Chemistry

Atom:     nucleus (protons + neutrons) + electron cloud
Element:  defined by number of protons (atomic number Z)

Subatomic particles:
  Proton:    +1 charge, mass ≈ 1 amu, in nucleus
  Neutron:   0 charge, mass ≈ 1 amu, in nucleus
  Electron:  -1 charge, mass ≈ 1/1836 amu, in orbitals

Isotopes:  same element, different neutron count
  Carbon-12: 6 protons, 6 neutrons
  Carbon-14: 6 protons, 8 neutrons (radioactive)

Shells:    K(2) L(8) M(18) N(32)
Orbitals:  s(2) p(6) d(10) f(14)

Silicon (Z=14): 1s^2 2s^2 2p^6 3s^2 3p^2
  4 valence electrons → semiconductor
  Doping with P (5 valence) → n-type
  Doping with B (3 valence) → p-type

Ionic:       electron transfer (NaCl — Na gives e- to Cl)
             Metals + nonmetals
             High melting point, conducts when dissolved

Covalent:    electron sharing (H2O, SiO2)
             Nonmetal + nonmetal
             Can be polar (unequal sharing) or nonpolar

Metallic:    electron sea model (Cu, Al, Fe)
             Metal atoms share delocalized electrons
             Conducts electricity, malleable, lustrous

Van der Waals: weak intermolecular forces
             Temporary dipoles
             Why geckos stick to walls

Silicon:     covalent crystal, semiconductor backbone
Copper:      metallic bonding, excellent conductor (PCB traces)
Solder:      Sn-Pb or Sn-Ag-Cu alloys (metallic)
Thermal paste: silicone + metal oxide (fills air gaps)
Li-ion battery: lithium ions move between electrodes
  Anode: graphite (LiC6)
  Cathode: LiCoO2 or LiFePO4
  Electrolyte: organic solvent with Li salt

Conservation of mass: atoms in = atoms out

Example (rust):
  4Fe + 3O2 → 2Fe2O3

Mole: 6.022 × 10^23 particles (Avogadro's number)
Molar mass: mass of one mole in grams
  H2O: 2(1.008) + 16.00 = 18.02 g/mol

pH scale: measures hydrogen ion concentration
  pH = -log_10[H+]
  pH 7 = neutral
  pH < 7 = acidic (battery acid ≈ 1)
  pH > 7 = basic (bleach ≈ 13)

Conductor:       low resistivity, free electrons (Cu, Ag, Au)
                 ρ_Cu ≈ 1.7 × 10^-8 Ω·m

Insulator:       high resistivity, no free carriers (glass, rubber)
                 ρ_glass ≈ 10^12 Ω·m

Semiconductor:   middle ground, controllable (Si, Ge, GaAs)
                 ρ_Si ≈ 640 Ω·m (intrinsic)
                 Doping changes resistivity by orders of magnitude

Superconductor:  zero resistance below critical temperature
                 Tc for YBCO ≈ 93 K (-180°C)