Chemistry
Atoms and the periodic table
Electron configuration fixes an element's position: group number reflects valence electrons, period reflects shells. Across a period, atomic radius decreases and electronegativity increases; down a group, the pattern reverses. Metals lose electrons to form cations; nonmetals gain electrons to form anions.
Bonding
Ionic bonding transfers electrons between a metal and a nonmetal, producing high melting points and conductive melts. Covalent bonding shares electrons and typically gives molecular compounds that conduct poorly.
Moles and stoichiometry
One mole equals 6.02 × 10²³ particles and equals the molar mass in grams. Balance the equation before applying mole ratios; the limiting reagent caps the product. Example: 36 g of water is 36 ÷ 18 = 2 mol of H₂O.
Acids, bases, and solutions
pH = −log[H⁺], so one pH unit is a tenfold change in hydrogen-ion concentration. Molarity is moles of solute per liter of solution.
Gases and energy
Ideal gas law: PV = nRT. At constant temperature, pressure and volume are inversely related. Exothermic reactions release heat and have negative ΔH.
Trap
The single most common error is balancing an equation by changing subscripts in a formula. Only coefficients may change.