FUNDAMENTAL DEFINITIONS:
  1 mole = 6.022 × 10²³ particles (Avogadro's number)
  Molar mass = mass of 1 mole in grams = molecular weight
  
  Moles = Mass (g) / Molar Mass (g/mol)
  Moles = Number of particles / 6.022 × 10²³
  Moles = Volume at STP (L) / 22.4 L/mol

STOICHIOMETRY STEPS:
  1. Write balanced chemical equation
  2. Convert given to moles
  3. Use mole ratio from balanced equation
  4. Convert to required units

PERCENTAGE COMPOSITION:
  % of element = (atomic mass × number of atoms / molar mass) × 100

EMPIRICAL FORMULA:
  % → grams (assume 100g) → moles → simplest ratio → empirical formula
  Molecular formula: empirical × n where n = molar mass / empirical mass

PERIODS (horizontal rows): 7 periods
  Valence electrons increase left to right
  Atomic size decreases left to right
  Ionisation energy increases left to right
  Metallic character decreases left to right

GROUPS (vertical columns): 18 groups
  s-block: groups 1-2 | d-block: groups 3-12 | p-block: groups 13-18
  f-block: lanthanides and actinides

KEY TRENDS:
  Atomic radius: increases down a group, decreases across a period
  Ionisation energy: decreases down a group, increases across a period
    Exceptions: IE₁(B) < IE₁(Be) and IE₁(O) < IE₁(N)
  Electron affinity: increases across period (most negative for halogens)
  Electronegativity: highest for F (3.98) | increases across period, decreases down group

IMPORTANT EXCEPTIONS:
  Li resembles Mg (diagonal relationship) | Be resembles Al
  Anomalous behaviour of first element of each group

IONIC BOND:
  Metal + Non-metal | Metal loses electrons, non-metal gains
  Crystal lattice | high melting point | conducts in solution or melt
  Cation size < anion size (for typical ionic compounds)

COVALENT BOND:
  Non-metal + Non-metal | electrons shared
  Types: single (σ), double (σ+π), triple (σ+2π)

HYBRIDISATION (VSEPR for shape):
  sp: linear 180° — BeCl₂, CO₂, C₂H₂
  sp²: trigonal planar 120° — BF₃, SO₃, C₂H₄, graphite (each carbon)
  sp³: tetrahedral 109.5° — CH₄, SiF₄, CCl₄ | 107° NH₃ | 104.5° H₂O
  sp³d: trigonal bipyramidal — PCl₅ | sp³d²: octahedral — SF₆

BOND PROPERTIES:
  Bond length: increases with bond order decrease | single > double > triple
  Bond energy: increases with bond order | triple > double > single
  Bond polarity: difference in electronegativity