Inorganic Chemistry of the s- and p-Block Elements
Inorganic Chemistry of the s- and p-Block Elements
The exam tests group trends, the reactions of Groups 1, 2, 3, 4 and 7, the acid-base character of oxides and chlorides, thermal stability, and the identification of halide ions.
s-Block trends
Reactivity increases down Groups 1 and 2 as ionisation energy falls. Group 2 hydroxides become more soluble down the group while sulfates become less soluble, which is why BaSO4 is used as a radiological contrast medium and CaSO4 is sparingly soluble. Thermal stability of carbonates and nitrates increases down the group: the larger cation has lower charge density, polarises the anion less, so decomposition needs a higher temperature. Beryllium is anomalous: BeCl2 is covalent and polymeric, BeO is amphoteric.
p-Block: Period 3 and the halogens
Oxides change from basic (Na2O, MgO) through amphoteric (Al2O3) to acidic (SiO2, P4O10, SO2, SO3, Cl2O7) as metallic character falls. Chlorides: NaCl is ionic; MgCl2 is ionic with covalent character; AlCl3 is covalent and dimerises as Al2Cl6; SiCl4 is covalent and hydrolyses vigorously to SiO2 and HCl. Halogen oxidising power decreases down the group, so Cl2 displaces Br2 and I2 from their salts. Silver halides: AgCl is white and dissolves in dilute ammonia; AgBr is cream and dissolves in concentrated ammonia only; AgI is yellow and insoluble in ammonia.
Worked example
Explain why barium carbonate needs a higher temperature to decompose than magnesium carbonate. Compare the cations: Ba2+ has a larger radius than Mg2+ and lower charge density, so it distorts the carbonate electron cloud less. Weaker polarisation leaves the C-O bonds less weakened, and the small oxide ion is less stabilised relative to the larger carbonate. Both effects make decomposition less favourable, so a higher temperature is needed.
Traps
Calling Al2O3 acidic; forgetting SiO2 is giant covalent while SO3 is molecular; writing a hydrolysis equation for SiCl4 without HCl; assuming the halide test needs acidified AgNO3 for all three halides; and describing Group 2 trends without the polarisation explanation.
Key ideas
- Group 2 hydroxide solubility rises down; sulfate solubility falls.
- Thermal stability of Group 2 carbonates and nitrates rises down the group.
- Period 3 oxides slide from basic through amphoteric to acidic.
- Oxidising power of halogens falls down the group.
- AgCl white/AgBr cream/AgI yellow with distinct ammonia solubility.