ChemistryMediumClass 12

Group 16 — Oxygen family, allotropes of S

p-block Elements (Class 12 — Groups 15, 16, 17, 18)

JEE Qs

Hard

min

Master the general trends across the group, understand the specific reasons for oxygen's anomalous behavior, and memorize the structures and key reactions of sulfur allotropes and ozone.

🧮 Key Formulas

O2 (Dioxygen)

O3 (Ozone)

S8 (Rhombic and Monoclinic Sulfur - puckered ring structure)

S6 (Cyclo-S6 - chair form)

S2 (Sulfur vapor, paramagnetic)

3O2(g) \u21cc 2O3(g) (Formation/Decomposition of Ozone)

PbS + 4O3 \u2192 PbSO4 + 4O2 (Oxidation of lead sulfide by ozone)

2KI + H2O + O3 \u2192 2KOH + I2 + O2 (Oxidation of potassium iodide by ozone)

✅ Key Points for JEE

1Oxygen exhibits anomalous behavior (small size, high electronegativity, absence of d-orbitals) leading to significant differences from other group 16 elements, such as diatomic nature, strong hydrogen bonding in H2O, and a tendency to form -2 oxidation state.
2Sulfur displays diverse allotropes: rhombic (\u03b1-sulfur, stable below 369K) and monoclinic (\u03b2-sulfur, stable above 369K), both containing S8 puckered ring structures but differing in crystal packing. Plastic sulfur is an amorphous polymeric form.
3Ozone (O3) is an angular molecule with resonance-stabilized identical O-O bond lengths, making it a powerful oxidizing agent. Its reactions with substances like KI (forming I2), PbS (forming PbSO4), and nitrogen oxides are important.
4General periodic trends for Group 16 elements include a decrease in ionization enthalpy and electronegativity down the group, an increase in metallic character, and an increase in melting/boiling points (except for O2 due to its small size and weaker van der Waals forces).
5Common oxidation states for Group 16 elements (except oxygen) are +2, +4, and +6, facilitated by the availability of d-orbitals. Oxygen typically shows -2, -1 (peroxides), -1/2 (superoxides), and +2 (in OF2).

⚠️ Common Mistakes

✕Confusing the stability temperatures and interconversion conditions for rhombic and monoclinic sulfur, or incorrectly describing their structures (both are S8, but differ in crystal system).
✕Incorrectly predicting the products or balancing reactions involving ozone as an oxidizing agent, especially with common reducing agents like KI or PbS.
✕Failing to adequately explain the anomalous behavior of oxygen by overlooking the combined effect of its small size, high electronegativity, and the absence of d-orbitals for expansion of covalency.

NCERT Chapters

Class 12 Chemistry Part 1 Ch 7: The p-Block Elements