Q.Boron behaves quite differently from the rest of the Group 13 elements (Al, Ga, In, Tl), even though all share the outer configuration $ns^2np^1$. Give three structural or chemical reasons for this anomalous behaviour of the first member of the group.

[!TLDR] Boron is exceptionally small, has no accessible d orbitals, and forms only covalent (never simple cationic) bonds — three features none of Al, Ga, In or Tl fully share. [!ANSWER] Boron's anomalous behaviour comes from (1) its unusually small atomic radius versus even aluminium, (2) the complete absence of low-lying d orbitals, capping its coordination number at 4, and (3) its exclusively covalent bonding, since its high ionisation enthalpy and small size prevent formation of a simple $\text{B}^{3+}$ cation.

Although boron, aluminium, gallium, indium and thallium all share the outer configuration $ns^2np^1$, boron's chemistry is markedly different from the four metals below it, for three closely related reasons. First, size: boron's atomic radius (about 85 pm) is exceptionally small, and the increase from boron to aluminium (about 143 pm) is unusually large by percentage compared with typical group-to-group increases lower in the group — boron sits at the classic 'first-member anomaly' seen at the top of several p-block groups, because it has no intervening $d$ or $f$ electrons to poorly shield a comparatively high effective nuclear charge for its size. Second, absence of d orbitals: boron's valence shell is $n=2$, which has only $s$ and $p$ sub-shells and no accessible $d$ orbitals, so boron can never expand its coordination number beyond 4 (as seen in $\text{BF}_4^{-}$); aluminium and the heavier members, by contrast, are larger and in various compounds can access higher coordination. Third, bonding character: boron's small size and correspondingly high ionisation enthalpy mean it never loses all three valence electrons to form a genuinely ionic $\text{B}^{3+}$ cation — its bonding is essentially always covalent (as in $\text{BF}_3$, $\text{B}_2\text{O}_3$, borates and boranes) — whereas aluminium's compounds show at least partial ionic character in many contexts. Together, these three features (extreme smallness, no $d$-orbital participation, and purely covalent bonding) explain why boron behaves as a nonmetal/metalloid with electron-deficient, Lewis-acidic chemistry, sharply distinct from the amphoteric-to-basic metallic chemistry of aluminium and its heavier congeners. [!ANSWER] Boron's small size, lack of accessible d orbitals (capping coordination at 4), and exclusively covalent bonding together set it apart from the more metallic, amphoteric chemistry of Al, Ga, In and Tl.

Compare boron's atomic radius, available orbitals and bonding character against the trend expected simply from a shared $ns^2np^1$ configuration, identifying the structural/electronic reasons its chemistry diverges from the rest of the group.

Attributing the anomaly only to size without also noting the d-orbital and covalent-bonding consequences; assuming all Group 13 elements form simple $M^{3+}$ ions the way boron notably does not.