Hydrogen, s-Block and p-Block Elements
Hydrogen, s-Block and p-Block Elements tests hydrides, hydrogen peroxide, alkali and alkaline-earth trends, and the descriptive chemistry of Groups 13–18. Expect questions on anomalous behavior, oxides, hydroxides, and thermal stability.
Core rules
- Hydrogen forms ionic hydrides with active metals, covalent hydrides with nonmetals, and interstitial hydrides with transition metals.
- H2O2 acts as both oxidant and reductant; it disproportionates catalytically.
- Alkali metals form mainly oxides, but Li gives Li2O, Na gives Na2O2, and K, Rb, Cs give superoxides.
- Group 2 hydroxides become more soluble and more basic down the group; carbonates become more thermally stable down the group.
- p-Block anomalies: Be and B are small and covalent; N, O, F show limited coordination and strong hydrogen bonding.
Worked example
Which is more thermally stable, MgCO3 or BaCO3? BaCO3 is more stable. The carbonate ion decomposes when a small, highly charged cation polarizes it strongly. Mg2+ is smaller than Ba2+, so Mg2+ polarizes CO3^2− more and lowers the decomposition temperature. Thermal stability therefore increases down Group 2: BeCO3 < MgCO3 < CaCO3 < SrCO3 < BaCO3.
Traps
Do not say all alkali metals form peroxides; Li forms mainly the normal oxide. Do not confuse peroxide O2^2− with superoxide O2^−. Do not expect inert-pair effects to be equally strong for all heavy p-block elements. Diagonal relationships can mislead if you ignore charge and size.