Revision Notes for Class 11 Chemistry P Block Elements
Class 11 Chemistry students should refer to the following concepts and notes for P Block Elements in standard 11. These exam notes for Grade 11 Chemistry will be very useful for upcoming class tests and examinations and help you to score good marks
P Block Elements Notes Class 11 Chemistry
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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