What NEET Asks
- Typically 2-3 questions, often tricky, directly from NCERT.
- Focus on general trends (atomic size, ionization enthalpy, electronegativity, electron gain enthalpy), anomalous behaviour of first elements, and oxidation states.
- Reactions and properties of hydrides, oxides, and halides, especially Group 15, 16, and 17 compounds, are frequently tested.
Key Points
- Inert Pair Effect: Stabilises lower oxidation states (e.g., +3 for Group 15, +4 for Group 14, +2 for Group 16) down the group for heavier elements.
- Anomalous Behaviour: First element (N, O, F) in each group differs significantly due to small size, high electronegativity, high ionization enthalpy, and absence of d-orbitals.
- Hydrides (Group 15-16): Thermal stability decreases down the group. Reducing character increases down the group. Basicity of Group 15 hydrides decreases, while acidic character of Group 16 hydrides increases down the group.
- Halogens (Group 17): Reactivity order: F₂ > Cl₂ > Br₂ > I₂. Bond dissociation enthalpy order: Cl₂ > Br₂ > F₂ > I₂ (Fluorine is an exception).
- Noble Gases (Group 18): Highly stable due to complete octet. Xenon forms compounds with highly electronegative elements like F₂ and O₂ (e.g., XeF₂, XeF₄, XeF₆, XeO₃, XeOF₄).
Must-Know Formula / Reaction
Ostwald Process for Nitric Acid (HNO₃):
4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g)(Catalytic oxidation of ammonia, Pt/Rh gauze, 500 K, 9 bar)2NO(g) + O₂(g) → 2NO₂(g)(Oxidation of nitric oxide)3NO₂(g) + H₂O(l) → 2HNO₃(aq) + NO(g)(Absorption of nitrogen dioxide in water)
Common Mistakes
- Students often confuse the trends for thermal stability, reducing power, and acidic nature of hydrides; remember them distinctly.
- Don't forget the anomalous behaviour of fluorine, especially its lower bond dissociation enthalpy compared to chlorine due to lone pair repulsions.
- Incorrectly predicting oxidation states or products in complex redox reactions involving p-block elements, particularly for those showing variable valency.
Rapid Revision
Focus on electronegativity, ionization enthalpy, and atomic radius trends. Remember the inert pair effect and anomalous behaviour of N, O, F. Quickly review important industrial processes (Haber, Ostwald, Contact) and the structures/hydrolysis reactions of noble gas compounds.