DEFINITIONS:
  Mole: 6.022 × 10²³ particles (atoms/molecules/ions) — Avogadro's number
  Molar mass: mass of 1 mole in grams = atomic/molecular weight in g
  Molarity (M): moles of solute per litre of solution | M = n/V(L)
  Molality (m): moles of solute per kg of solvent | m = n/w(kg)

CALCULATIONS:
  Moles = given mass / molar mass
  Moles = given number of particles / 6.022 × 10²³
  Number of moles in V litres of gas at STP = V/22.4

STOICHIOMETRY:
  Balance equation first
  Find limiting reagent: the reactant that gives minimum product
  Theoretical yield from limiting reagent
  Percentage yield = (actual yield / theoretical yield) × 100

EMPIRICAL AND MOLECULAR FORMULA:
  Step 1: Convert % to grams (assume 100g sample)
  Step 2: Convert grams to moles (divide by molar mass)
  Step 3: Find simplest ratio (divide by smallest)
  Step 4: Empirical formula
  Step 5: Molecular formula = empirical × n where n = molar mass / empirical mass

BOHR'S MODEL:
  rₙ = n²a₀/Z where a₀ = 0.529 Å (Bohr radius)
  Eₙ = -13.6Z²/n² eV (energy of nth orbit)
  
  Energy levels for hydrogen: -13.6, -3.4, -1.51, -0.85 eV (n=1,2,3,4)
  For emission: ΔE = E_higher - E_lower (positive)
  Wavelength: 1/λ = RZ²(1/n₁² - 1/n₂²) where R = Rydberg constant

QUANTUM NUMBERS:
  n: principal (1,2,3...) — energy level, shell
  l: azimuthal (0 to n-1) — subshell (0=s, 1=p, 2=d, 3=f)
  ml: magnetic (-l to +l) — orbital orientation
  ms: spin (+½ or -½)
  
  Rules: Aufbau (fill lower energy first) | Pauli (no two electrons same 4 QN)
         Hund (maximize unpaired electrons in same subshell)
  
  Electronic configuration: 1s², 2s², 2p⁶, 3s², 3p⁶, 3d¹⁰, 4s²...
  Exceptions: Cr = [Ar]3d⁵4s¹ | Cu = [Ar]3d¹⁰4s¹ (stability of half-filled/full)

TYPES OF BONDS:
  Ionic: metal + non-metal | large EN difference (> 1.7)
  Covalent: non-metals | share electrons | small EN difference
  Coordinate/Dative: one atom provides both electrons (e.g., NH₄⁺, BF₃.NH₃)
  Metallic: sea of electrons | delocalized | explains conductivity, luster

VSEPR THEORY:
  Count: bonding pairs + lone pairs on central atom
  2 bp: linear (180°) | 3 bp: trigonal planar (120°)
  4 bp: tetrahedral (109.5°) | 5 bp: trigonal bipyramidal | 6 bp: octahedral
  Lone pairs repel more: NH₃ (107°) | H₂O (104.5°) — reduced from 109.5°

HYBRIDISATION:
  sp: linear (BeCl₂, CO₂) | sp²: trigonal planar (BF₃, ethene)
  sp³: tetrahedral (CH₄, NH₃, H₂O) | sp³d: trigonal bipyramidal (PCl₅)
  sp³d²: octahedral (SF₆)