Elements of Groups 16, 17 and 18
Elements of Groups 16, 17 and 18 tests periodic trends, preparation and properties of oxygen, sulphur, halogens and noble gases, oxoacids and interhalogen compounds.
Core rules
- Down Group 16: atomic radius increases, ionisation enthalpy decreases, metallic character increases; oxygen is a gas, sulphur a solid.
- Halogens: oxidising power F2 > Cl2 > Br2 > I2; fluorine shows only -1 and 0 in compounds, never positive oxidation states.
- Acid strength of oxoacids increases with more oxygen and greater stability of the conjugate base: HClO4 > HClO3 > HClO2 > HClO.
- Interhalogens: ClF3, BrF5, IF7; the more electronegative halogen carries the negative oxidation state.
- Noble gases: Xe forms XeF2, XeF4 and XeF6; He, Ne and Ar form no stable compounds under ordinary conditions.
Worked example
Explain why H2O is liquid but H2S is gas. Oxygen is small and highly electronegative, so water forms extensive hydrogen bonds; sulphur is larger and less electronegative, so H2S has only weak van der Waals forces. Reasoning: boiling point depends on intermolecular forces, not on molar mass alone.
Traps
Assigning a positive oxidation state to fluorine, naming OF2 as an oxide of fluorine instead of oxygen difluoride (O is +2), and assuming all noble gases form compounds. Acid strength follows conjugate-base stability, not the number of hydrogen atoms.