p-Block Elements and Hydrogen
This domain tests trends and important compounds of groups 15–18, oxoacid strength and structure, interhalogens, noble gas chemistry, and the isotopes, hydrides and bonding of hydrogen.
Group 15
NH₃ is pyramidal with a lone pair and strong hydrogen bonding. HNO₃ is made by the Ostwald process (NH₃ → NO → NO₂ → HNO₃), is a strong oxidising acid, gives NO₂ with copper and passivates Al and Fe when concentrated. White phosphorus (P₄, tetrahedral) is stored under water; red and black phosphorus are polymeric and less reactive. In oxoacids the number of P–OH groups, not P–H bonds, fixes basicity, so H₃PO₃ is diprotic and H₃PO₄ triprotic.
Group 16 and 17
Ozone is bent, diamagnetic and a powerful oxidant. H₂SO₄ is made by the contact process (V₂O₅, about 720 K, 2 bar); concentrated H₂SO₄ dehydrates and oxidises, gives SO₂ with Cu and passivates iron. Oxidising power: F₂ > Cl₂ > Br₂ > I₂. Acid strength of chlorine oxoacids rises with the number of terminal oxygens: HClO < HClO₂ < HClO₃ < HClO₄. Interhalogens include ClF₃, BrF₅ and IF₇; bleaching powder is Ca(OCl)₂·CaCl₂·Ca(OH)₂·2H₂O.
Group 18 and hydrogen
Xenon forms XeF₂, XeF₄ and XeF₆ with fluorine under pressure and forms clathrates with water. Hydrogen has three isotopes, ortho and para forms, and ionic, covalent and interstitial hydrides; HF shows the strongest hydrogen bond, and H₂O₂ both oxidises and reduces.
Worked example
Find the oxidation state of chlorine in HClO₄ and HClO, then decide which is the stronger acid. For HClO₄: 1 + x + 4(−2) = 0, so x = +7. For HClO: 1 + x − 2 = 0, so x = +1. The extra terminal oxygens delocalise the negative charge of the conjugate base ClO₄⁻, so HClO₄ is much the stronger acid.
Traps that cost marks
- Counting P–H bonds when deciding the basicity of H₃PO₃.
- Assuming fluorine can expand its octet or show variable oxidation states.
- Believing F₂ has the highest bond dissociation enthalpy (Cl₂ does, among the halogens).
- Writing bleaching powder as pure Ca(OCl)₂.