Physical Science: Chemical Elements, Compounds and Reactions, Molecular Structure and Bonding
This domain tests the periodic table, bonding and molecular shape, reaction types, and quantitative stoichiometry.
Periodic table and trends
Elements are ordered by atomic number. Groups share valence counts: alkali metals (1), alkaline earths (2), halogens (7), noble gases (usually 8 valence electrons, with helium having 2). Across a period, atomic radius decreases while ionization energy and electronegativity increase; down a group, radius increases and ionization energy decreases.
Bonding and structure
Ionic bonds transfer electrons between metals and nonmetals, forming lattices with high melting points. Covalent bonds share electrons; unequal sharing gives polar bonds and dipoles. Metallic bonding delocalizes electrons. VSEPR predicts shape from electron domains: 2 = linear, 3 = trigonal planar, 4 = tetrahedral, with lone pairs giving trigonal pyramidal or bent. Hydrogen bonding helps explain water's relatively high boiling point. There is no universal strength ordering across all substances: dispersion grows with polarizability and molecular size and can dominate comparisons.
Reactions and stoichiometry
Balance equations to conserve atoms. One mole = 6.022 × 10²³ particles; molar mass sums atomic masses. Types include synthesis, decomposition, single and double replacement, combustion, neutralization, and redox (oxidation numbers change). The limiting reagent is consumed first and sets the yield. pH = −log[H+]; strong acids dissociate fully. Le Châtelier's principle predicts equilibrium shifts.
Worked example
Mix 4 g H2 (molar mass 2, so 2 mol) with 16 g O2 (molar mass 32, so 0.5 mol) for 2H2 + O2 → 2H2O. The ratio needs 2 mol H2 per 1 mol O2, so 0.5 mol O2 consumes only 1 mol H2 - oxygen is limiting and hydrogen is in excess. Product: 2 × 0.5 = 1 mol H2O = 18 g. Reasoning: convert to moles first, then divide by coefficients.
Traps
Comparing masses rather than moles to find the limiting reagent; forgetting H2, O2, N2, F2, Cl2, Br2, and I2 are diatomic; treating pH as linear (pH 3 is 100× more acidic than pH 5); assuming nonpolar molecules lack intermolecular forces (dispersion always exists); ignoring lone pairs in VSEPR.