Grade 10

Halogens, oxygen–sulfur, nitrogen–phosphorus, carbon–silicon

Compare the chemistry of groups 17–14 through their valence patterns, characteristic molecules and industrially important compounds, then relate halogen redox behavior to thermodynamics.

IntuitionA map of the nonmetals

Elements in groups 17, 16, 15 and 14 share outer-electron patterns. Moving down a group adds an electron shell, changing size, bonding and reactivity; neighboring groups still show distinct chemical families.

This article compares their representative molecular forms and compounds: halogens, oxygen and sulfur, nitrogen and phosphorus, then carbon and silicon.

SchoolValence patterns and group trends

Down a group, atomic radius generally increases and electronegativity decreases. In group 17 the aqueous standard reduction potentials fall from F₂/F⁻ to I₂/I⁻, although phase, solvent and bond-energy terms all contribute.

SchoolRepresentative chemistry across the families

Halogens are diatomic. At room conditions F₂ is a pale-yellow gas, Cl₂ a yellow-green gas, Br₂ a red-brown liquid and I₂ a dark grey-violet solid (violet vapour). A more reactive halogen displaces a less reactive halide from solution.

Hydrogen-halide acid strength in water increases HF < HCl < HBr < HI as the H–X bond becomes easier to break. As reducing agents, the halide ions strengthen in the order F⁻ < Cl⁻ < Br⁻ < I⁻; do not confuse acid strength with the oxidising strength of X₂.

Silver nitrate gives AgCl white, AgBr cream and AgI yellow precipitates; dilute ammonia dissolves AgCl, concentrated ammonia dissolves AgBr less readily, and AgI is insoluble. Chlorine reacts reversibly with water to form hydrochloric acid and hypochlorous acid; hypochlorous acid provides bleach action.

Oxygen has O₂ and O₃ allotropes; ozone is a reactive, protective trace gas in the stratosphere. Elemental sulfur commonly occurs as S₈ rings. Burning sulfur forms pungent SO₂; catalytic oxidation to SO₃ and absorption in concentrated sulfuric acid are central to the contact process. Concentrated H₂SO₄ is a strong acid and, depending on conditions, a dehydrating and oxidising agent. H₂S is a toxic weak acid and reducing agent.

Nitrogen gas is relatively inert because its N≡N bond is very strong. The Haber–Bosch process combines N₂ and H₂ reversibly to make NH₃; the Ostwald process oxidises NH₃ to nitric acid. NO₂ is brown and dimerises to colourless N₂O₄ more at lower temperature. White phosphorus consists of P₄ molecules and is dangerously reactive; phosphoric acid H₃PO₄ and phosphate salts are important in fertilisers.

Carbon forms CO, a colourless toxic gas that binds haemoglobin, and CO₂, a linear greenhouse gas and product of complete combustion. Silicon dioxide is a giant covalent solid; silicates built from linked SiO₄ tetrahedra dominate many rocks. Crystalline silicon is a semiconductor whose conductivity can be tuned by doping.

UndergraduateRedox potentials and bond energetics

The Frost-diagram idea plots oxidation state against standard reduction potential (often through $nE^\circ$): the thermodynamically favored redox direction corresponds to a downhill change in free energy. For halogens, the large positive potentials make X₂ strong oxidants, with the trend F₂ > Cl₂ > Br₂ > I₂ in water.

Fluorine has an unusually weak F–F bond because the very short bond brings three lone pairs on each small atom into strong repulsion. The net standard potential also includes atomisation, electron-affinity and hydration contributions; the high hydration of F⁻ helps make $E^\circ(\ce{F2/F-})=+2.87\,V$ exceptionally positive. Thus the weak bond helps explain, but alone does not determine, fluorine’s oxidising strength.

UndergraduateQuantitative stoichiometry in the sulfur cycle

UndergraduateOxidation-state bookkeeping

Oxidation states are formal electron-bookkeeping assignments, not literal charges in covalent molecules. In the Ostwald first step N changes from −3 in NH₃ to +2 in NO, while O changes from 0 in O₂ to −2; later oxidation of NO gives NO₂, followed by absorption and reactions that produce nitric acid.