Revision: Class 12 >> p-Block Elements NEET (UG) p-Block Elements

General electronic configuration: ns² np¹.

• All group 13 elements predominantly show a +3 oxidation state.
• Aluminium is the third most abundant element in Earth's crust (after oxygen and silicon).
• Boron is non-metallic; it does not react with non-oxidising acids but dissolves in strong oxidising acids like HNO₃ (conc.) and H₂SO₄ (conc.).

Trends in Physical Properties:

Trends in Chemical Properties:

• Group 13 elements are less reactive than Group 1 and 2 elements.
• All Group 13 elements form ionic compounds except boron, which forms covalent compounds.
• Reaction with oxygen: Boron forms a mixture of oxide (B₂O₃) and nitride (BN) when heated at 700°C in air. Oxygen has no effect on aluminium in normal conditions, but in moist air, a thin protective oxide layer forms on its surface.
• Reaction with water: Except for boron, Group 13 elements decompose in boiling water to produce hydrogen gas. Example: 2Al + 6H₂O → 2Al(OH)₃ + 3H₂↑
• Reaction with halogens: All Group 13 elements react with halogens to form trihalides (MX₃), except thallium (which prefers monohalides due to the inert pair effect).

Anomalous Properties of Boron:

Boron behaves differently from the rest of Group 13 because of:

• Small size – high polarising power
• High ionisation energy – does not form B³⁺ ions easily
• High electronegativity
• Absence of vacant d-orbitals – cannot expand its coordination number beyond 4

General electronic configuration: ns² np²

• Size & Energy Trends: Down the group, atomic and ionic radii increase, while ionisation energy decreases.
• Oxidation States: Show +4 and +2 states → +4 stable at top (C, Si), +2 becomes stable down the group (Sn, Pb) due to inert pair effect.
• Metallic Character: Increases down the group → C is non-metal, Si/Ge metalloids, Sn/Pb metals.
• Covalent Nature: Tendency to form covalent compounds decreases down the group as metallic character increases.
• Anomalous Behaviour of Carbon: Due to small size, high electronegativity, absence of d-orbitals, and high catenation.
• Catenation & Bonding: Carbon shows maximum catenation and strong covalent bonding, forming chains and rings.

Allotropes of Carbon:

• Crystalline: Diamond (hardest), Graphite (conducts electricity), Fullerene (spherical)
• Amorphous: Coke, Charcoal (adsorbent), Lampblack (~98–99% carbon)

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: