CBSE Class 11 Chemistry The P Block Elements Notes Set C

CLASS: 11 CHAPTER 11: THE P-BLOCK ELEMENTS

IMPORTANT POINTS:

 p-block elements: last electron enters outermost p-orbital

 6 groups (Gp 13-18)

 General electronic configuration: ns2np6

 Inert pair effect: Reluctance of the s-electrons of the valence shell to take part in bonding. Due to poor or ineffective shielding of the ns2 electrons, by the intervening d- and/or f-electrons

 Inert pair effect increases down a group

 Oxidation states: Elements down the group show lower oxidation states (2 units less) than highest group oxidation state

 1st element of the group differs from rest, due to

 small size

 large (charge/radius) ratio

 high ionization enthalpy

 high electronegativity

 unavailability of d-orbitals in valence shell

 ability to form pπ-pπ bond to itself or to other second row elements

Group-13 Elements (Boron family)

√ Electronic configuration: ns²np¹

√ Atomic radius increases down the group, but atomic radius of Ga is less than Al. This is due to poor shielding effect of the d-orbitals

√ Inert pair effect is more pronounced in Tl

√ Oxidation state: B and Al show state of +3 only

• In and Tl show states of both +1 and +3

•The stability of +3 oxidation state decreases while that of +1 oxidation state increases as we move down the group

• For Tl, +1 oxidation state is more stable than +3 (redox potential data)

Tl³⁺ (aq) + 2e^- → Tl⁺ (aq) E° = +1.25 V

• Stability of +1 oxidation state follows the trend: Al< Ga < In < Tl

√ Melting point of Boron is very high due to strong crystalline lattice

√ Halides: All the elements of group 13 (except Tl which forms thallous monohalides) form trihalides of the general formula MX3 where X = F, Cl, Br and I.

•All boron trihalides, i.e. BF₃, BCl₃, BBr₃ and BI₃ and aluminium trihalides i.e. AlCl₃, AlBr₃ and AlI₃ (AlF3 being ionic) are covalent compounds.

• Boron trihalides exist as only monomers, aluminium trihalides exist as dimmers

√ Nature of MX₃: have only six electrons in the valence shell, electron-deficient

• have strong tendency to accept a pair of electrons to acquire the nearest inert gas configuration

• behave as Lewis acids

• on hydrolysis, trichlorides form tetrahedral species [M(OH)4]-

√ Reaction with oxygen: Forms oxides with formula E2O3.

• B₂O₃ is acidic and reacts with basic oxides to form metal borates

• Al₂O₃ forms a thin layer over aluminium surface and prevents further oxidation

 Reaction with acids and alkalies: Aluminium shows amphoteric behaviour with acids and alkalies.

• Al dissolves in dilute HCl: 2Al + 6HCl → 2 Al³⁺ + 6 Cl^- + 3H₂

• Al reacts with alkali: 2Al + 2NaOH + 6H₂O → 2Na[Al(OH)₄]- + 3H₂

• With conc. HNO₃, forms a passive and protective oxide layer

√ Borax: (Na₂B₄O₇. 10 H₂O), It contains the tetranuclear units [B₄O₅ (OH)₄]^2- so, the correct formula is Na₂[B₄O₅ (OH)₄].8 H₂O

• dissolves in water to form an alkaline solution

Na₂B₄O₇ + 7 H₂O → 2NaOH + 4 H₃BO₃

•On heating it loses water and swells up, turns into transparent liquid which solidifies into glass like bead. Used in borax-bead test

Na₂B₄O₇. 10 H₂O (heat) → Na₂B₄O₇ (heat) →2NaBO₂ + B₂O₃

√ Orthoboric acid (H3BO3): prepared by acidifying aqueous solution of borax 

Na₂B₄O₇ + 2HCl + 5 H₂O → 2NaCl + 4 B(OH)₃

• Layered structure with planar BO₃ atoms joined by H-bonds

• Weak, monobasic, aprotonic lewis acid B(OH)₃ + 2HOH → [B(OH)₄]- + H₃O+

• On heating, B(OH)₃ forms metaboric acid (HBO₂) on further heating forms boric oxide (B₂O₃)

√ Diborane (B2H6): prepared by treating BF₃ with LiAlH₄

• Catches fire spontaneously in air, forms B₂O₃

• Readily hydrolysed to give boric acid

• Undergoes cleavage reactions with Lewis bases to give adducts B₂H₆ + 2 NMe₃ → 2 BH₃.NMe₃

• Further heating of B₂H₆.2 NH₃, gives Borazine (inorganic benzene) B₃N₃H₆

3 B₂H₆ + 6 NH₃ → 3[BH₂(NH₃)2]+[BH₄]- heat 2 B₃N₃H₆ + 12 H₂

• Structure: 4 terminal H-atoms and 2 B-atoms lie in one plane (regular 2 centre-two e- bond) with 2 bridging H-atoms above and below this plane (3 centre-two e- banana bond)

√ Uses of B and Al: Boron fibres are used in bullet-proof vest and light composite material for aircraft. absorb neutrons in nuclear reactors, heat resistant glasses, glasswool, antiseptic (orthoboric acid)

 Al forms alloys, in packing , utensils and construction

Group-14 Elements (Carbon family)

√ Electronic configuration: ns2np2

√ Atomic radius increases down the group, large increase from C to Si, small increase

afterwards, due to presence of completely filled d- and f-orbitals

√ Physical properties: C is non-metal, Si and Ge are metalloid and Sn and Pb are metals 

√ Inert pair effect is more pronounced in Sn and Pb

√ Oxidation state: Generally state of +2 and +4

• +4 state is shown by all elements. The stability of +4 oxidation state decreases while that of +2 oxidation state increases as we move down the group due to inert pair effect
• The stability of +2 state increases in the sequence Si<Ge< Sn < Pb.
• Compounds with +4 oxidation state are covalent in nature and are electron-precise molecules

√Covalency: Compounds having elements in oxidation state of +4 are covalent while with oxidation of +2 are ionic

• On going down the group, the tendency to form covalent compounds decreases
• Covalency of carbon is four as only s and p orbitals are present. Covalency is expanded in other elements as d-orbitals are present

√Catenation: Property of self-linking of atoms of an element through covalent bonds to form straight or branched chains and rings of different sizes is called catenation.

• Depends on the strength of the element-element bond
• The C-C bond strength is highest so, carbon show maximum tendency of catenation.
• On moving down the group, the size increases and electronegativity decreases. So, the tendency for catenation have the order C > > > Si > Ge ≈ Sn > > Pb

• Pb do not show any tendency for catenation

√pπ-pπ and pπ-dπ multiple bonding: Carbon has a unique ability to form pπ-pπ multiple bonds with itself (e.g. C=C, C≡C) and with other elements especially nitrogen and oxygen (C=O, C≡N).

• Other elements have negligible tendency to form multiple bonds as size of atomic orbitals increases

√Reaction with oxygen: Two types of oxides : (i) monoxides and (ii) dioxide

• Monoxide (MO): i.e. CO, SiO, GeO, SnO and PbO. SiO is stable at high temperature only
• CO is neutral, GeO is acidic and SnO and PbO are amphoteric
• Dioxides (MO2): i.e. CO2, SiO2, GeO2, SnO2 and PbO2.
• CO2, SiO2 and GeO2 are acidic and SnO2 and PbO2 are amphoteric

√Reaction with water: Sn decomposes water to form SnO2 and H2

• Pb not affected by water, as protective oxide film is present

√Reaction with halogen: Two types of halides: (i) dihalide and (ii) tetrahalide

• Dihalide (MX2): Stability of these dihalides increases as we move down the group from C to Pb. So, SnCl2 and PbCl2 are quite stable
• Generally ionic in nature and behave as reducing agent
• Tetrahalides (MX4): Covalent compounds except SnF4 and PbF4, tetrahedral structure.
• Stability of these decreases as we move from C to Pb, i.e. CCl4 > SiCl4 > GeCl4 > SnCl4 > PbCl4

• Except CCl4, other tetrachlorides are hydrolysed by water as the central atom can accommodate lone pair of electrons from oxygen in d-orbital

√Allotropes of carbon: Two types: crystalline (diamond, graphite and fullerenes) and amorphous (coal, charcoal and coke)

• Diamond: Each C is sp3 hybridised, linked to 4 other C atoms, forms 3- D, rigid, tetrahedron structure. Hardest substance due to extensive covalent bonding
• Graphite: Each C is sp2 hybridised, forms planar hexagonal rings which forms layered structure held by Van der Waal’s forces. This leads to soft and slippery nature. One electron on each C-atom is delocalised in π-bond. This conducts electricity.

• Fullerenes: Only pure form of carbon, cage-like molecules formed by heating of graphite in electric arc in presence of inert gases. E.g. Buckminsterfullerene with 20 six-membered rings and 12 five- membered rings. Aromatic with sp2 hybridised carbon atoms

√Carbon-monoxide (CO): Powerful reducing agent, so used in extraction of metals

• On commercial scale, prepared by passage of steam over hot coke. Mixture of CO and H2 is water gas or synthesis gas.

• Mixture of CO and N2 is producer gas
• :C≡O: have one σ and two π bonds. Due to lone pair of e-, acts as donor in metal carbonyls.
• Highly poisonous as forms stable complex with haemoglobin

√Carbon-dioxide (CO2): Monomeric, linear molecule with sp hybridisation, pπ-pπ bonding and exists as a gas

• Forms H2CO3 (weak dibasic acid) with water
• Helps in photosynthesis, causes green-house effect and global warming
• Dry ice (solid CO2) as refrigerant

√Silicon dioxide (SiO2): Covalent, 3-D network having each Si atom covalently bonded to oxygen in a tetrahedral manner 

• Generally non-reactive due to high bond enthalpy of Si-O bond
• Used as piezoelectric material in form of quartz

√ Silicones: Synthetic organosilicon compounds with repeating R2SiO units held by Si- O-Si linkage

• Prepared by hydrolysis of alkyl or aryl substituted silicon chlorides (RnSiCl(4-n))

• Water repellent, heat resistant, chemically inert, resistant to oxidation and attack by organic acids, good electrical insulators and biocompatible
• Used for making sealant, greases, water-proofing, insulators and surgical and cosmetic implants 

√Silicates: SiO44- having Si atom bonded to 4 Oxygen atoms in tetrahedron fashion

• -ve charge is neutralised by +vely charged metal ions
• E.g. glass, cement etc.

√Zeolites: Aluminosilicates having a negative charge

• Used as catalyst in cracking of hydrocarbons and isomerisation
• E.g. ZSM-5

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