Oxygen Family Chemistry
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Direct answer
Dioxygen is paramagnetic — two unpaired electrons sit in its antibonding pi orbitals, a fact molecular orbital theory explains and Lewis structures cannot — and that single property anchors the whole oxygen corner of Group 16. The hydrides order themselves by hydrogen bonding: H2O boils at 373 K, far above H2S's 213 K, though thermal stability and acidity then move the other way, H2Te being the least stable and most acidic. Sulphur's dioxide bleaches by reduction (a temporary bleach that oxygen reverses) and its trioxide converts to sulphuric acid in the contact process over V2O5 — the "king of chemicals" whose dibasic strength, dehydrating action on sugars and charring of paper are bench classics. Ozone, the stratosphere's ultraviolet shield, is diamagnetic with a bent structure and delocalised bonds between its two resonance forms.
What you must remember
- O2 paramagnetism: liquid oxygen sticks to a magnet; explained by two unpaired electrons in π* molecular orbitals (bond order 2).
- Hydride ladder: boiling points H2O >> H2S < H2Se < H2Te (hydrogen bonding first, then molecular mass); acidity and reducing character increase down; thermal stability decreases down.
- Contact process: 2SO2 + O2 ⇌ 2SO3 over V2O5 at about 720 K and 2 atm, then oleum H2S2O7 and dilution to H2SO4 — the standard NCERT route.
- Sulphuric acid bench behaviour: dibasic, charring sucrose to a black column of carbon, dehydrating copper sulphate crystals white, and highly exothermic dilution (acid into water, never the reverse).
- SO2 ambivalence: reducing toward KMnO4 (decolourising it), yet oxidising in other settings; its bleaching of moist litmus-type dyes is temporary because air re-oxidises the reduced dye.
- Ozone facts: bent molecule (117°), diamagnetic, resonance between two structures, formed in the stratosphere from O2 photolysis; a pollutant at ground level but essential aloft; oxidises iodide to iodine (the starch-iodide test paper reaction).
- Hydrogen peroxide strength: sold as "volume" hydrogen peroxide — 20 volume releases 20 times its volume of O2; H2O2 is stored in wax-lined glass with stabilisers because it disproportionates, and its oxidising power restores darkened paintings by converting black PbS to white PbSO4, the NCERT-cited application.
- Sulphur allotropy: rhombic (stable below 369 K) transforms to monoclinic — a classic equilibrium-temperature fact.
Reasoning the hydride anomalies
Ask which Group 16 hydride boils highest and the wrong answer is H2Te, the heaviest. Water wins because its O–H bonds are polar and short enough for strong hydrogen bonding, worth dozens of kelvin over its mass-proper place; below H2S, normal dispersion forces take over and boiling points climb with molecular mass. Now switch the question to acidity: H2Te is the strongest acid, because the long, weak H–Te bond releases its proton most easily. Thermal stability inverts again — H2Te decomposes on mild heating while H2O needs thousands of kelvin. Three properties, three different ladders, and NEET's matching columns deliberately set them side by side to catch single-ladder memorisers.
The same doublethink explains SO2's apparently contradictory redox résumé: in the presence of stronger oxidants (KMnO4, halogens), its +4 sulphur surrenders electrons and acts reduced to +6; facing stronger reductants, it accepts them. Oxidation state +4 sits mid-ladder, so direction depends on the partner — the exam's favourite "ambivalence" line.
Where the paper sets its Group 16 traps
Four fixtures recur. The paramagnetism of O2 arrives as an assertion-reason with molecular orbital theory as the reason — both true, both NCERT lines. The contact process conditions (about 720 K, 2 atm, V2O5) are asked as a matching pair with Haber's (about 700 K, 200 atm, iron), and candidates swap the pressures. Ozone questions split between structure (bent, 117°, resonance) and environment (it absorbs ultraviolet in the stratosphere; at ground level it is a component of photochemical smog) — the same molecule as shield above and pollutant below. Hydrogen peroxide items quote the volume definition, its disproportionation, and its NCERT-cited painting-restoration use: blackened lead sulphide converts to white lead sulphate (PbS + 4H2O2 → PbSO4 + 4H2O).
Frequently asked questions
Why is oxygen paramagnetic?
Two unpaired electrons occupy the degenerate π* antibonding orbitals, as molecular orbital theory shows — the property that invalidates a simple double-bond Lewis picture.
Why does water boil far above hydrogen sulphide?
Strong intermolecular hydrogen bonding between H2O molecules requires extra thermal energy to break; below water, group hydrides boil in order of molecular mass.
What conditions run the contact process?
Sulphur dioxide and air pass over a V2O5 catalyst at about 720 K and 2 atm; the SO3 formed is absorbed in concentrated H2SO4 to give oleum, then diluted.
Why is bleaching by sulphur dioxide temporary?
SO2 reduces the dye by removing its oxygen in the presence of moisture, and atmospheric oxygen slowly re-oxidises the colourless reduced product, restoring colour.
What does "20 volume" hydrogen peroxide mean?
One volume of the solution decomposes to liberate twenty volumes of oxygen at STP — the trade measure of H2O2 strength used for laboratory and pharmaceutical grades.