Chemistry NCERT Exemplar Solutions Class 11th Chapter Eleven

This is a Long Answers Type Questions as classified in NCERT Exemplar

In BF₃, due to n−pπ back bonding between the vacant p-orbital of boron and filled p-orbital of fluorine. This Π− pπ back bonding is absent in case of hydrogen as it is a single electron element.

Two BH₃ molecules dimerise to form diborane.

In B₂H₆ There are two types of hydrogens present.

(I) Four hydrogens that are terminally bonded to each of two boron atoms.

(II) Two hydrogens that are bonded to both boron atoms forming a bridge in between.

The four terminal hydrogen atoms and two boron atoms lie in the same plane while bridging hydrogen lies in a plane perpendicular to them.

Two hydrogens forming a bridge in B₂H₆ are peculiar in bonding and can be termed as 3 -centered-2-electron bond or banana bond. 1sorbital  of each hydrogen overlaps with the hybrid orbital of one of the boron then delocalising the 2e−over three atoms making 3 -centered- 2 -electron bond.

This is a Long Answers Type Questions as classified in NCERT Exemplar

(i) ln? AlCl₃, Aluminium is bonded to 3 chlorine atoms through three covalent bonds and six electrons are shared in the structure. To have complete Octet, AlCl₃ lacks two electrons & hence act as an electron acceptor substance or Lewis acid.

(ii) Undoubtedly, fluorine has more electronegativity than chlorine, BF₃ is stronger Lewis acid than BCl₃ because of n−p π back bonding in BF₃, both the constituting atoms boron and fluorine are involve p-orbital in back bonding. On moving down the group, the size of halogen atoms increases, back bonding decreases and Lewis acid character also decreases.

_(iii) PbO₂, lead is in +2 oxidation state and can be reduced to its most stable oxidation state of +2. Hence PbO₂ can be further exidised or act as a strong oxidising agent. and in SnO, Sn is and +2 oxidation state and can extend its oxidation state up to +4. Thus, SnO can be used as a reducing agent.

_(iv) As we move down the group in group 13 the participation of s-electrons in bond formation decreases the primary reason behind this is the inert pair effect. In this the p-electrons take part in bond formation and more energy is required to unpair the valence electrons to make them participate in bonding. Due to this the lower oxidation state of elements becomes stable than the higher oxidation state. As for thallium, +1 oxidation state is more stable than +3.

This is a Long Answers Type Questions as classified in NCERT Exemplar

(i) Grp-13 The atomic size of the boron family follows the irregular trend. Generally, down the group the size increases but Gallium has a smaller atomic radius than Aluminium due to the poor shielding effect of 3d-orbitals.

Order: B

Grp-14 The size of the carbon family is smaller than the modern family and as we move down the group the atomic size increases regularly. The increase in covalent radius from carbon to silicon is prominent while from Silicon to lead a small increase in covalent radius is observed; this is due to the presence of completely filled D and f-orbital in the heavier members.

Order: C

(ii) Ionization enthalpy: In group 13, the standardised trend of decrease of Ionisation enthalpy is not monitored. 

As we move from Boron to Aluminium the atomic size increases and ionisation enthalpy decreases but when we move ahead from Aluminium to Gallium, the screening effect of 3d electrons comes into play. The poor shielding effect of the electrons lead to the increase in nuclear charge on the valence electrons and results in increase of ionisation enthalpy.

Moving from gallium to Indium, due to The shielding effect of 4d electrons the ionisation enthalpy decreases. 

From Indium to thallium, 4f electrons come into action their poor shielding effect increases the effective nuclear charge on valence electrons hence the ionisation of Thallium energy further increases. 

The decreasing order of ionization enthalpy for group-13 elements will be: B>Ti>Ga>Al>In

As we move down in group 14 the ionisation enthalpy decreases in the order: C>Si>Ge>Pb>Sn

On moving down the group the size of the atom increases which results in the large decrease in ionisation energy from carbon to silicon but as we move from silicon to heavier metals small decrease in the ionisation energy is observed. This is mainly due to the less screening effect of d-electrons in germanium and tin, and due to fully filled d & f electrons in lead.

(iii) Metallic character

The metallic character in group 13 follows an unusual trend. Metallic character First increases from Boron to aluminium and then decreases from aluminium to thallium. This decrease is majorly due to the poor shielding effect of d and f electrons present in the heavier metals.

In group-14, as we move down the group, the metallic character increases.

(iv) Possible oxidation states Group 13 elements are from +3 to -5. But the stability of +1 oxidation state increases down the group due to inert their effect than the stability of +3 oxidation state. 

Similarly, in group 14, The common oxidation states shown by the carbon family are +2 and +4. Due to the inert pair effect, stability of +2 oxidation state increases then the stability of +4.

(v) Except thallium, All the elements of the boron family combine with three atoms of halogens to form trihalides. All these halides exist as molecular species with hybridisation of sp². All the trihalides are Lewis acid and their acidic strength of trihalides of boron is in order: Bf₃33 hybridisation state and corresponding tetrahedral shape.

There are few exceptions also, SnF₄ and Pf₄ are ionic in nature, while the rest are covalent compounds.

This is a Long Answers Type Questions as classified in NCERT Exemplar

Grp-13 The atomic size of the boron family follows the irregular trend. Generally, down the group the size increases but Gallium has a smaller atomic radius than Aluminium due to the poor shielding effect of 3d-orbitals.

Order: B

Grp-14 The size of the carbon family is smaller than the modern family and as we move down the group the atomic size increases regularly. The increase in covalent radius from carbon to silicon is prominent while from Silicon to lead a small increase in covalent radius is observed; this is due to the presence of completely filled D and f-orbital in the heavier members.

Order: C