p-Block Elements: Halogens and Noble Gases (Group 17 & 18)

Understand oxidizing power trends, interhalogen structures, and xenon compound hybridisation for CBSE Boards and NEET/JEE.

Groups 17 (halogens: F, Cl, Br, I) and 18 (noble gases: He, Ne, Ar, Kr, Xe, Rn) close out the p-Block and are high-yield for NEET and JEE because their trends are sharp and highly predictable, from halogen reactivity to the once-surprising discovery that xenon forms real chemical compounds.

Halogens exist as diatomic molecules and show decreasing oxidising power down the group: F2 > Cl2 > Br2 > I2. This holds even though the F-F bond is unusually weak (due to strong lone pair-lone pair repulsion between the small fluorine atoms), because the overall process is dominated by the very high hydration and lattice energies of the small fluoride ion. Chlorine's oxoacids increase in strength as the oxidation state of chlorine rises: HOCl < HClO2 < HClO3 < HClO4, since more oxygen atoms stabilise the conjugate base through delocalisation. Chlorine gas is commonly prepared in the laboratory by heating MnO2 with concentrated HCl, and industrially by electrolysis of brine.

Interhalogen compounds form between two different halogens with the general formula XX'n (n = 1, 3, 5, or 7), such as ClF, BrF3, ClF3, IF5 and IF7, where X is the larger and less electronegative halogen. These compounds are more reactive than either parent halogen because the X-X' bond is weaker than the X-X bond in the corresponding halogen molecule. Their shapes follow VSEPR theory directly - BrF3, for instance, is bent T-shaped because bromine carries two lone pairs in its sp3d hybrid orbitals.

Noble gases owe their general inertness to a stable, filled ns2np6 valence configuration, but xenon - being the largest easily available noble gas with the lowest ionisation enthalpy - forms genuine compounds with strongly electronegative elements like fluorine and oxygen. XeF2 is linear (sp3d hybridisation), XeF4 is square planar (sp3d2), and XeF6 is distorted octahedral (sp3d3); all three are prepared by direct reaction of xenon with fluorine gas under different temperature, pressure and Xe:F2 ratio conditions. Dr. Aarzoo Saini walks students through these VSEPR-based structures step by step at We-Gyaan Classes Roorkee, since they are a near-guaranteed NEET/JEE structure question.