles tightly packed
  • Strong intermolecular forces
  • Particles vibrate but don't move freely
  • Examples: Ice, wood, metal

Liquid (द्रव):

  • Definite volume, but takes shape of container
  • Particles loosely packed
  • Weaker intermolecular forces than solids
  • Particles move around but stay together
  • Examples: Water, oil, mercury

Gas (गैस):

  • No definite shape or volume
  • Particles very far apart
  • Very weak intermolecular forces
  • Particles move rapidly and randomly
  • Examples: Air, oxygen, carbon dioxide

Phase Changes:

  • Melting: Solid → Liquid (temperature increase)
  • Freezing: Liquid → Solid (temperature decrease)
  • Evaporation: Liquid → Gas (temperature increase)
  • Condensation: Gas → Liquid (temperature decrease)
  • Sublimation: Solid → Gas (temperature increase)
  • Deposition: Gas → Solid (temperature decrease)

Heating and Cooling Curves:

  • Temperature increases linearly with heat addition (solid, then liquid, then gas)
  • Plateaus during phase changes (latent heat)
  • Latent heat of fusion: Energy for melting
  • Latent heat of vaporization: Energy for evaporation

2. Atomic Structure​

Atoms (परमाणु):

  • Smallest unit of element
  • Composed of nucleus and electron cloud
  • Nuclear radius ~10⁻¹⁵ m, atomic radius ~10⁻¹⁰ m

Nucleus:

  • Composed of protons and neutrons
  • Protons: Positive charge (+1), mass ~1 amu
  • Neutrons: No charge, mass ~1 amu
  • Nucleus contains most mass, very small volume

Electrons (इलेक्ट्रॉन):

  • Negative charge (-1)
  • Negligible mass
  • Orbit nucleus in electron shells
  • Number equals protons (neutral atom)

Atomic Number (परमाणु क्रमांक):

  • Number of protons
  • Defines the element
  • Determines number of electrons (neutral)
  • Determines chemical properties

Mass Number:

  • Total protons + neutrons
  • Defines isotope
  • Varies for same element

Isotopes (समस्थानिक):

  • Same atomic number, different mass number
  • Same element, different neutrons
  • Examples: Carbon-12 and Carbon-14
  • Some isotopes radioactive

Energy Levels:

  • Electrons occupy discrete energy levels
  • Closer to nucleus = lower energy
  • Level 1: 2 electrons maximum
  • Level 2: 8 electrons maximum
  • Level 3: 18 electrons maximum
  • Outer level (valence) determines chemical behavior

Electron Configuration:

  • Shows distribution of electrons
  • Example: Oxygen (O) = 1s² 2s² 2p⁴
  • Shows number and type of orbital, electrons present
  • Valence electrons on outermost shell

3. Chemical Bonding​

Ionic Bonding (आयनिक बंधन):

  • Electrons transferred from one atom to another
  • Forms cations (positive) and anions (negative)
  • Electrostatic attraction between oppositely charged ions
  • Example: NaCl - Na⁺ and Cl⁻
  • Occurs between metals and nonmetals
  • Forms ionic compounds (crystalline solids typically)

Covalent Bonding (सहसंयोजक बंधन):

  • Electrons shared between atoms
  • Both atoms attracted to shared electrons
  • Atoms held together by shared electrons
  • Occurs between nonmetals
  • Single bond: 2 shared electrons
  • Double bond: 4 shared electrons
  • Triple bond: 6 shared electrons

Metallic Bonding (धात्विक बंधन):

  • Metal atoms lose valence electrons
  • Electrons delocalized in "sea"
  • Cations held in electron sea
  • Explains metal properties (conductivity, malleability)
  • Electrons free to move

Electronegativity:

  • Ability to attract electrons
  • More electronegative atom pulls electron density
  • Polar bond: Unequal sharing (electronegativity difference)
  • Nonpolar bond: Equal sharing (no electronegativity difference)
  • Determines polarity of molecules

4. Elements and Compounds​

Elements (तत्व):

  • Pure substance of one type of atom
  • 118 known elements
  • Periodic table organizes by atomic number
  • Metals (left), nonmetals (right), metalloids (between)

Compounds (यौगिक):

  • Pure substance of two or more elements
  • Fixed ratio of elements
  • Different properties from component elements
  • Example: Water (H₂O) ≠ hydrogen + oxygen
  • Chemical formula shows element ratio

Mixtures:

  • Combinations of substances not bonded
  • Variable ratio of components
  • Properties intermediate to components
  • Separable by physical means
  • Examples: Air, salt water, rocks

Naming Compounds:

  • Ionic: Cation name + anion name (sodium chloride = NaCl)
  • Covalent: Prefixes show atom count (carbon monoxide = CO, carbon dioxide = CO₂)
  • Acids: Hydrogen + nonmetal (hydrochloric acid = HCl)

Chemical Reactions​

1. Types of Reactions​

Synthesis Reaction (संश्लेषण):

  • Two or more substances combine to form one product
  • General form: A + B → AB
  • Example: 2H₂ + O₂ → 2H₂O
  • Usually exothermic (releases energy)

Decomposition Reaction (विघटन):

  • One compound breaks into two or more compounds
  • General form: AB → A + B
  • Example: 2H₂O₂ → 2H₂O + O₂
  • Often requires energy input

Single Displacement (एकल विस्थापन):

  • One element replaces another in compound
  • General form: A + BC → AC + B
  • Example: Zn + 2HCl → ZnCl₂ + H₂
  • Reactivity series determines occurrence

Double Displacement (द्विविस्थापन):

  • Cations and anions exchange
  • General form: AB + CD → AD + CB
  • Example: HCl + NaOH → NaCl + H₂O
  • Often produces precipitate, gas, or water

Combustion Reaction (दहन):

  • Substance reacts with oxygen
  • Produces oxides and releases lots of energy
  • Example: CH₄ + 2O₂ → CO₂ + 2H₂O
  • Always exothermic

2. Reaction Rates and Equilibrium​

Reaction Rate (अभिक्रिया दर):

  • Speed at which reactants converted to products
  • Measured in concentration change per unit time
  • Factors affecting:
    • Concentration: Higher concentration increases rate
    • Temperature: Higher temperature increases rate
    • Pressure: Higher pressure increases rate (for gases)
    • Catalyst: Increases rate without being consumed
    • Surface area: Greater area increases rate

Reversible Reactions (उत्क्रमणीय अभिक्रिया):

  • Reaction can go both forward and backward
  • Products convert back to reactants
  • Equilibrium reached when rates equal

Chemical Equilibrium (रासायनिक संतुलन):

  • Concentrations of reactants and products constant
  • Forward and reverse rates equal
  • Equilibrium constant (K) characterizes position
  • Equilibrium position can shift with conditions

Le Chatelier's Principle:

  • If equilibrium disturbed, system shifts to counteract
  • Increasing reactant concentration: Shifts right
  • Decreasing reactant concentration: Shifts left
  • Increasing temperature: Shifts toward endothermic direction
  • Decreasing temperature: Shifts toward exothermic direction
  • Increasing pressure: Shifts toward fewer gas molecules

3. Energy in Reactions​

Exothermic Reaction (उष्मा अवशोषक):

  • Releases heat/energy
  • Products lower energy than reactants
  • Temperature increases in surroundings
  • ΔH negative
  • Example: Combustion, neutralization, condensation

Endothermic Reaction (उष्मा अवशोषी):

  • Absorbs heat/energy
  • Products higher energy than reactants
  • Temperature decreases in surroundings
  • ΔH positive
  • Example: Melting, evaporation, photosynthesis

Activation Energy:

  • Minimum energy needed for reaction to occur
  • Catalyst lowers activation energy
  • Heat provides activation energy
  • Higher activation energy = slower reaction

Bond Energy:

  • Energy stored in chemical bonds
  • Breaking bonds requires energy (endothermic)
  • Forming bonds releases energy (exothermic)
  • Net energy = bonds broken - bonds formed

4. Stoichiometry (रासायनिक मात्रा)​

Balancing Equations:

  • Mass conserved (same atoms each side)
  • Balance atoms by adding coefficients
  • Coefficients show mole ratios
  • Example: 2H₂ + O₂ → 2H₂O

Mole Concept:

  • 1 mole = 6.02 × 10²³ particles (Avogadro's number)
  • Molar mass: Mass of one mole in grams
  • Atomic masses from periodic table
  • Example: Molar mass H₂O = 2(1) + 16 = 18 g/mol

Stoichiometric Calculations:

  • Mass to moles: Divide by molar mass
  • Moles to moles: Use coefficients from equation
  • Moles to mass: Multiply by molar mass
  • Limiting reactant: Runs out first
  • Theoretical yield: Maximum possible product

Summary​

Matter, atoms, and chemistry explain:

  • States of Matter: Solid, liquid, gas with different properties and phase changes
  • Atomic Structure: Nucleus with electrons in shells determining properties
  • Bonding: Ionic, covalent, metallic holding atoms together
  • Reactions: Various types with energy changes and rates
  • Stoichiometry: Quantitative relationships in reactions

Chemical understanding fundamental to material science, medicine, environmental science, and technology.