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p-Block Elements: Part I (Group 13 & 14) revision notes

Every idea in the p-Block Elements: Part I (Group 13 & 14) chapter for JEE Main, with the usual mistakes and the fast methods. Topic 1 is open to everyone; sign in free to read all 14 and download the PDF.

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Group 13: trends and oxidation states

Group 13 at a glance
11× asked, last 2025
r (pm): B 85<Ga 135<Al 143<In 167<Tl 170r(MX3+): B 27<Al 53.5<Ga 62<In 80<Tl 88.5ΔiH1 (kJ mol−1): B 801>Tl 589>Ga 579>Al 577>In 558ΔiH2 (kJ mol−1): B 2427>Ga 1979>Tl 1971>In 1820>Al 1816m.p. (K): B 2453>Al 933>Tl 576>In 430>Ga 303\begin{gathered}r\ (\text{pm}):\ \text{B }85<\text{Ga }135<\text{Al }143<\text{In }167<\text{Tl }170\\ r(\ce{M^{3+}}):\ \text{B }27<\text{Al }53.5<\text{Ga }62<\text{In }80<\text{Tl }88.5\\ \Delta_iH_1\ (\text{kJ mol}^{-1}):\ \text{B }801>\text{Tl }589>\text{Ga }579>\text{Al }577>\text{In }558\\ \Delta_iH_2\ (\text{kJ mol}^{-1}):\ \text{B }2427>\text{Ga }1979>\text{Tl }1971>\text{In }1820>\text{Al }1816\\ \text{m.p. (K)}:\ \text{B }2453>\text{Al }933>\text{Tl }576>\text{In }430>\text{Ga }303\end{gathered}
ns2np1ns^2np^1. EN does not fall steadily: B 2.0, Al 1.5, Ga 1.6, In 1.7, Tl 1.8, from poor dd/ff shielding.
rr (pm)BAlGaInTl50100150atomMX3+\ce{M^{3+}}
Atoms (green): Ga below Al. Ions MX3+\ce{M^{3+}} (amber): normal order
kJ mol⁻¹BAlGaInTl5001000150020002500ΔiH1\Delta_iH_1ΔiH2\Delta_iH_2
ΔiH2\Delta_iH_2 (amber): Ga above Al; Al lowest of all
2453B933Al303Ga430In576Tlm.p. (K)
Ga melts at 303 K (yet boils at 2676 K)
  • Ga melts at 303 K but boils at 2676 K, a very wide liquid range, so it fills high-temperature thermometers.
  • Not Tl > In > Ga > Al > B for atomic radius. Ga (135 pm) is smaller than Al (143), as its ten 3d3d electrons shield poorly. Ions keep the normal order: GaX3+\ce{Ga^{3+}} (62) > AlX3+\ce{Al^{3+}} (53.5), one more filled shell.
  • Not B > Al > Ga for ΔiH2\Delta_iH_2. Ga (1979) exceeds Al (1816), as its ten 3d3d electrons shield poorly. Al has the lowest ΔiH2\Delta_iH_2 in the group.
Boron: the anomalous first member
5× asked, last 2024
4 B+3 OX2→Δ2 BX2OX3,2 B+NX2→Δ2 BN\ce{4B + 3O2 ->[\Delta] 2B2O3},\qquad \ce{2B + N2 ->[\Delta] 2BN}
Isotopes 10^{10}B (19%), 11^{11}B (81%). Hard black non-metal, covalent network solid. Boron fibres go into bullet-proof vests.
[BFX4]X−\ce{[BF4]-}: B has no dd orbitals, so 4 at most
[AlFX6]X3−\ce{[AlF6]^{3-}}: Al uses 3d3d, so 6
  • Not 'B is too small' for the missing [BFX6]X3−\ce{[BF6]^{3-}}. B has no dd orbitals, so its covalency stops at 4 ([BFX4]X−\ce{[BF4]-}). Al uses 3d3d orbitals and forms [AlFX6]X3−\ce{[AlF6]^{3-}}.
Oxidation states and the inert pair effect
8× asked, last 2025
stability of +1: Al<Ga<In<Tl,TlX+>TlX3+E∘(AlX3+/Al)=−1.66 V,E∘(TlX3+/Tl)=+1.26 V\begin{gathered}\text{stability of }+1:\ \text{Al}<\text{Ga}<\text{In}<\text{Tl},\qquad \ce{Tl+}>\ce{Tl^{3+}}\\ E^\circ(\ce{Al^{3+}/Al})=-1.66\ \text{V},\qquad E^\circ(\ce{Tl^{3+}/Tl})=+1.26\ \text{V}\end{gathered}
Down the group the ns2ns^2 pair stays out of bonding (poor dd/ff shielding). B and Al are +3 only; +1 is Tl's main state.
-2-10+1+2AlX3+/Al\ce{Al^{3+}/Al} −1.66 VAl³⁺: stableTlX3+/Tl\ce{Tl^{3+}/Tl} +1.26 VTl³⁺: oxidant → Tl⁺E∘E^\circ (V)
Down the group the ns2ns^2 pair holds back: +3 gives way to +1 at Tl
  • AlX3+\ce{Al^{3+}} is very stable and hard to reduce. TlX3+\ce{Tl^{3+}} is unstable in solution and a strong oxidising agent, as it readily drops to the stable TlX+\ce{Tl+}. So 'both AlX3+\ce{Al^{3+}} and TlX3+\ce{Tl^{3+}} are stable' is false.
  • Quick method. For an odd formula, find the stable anion first, then give the leftover metal its charge: TlIX3\ce{TlI3} is TlX+ IX3X−\ce{Tl+ I3-}, not TlX3+\ce{Tl^{3+}} (IX−\ce{I-} would reduce TlX3+\ce{Tl^{3+}}).
Trihalides: Lewis acidity and dimers
4× asked, last 2024
Lewis acidity: BFX3<BClX3<BBrX3<BIX3(back-bonding the reverse)BFX3+NHX3→FX3B ⋅ NHX3,2 AlClX3→AlX2ClX6\begin{gathered}\text{Lewis acidity: }\ce{BF3}<\ce{BCl3}<\ce{BBr3}<\ce{BI3}\quad(\text{back-bonding the reverse})\\ \ce{BF3 + NH3 -> F3B*NH3},\qquad \ce{2AlCl3 -> Al2Cl6}\end{gathered}
MXX3\ce{MX3} has an empty pp orbital, so it is a Lewis acid. AlClX3\ce{AlCl3} dimerises: two Cl bridge, each Al sp3sp^3.
BNFFFHHHlone pairB: sp2sp^2 → sp3sp^3
FX3B←NHX3\ce{F3B<-NH3}: N gives its lone pair
AlAlClClClClClClbridging Cl
AlX2ClX6\ce{Al2Cl6}: two bridging Cl (amber); each Al has 4 Cl
  • Covalent MXX3\ce{MX3} hydrolyse in water to tetrahedral [M(OH)X4]X−\ce{[M(OH)4]-} (M sp3sp^3). In acidified water AlClX3\ce{AlCl3} gives octahedral [Al(HX2O)X6]X3+\ce{[Al(H2O)6]^{3+}}, where Al uses its 3d3d orbitals (sp3d2sp^3d^2).
  • Not BFX3\ce{BF3} strongest because F is most electronegative. F's 2p2p matches B's 2p2p, so back-bonding fills the empty orbital most. The large 5p5p of I overlaps least, making BIX3\ce{BI3} the strongest acid.

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Practise p-Block Elements: Part I (Group 13 & 14)

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Boron compounds, Aluminium and alums, Group 14: trends and oxidation states, Allotropes of carbon and the oxides and Silicon compounds. Read the first part free. Sign in free with Google to read the rest and download the PDF.

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Boron compounds

  • Borax and boric acid5×
  • Diborane and borazine6×

Aluminium and alums

  • Aluminium and alums3×

Group 14: trends and oxidation states

  • Group 14 at a glance4×
  • Catenation and covalency3×
  • +2 versus +4: the inert pair again4×

Allotropes of carbon and the oxides

  • Allotropes of carbon2×
  • Nature of the oxides; CO and CO27×
  • Carbon monoxide and carbon dioxide

Silicon compounds

  • Silicon compounds and silicones2×

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