Group 16 (Oxygen family) — allotropes of O (O₂, O₃); ozone chemistry, ozone layer

1. Allotropy — WHAT it is

WHY they differ: same atoms, but different number of atoms per molecule and different bond arrangement → different bond order, energy, colour, reactivity.

2. Dioxygen O₂

MOT electron filling for O₂ (16 electrons): $\sigma_{1s}^2\,\sigma_{1s}^{*2}\,\sigma_{2s}^2\,\sigma_{2s}^{*2}\,\sigma_{2p_z}^2\,\pi_{2p_x}^2\pi_{2p_y}^2\,\pi_{2p_x}^{*1}\pi_{2p_y}^{*1}$

3. Ozone O₃ — structure & bonding

HOW it's made (lab): $3O_2 \xrightarrow{\text{silent electric discharge}} 2O_3 \qquad \Delta H = +142\ \text{kJ mol}^{-1}\ (\text{endothermic})$

Figure — Group 16 (Oxygen family) — allotropes of O (O₂, O₃); ozone chemistry, ozone layer

4. Ozone chemistry — key reactions (WHY each works)

All driven by $O_3 \to O_2 + [O]$ , supplying oxidising power.

(a) Oxidises lead sulphide → lead sulphate (no [O] symbol needed): $PbS + 4O_3 \rightarrow PbSO_4 + 4O_2$

(b) Oxidises iodide → iodine (basis of estimation): $2KI + O_3 + H_2O \rightarrow 2KOH + I_2 + O_2$ Why this matters: liberated $I_2$ is titrated with thiosulphate → quantitative estimation of ozone.

(c) Oxidises moist potassium iodide / mercury (loses meniscus — "tailing of mercury"): $Hg + O_3 \rightarrow$ surface oxidation makes Hg stick to glass.

(d) Acts as bleach & germicide because of [O].

5. The Ozone Layer — WHY it protects us

6. Common mistakes (Steel-man + fix)

7. Active Recall

Recall Quick self-test (cover answers)

Bond order of O₂? → 2, paramagnetic.
Bond order of each O–O in O₃? → 1.5.
Shape & angle of O₃? → bent, ~117°.
Why is O₃ a strong oxidant? → releases nascent [O].
Net CFC catalytic equation? → $O_3 + O \to 2O_2$ , Cl regenerated.

Recall Feynman: explain to a 12-year-old

Oxygen is like LEGO. Two blocks together = the air you breathe (O₂). Three blocks stuck together = ozone (O₃), which is a bit grumpy and falls apart easily, throwing off one "angry" oxygen block that grabs onto other things (it bleaches and kills germs). Way up in the sky a blanket of these three-block pieces eats up the Sun's harmful invisible rays so we don't get burned. Some chemicals from old fridges and sprays sneak up there and smash the blanket — one troublemaker chemical keeps wrecking piece after piece, like a kid who never gets tired of knocking down towers.

8. Flashcards

What is allotropy?

Existence of an element in two or more forms in the same physical state (e.g. O₂ and O₃).

How is ozone prepared in the lab?

Passing O₂ through a silent electric discharge: 3O₂ → 2O₃ (endothermic).

Bond order and magnetism of O₂ from MOT?

Bond order 2; paramagnetic (2 unpaired e⁻ in π*₂p).

Shape, angle and bond order of O₃?

Bent V-shape, ~117°, each O–O bond order 1.5 (resonance hybrid).

Why is ozone a powerful oxidising agent?

It decomposes O₃ → O₂ + [O], releasing reactive nascent oxygen.

Reaction of O₃ with KI?

2KI + O₃ + H₂O → 2KOH + I₂ + O₂ (used to estimate ozone).

Why is ΔH of ozone formation positive?

Formation is endothermic; ozone is thermodynamically unstable (stores energy).

Chapman reactions forming stratospheric ozone?

O₂ →(UV) 2O; O + O₂ → O₃; O₃ →(UV) O₂ + O.

How do CFCs destroy ozone?

UV releases Cl•; Cl•+O₃→ClO•+O₂; ClO•+O→Cl•+O₂; Cl regenerated (catalytic).

Net CFC ozone-destruction equation?

O₃ + O → 2O₂ with Cl acting as catalyst.

Difference between stratospheric and tropospheric ozone?

Stratospheric = protective UV shield; tropospheric = harmful pollutant.

Why are O₃'s two bonds equal length?

It is a resonance hybrid, not a single fixed structure, so both bonds are identical (~128 pm).

9. Connections

Molecular Orbital Theory — explains O₂ paramagnetism & bond order
Resonance and Delocalisation — basis of O₃ structure
Oxidation and Reducing Agents — ozone as oxidant via [O]
Free Radical Chain Reactions — CFC–ozone catalytic cycle
Environmental Chemistry — Air Pollution — ozone hole, smog
Group 16 — Oxides and Oxoacids — wider oxygen family chemistry

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, oxygen do roop mein aata hai — yeh hi allotropy hai. Normal saans wali gas O₂ hai, aur teen atom wali O₃ yaani ozone hai, jo halki neeli, smelly aur bahut reactive hoti hai. O₂ ka structure MOT se samjho: bond order 2 nikalta hai aur do unpaired electrons π* orbital mein baithe hote hain, isliye O₂ paramagnetic hai — yeh Lewis structure se samajh nahi aata, sirf MOT batata hai.

Ozone bent hota hai (~117°), kyunki central oxygen pe 2 bond pair + 1 lone pair hain (sp²). Iske dono O–O bonds barabar hote hain kyunki yeh resonance hybrid hai, bond order 1.5. Important baat: ozone banana endothermic hai (ΔH +ve), matlab usme extra energy stored hai, isliye woh aasaani se toot ke $O_3 \to O_2 + [O]$ deta hai. Yeh nascent oxygen [O] hi usko powerful oxidising agent aur bleach/germicide banata hai. KI ke saath reaction se I₂ nikalta hai jisse ozone ki estimation hoti hai.

Ab ozone layer ki baat — stratosphere mein UV se O₂ tootta hai, atoms milke O₃ banate hain, phir UV usse todta hai (Chapman cycle). Iss process mein harmful UV-B/UV-C absorb ho jaata hai, toh hum surakshit rehte hain. Lekin CFC (fridge/spray gases) upar jaake UV se Cl radical chhodte hain. Yeh Cl ek catalyst ki tarah baar-baar O₃ todta hai aur khud wapas ban jaata hai — isliye thoda sa CFC hazaaron ozone molecules barbaad kar deta hai, aur ozone hole ban jaata hai. Yaad rakho: upar wala ozone protector, neeche zameen wala ozone pollutant.

Group 16 (Oxygen family) — allotropes of O (O₂, O₃); ozone chemistry, ozone layer

1. Allotropy — WHAT it is

2. Dioxygen O₂

3. Ozone O₃ — structure & bonding

4. Ozone chemistry — key reactions (WHY each works)

5. The Ozone Layer — WHY it protects us

6. Common mistakes (Steel-man + fix)

7. Active Recall

8. Flashcards

9. Connections

Concept Map

Hinglish (regional understanding)

Go deeper — visual, from zero

Test yourself — p-Block

Connections