Revision: p-block Elements - II Chemistry HSC Science Class 12 Tamil Nadu Board of Secondary Education

General electronic configuration: ns² np³

• General Trends: Down the group → atomic size, density, boiling point increase, while ionisation enthalpy decreases.
• Melting Point Trend: Increases from N → As, then decreases from As → Bi.
• Electron Gain Enthalpy: Generally increases down the group (becomes more negative).
• Oxidation States: Show −3, +3, +5 → +5 stability decreases down the group due to the inert pair effect.

Important Compounds/Exceptions:

• PCl₅ exists as ionic solid → [PCl₄]⁺[PCl₆]⁻
• NF₃ is stable, while other trihalides are unstable
• Bond strength: PCl₃ > AsCl₃ > SbCl₃

Special Reactions:

• PH₃ is not spontaneously inflammable (unlike impure phosphine)
• With AgNO₃: forms Ag₃P (black ppt)
• With NH₃: forms (NH₄)₂PtCl₆ (yellow ppt)

Phosphorus Allotropes:

• White P: Reactive, glows in the dark (chemiluminescence), soluble in CS₂
• Red P: Stable, no glow
• Black P: Most stable, highest density, polymeric

General electronic configuration: ns² np⁴

• General Trends: Down the group → atomic size increases, while electronegativity and ionisation energy decrease.
• Oxidation States: Common states → −2, +2, +4, +6 (oxygen mainly −2; heavier elements show positive states).
• Chemical Reactivity: Decreases down the group → O > S > Se > Te > Po.

Hydrides Trend (H₂E):

• Bond angle decreases: H₂O > H₂S > H₂Se > H₂Te
• Thermal stability decreases down the group
• Acidic character increases: H₂O < H₂S < H₂Se < H₂Te

Sulphur Allotropes:

• Rhombic (α): Most stable, prepared from CS₂ solution
• Monoclinic (β): Formed by melting sulphur
  Both consist of S₈ ring structure

Important Compounds & Reactions:

• H₂S: Poisonous, reducing agent
• SO₂: Acts as reducing + oxidising + bleaching agent
• O₃ (Ozone): Strong oxidising agent; converts KI → KIO₃/KIO₄
• O₂/O₃: Show allotropy

Sulphuric Acid (H₂SO₄):

• Dibasic acid
• Strong dehydrating agent
• Strong oxidising agent
• Low volatility & highly corrosive

General electronic configuration: ns² np⁵ 

Trends in Different Properties:

Electronic Configurations:

• General electronic configuration: ns² np⁶ (He = 1s²)
• Elements: He, Ne, Ar, Kr, Xe, Rn
• Noble gases have very high ionisation enthalpies and positive electron gain enthalpies — so they generally do not form compounds.
• First noble gas compound prepared by Neil Bartlett: \[\mathrm{O}_{2}(\mathrm{g})+\mathrm{PtF}_{6}(\mathrm{g})\longrightarrow\mathrm{O}_{2}^{+}[\mathrm{PtF}_{6}]^{-}(\mathrm{s})\] — since the ionisation enthalpy of Xe ≈ that of O₂, Xe was reacted similarly.

Shapes and Hybridisation of Xenon Compounds: