Explain the following :
(i) Gallium has higher ionisation enthalpy than aluminium.
(ii) Boron does not exist as B^{3+} ion.
(iii) Aluminium forms[AlF_{6}]^{3-}ion but boron does not form[BF_{6}]^{3-} ion.
(iv)PbX_{2} is more stable than PbX_{4}
(v) Pb^{4+} acts as an oxidising agent but Sn^{2+} acts as a reducing agent.
(vi) Electron gain enthalpy of chlorine is more negative as compared to fluorine.
(vii)Tl (NO_{3})_{3} acts as an oxidising agent.
(viii) Carbon shows catenation property but lead does not.
(ix) BF_{3} does not hydrolyse.
(x) Why does the element silicon, not form a graphite like structure whereas carbon does.

Answers (1)

(i) The ionization energy of Al is lower than that of Ga and Ga has less d and f orbital electrons and thus has an increases nuclear charge in order to balance the screening effect.

(ii) The atomic size of Boron is small, and the energy of hydrogen is immensely high. Therefore Boron cannot ionize to form B3+  but instead forms covalent compounds.

(iii) The vacant orbitals in Al are the d orbitals which can expand, and the coordination number can rise to 6; thus Al undergoes sp3d2 hybridization to form [AlF6]3- ion, which is octahedral. But only 4 as the coordination number is possible and thus forms [BF4]  not [BF6]3-.

(iv) The +4 oxidation state of Pb is less stable than its +2 oxidation state given the inert pair effect.  PbX4 has the oxidation state of Pb is +4 and is less stable than PbX2, which has the oxidation state of Pb is +2.

(v) Pb has a more dominant inert pair effect than Sn. Pb4+   can gain 2 electrons to become Pb 2+  which is more stable and then Sn2+  when it loses 2 electrons. Thus Pb is an oxidizing agent, and Sn is a reducing one.

(vi) F has a small atomic size, and the repulsion between the electrons is quite strong, and any extra electron is not accepted and as the Cl atom where the repulsion is weak. Thus the energy given out by Cl through gaining an electron is much lesser and negative as compared to F.

(vii)The +3 oxidation state of Tl is less stable than its +1 oxidation state due to the inert pair, and it is a strong effect. In Tl(NO3)3, Tl’s oxidation state is +3; therefore, it can accept two electrons to form TlNO3. This will make the oxidation state of Tl is +1. Thus it is an oxidizing agent.

(viii)The strength of element-element bond determines the strength and properties of catenation. This also depends on the atomic size and thus if carbon has a small atomic size in comparison to Pb, then the strength between the carbon bonds is much higher than that of Pb-Pb bonds which explains carbon’s higher tendency for catenation.

(ix)BF_{3} + 3H_{2}O \rightarrow H_{3} BO_{3} + 3HF (x4)

H_{3} BO_{3} + 4HF \rightarrow H^{+} + BF_{4} ^{-} + 3H_{2}O (x3)

4 BF_{3} + 3H_{2}O \rightarrow H_{3} BO_{3} + 3[ BF_{4} ^{-}] + 3H^{+}       

BF_{3}  does not completely hydrolyze, and it forms boric and fluoroboric acid instead.

(x) Carbon is sp^{2}-hybridised to form graphite, and it forms hexagonal rings and links 3 other atoms. This leaves one p orbital which is unhybridized and forms double bonds instead. Thus this forms a layered structure and rings are fused together. Silicon is not formed out of carbon as it does not undergo the same hybridization or form p-p bonds but has three-dimensional network due to sp^{3}-hybridisation.

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