Units 1 and 2: Foundations of Chemistry
What you must understand
Units 1 and 2 build the language of chemistry: the structure of the atom, the periodic table, ionic and covalent bonding, the mole concept, stoichiometry, and the behaviour of metals, ionic compounds, water and gases. You need to move fluently between the macroscopic (masses, volumes, observations), the symbolic (balanced equations, formulas) and the particulate (atoms, ions, molecules) levels.
Core concepts
- Atomic structure and the periodic table: electron configurations explain trends in atomic radius, ionisation energy and electronegativity.
- Bonding: metals (lattice of cations in a sea of delocalised electrons), ionic (electrostatic lattice), covalent (shared electron pairs), and the intermolecular forces that determine melting and boiling points.
- The mole: n = m/M and n = N/N_A; concentration c = n/V; the molar volume of a gas at STP.
- Stoichiometry: mole ratios from balanced equations, limiting reagents and percentage yield.
- Reactions of metals and ionic solutions: displacement, acid–metal, acid–carbonate and precipitation reactions, written as full and net ionic equations.
Traps
- Forgetting to balance equations before using mole ratios.
- Mixing units: cm³ must be converted to L before c = n/V.
- Confusing empirical and molecular formulas.
- Treating all covalent substances as low-melting; network covalent solids are exceptions.
- Writing net ionic equations with spectator ions left in.
Rule of thumb
Always convert to moles first, then use the balanced equation, then convert back to the requested quantity. Track units throughout.
Worked example
What mass of CO₂ forms when 5.00 g of CaCO₃ reacts completely with excess HCl? M(CaCO₃) = 100.1 g/mol, M(CO₂) = 44.0 g/mol. n(CaCO₃) = 5.00/100.1 = 0.0500 mol. The equation CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂ gives a 1:1 ratio, so n(CO₂) = 0.0500 mol. m = 0.0500 × 44.0 = 2.20 g.