NAVODAYA VIDYALAYA SAMITI, NOIDA

E-Content for Class 11 Chemistry

Chapter-11

P-Block Elements

Prepared by:

R. Srinivasan

PGT Chemistry

JNV Karaikal, Puducherry(UT)

P Block Elements

General Characteristics

• The general electronic configuration of p block elements is ns² np^1-6
• The common groups are group – 13(Boron family), group-14 (carbon family), group-15 (Nitrogen family), group-16 (oxygen family) group-17(halogens) and group 18 (noble gases)
• Group oxidation state of this group are +3, +4,+5,+6,+7 and 0
• There is a difference in inner core electronic configuration due to presence of (n-1)d and (n-2)f electrons and this have an impact on change in atomic size, IE and density along the group.

General Characteristics

• The non-participation of ns orbital electrons in the valency due to poor shielding of (n-1)d and (n-2)f electrons is known as inert pair effect.
• Due to this the lower oxidation state become stable for the elements in the bottom of the group.
• For example aluminium can form +3 ion whereas the stable oxidation state of thallium is +1
• The Lighter elements in the top of the group are nonmetals while heavier elements are metals.

General Characteristics

General Characteristics- Boron Family

• Boron is nonmetal while aluminium, gallium, indium and thallium are metals.
• Boron is rare element while aluminium is third most abundant element in the earth.
• General electronic configuration ns² np¹
• Atomic radius is increasing down the group.
• But atomic size of Al>Ga due to presence of inner core d and f electrons with poor shielding effect which increases effective nuclear charge

General Characteristics- Boron Family

• The first ionization enthalpy and electronegativity is irregular in the group due to presence of (n-1)d and (n-2)f electrons which increases effective nuclear charge by poor shielding.
• Boron is hard element with high melting point.
• Aluminium to thallium are soft metals.
• Gallium is having very low melting point (303K). It is used in thermometers

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Boron Family- Chemical Properties

• Boron cannot form +3 ions due to high ionization enthalpy. So, all the boron compounds are covalent.
• Aluminium can form +3 ion while for other elements the +1 oxidation state is stable due to inert pair effect
• The elements form electron deficient molecule due to incomplete octet. So they will function as Lewis acid and accept a pair of electrons.

Boron Family- Chemical Properties

• The compounds can be easily hydrolyzed due to formation of [M(OH)₄]^- or [M(H₂O)₆]³⁺ ions.
• They react with oxygen to form oxides with formula M₂O₃. Boron, aluminium resist oxidation.
• B₂O₃ is acidic while Al₂O₃ and Ga₂O₃ are amphoteric.

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Boron Family- Chemical Properties

• Boron will not react with acid or alkali while other elements will react with acid to form salts.
• Aluminium and gallium are amphoteric and react with both acid and base.
• Aluminium will not react with concentrated nitric acid due to formation of inert Al₂O₃.
• They react with halogens to form trihalides (MX₃)
• Boron cannot expand the valency due to absence of d orbital. So BCl₃ is strong Lewis acid. AlCl₃ and other halides dimerizes to manage the octet.

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Borax-Preparation and Properties

• Borax (Na₂B₄O₇. 7H₂O) is white crystalline solid.
• Borax dissolves in water to give alkaline solution.
• Na₂B₄O₇+7H₂O 🡪 2NaOH+4H₃BO₃
• Borax on heating loses the water of crystallization and on further heating gives sodium metaborate and boric oxide.
• Na₂B₄O₇.7H₂O 🡪 Na₂B₄O₇ 🡪 2NaBO₂+B₂O₃
• Metaborates of transition elements are coloured and this experiment is used as borax bead test.

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Ortho boric acid�Preparation and Properties

• Orthoboric acid is white crystalline solid with soapy touch.
• It is prepared by treating borax with HCl
• Na₂B₄O₇+2HCl+5H₂O 🡪 4H₃BO₃+2NaCl
• It is also prepared by hydrolysis of boron halides
• BCl₃+3H₂O 🡪 H₃BO₃+3HCl
• The structure of boric acid contains layer structure in which BO₃ units are joined by hydrogen bonds

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Orthoboric acid�Preparation and Properties

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• Boric acid functions as Lewis acid by accepting OH^- from water. Hence the solution of boric acid will be acidic.
• B(OH)₃+H₂O 🡪 [B(OH)₄]^- + H⁺
• Boric acid on heating gives meta boric acid which on further heating give boric oxide
• H₃BO₃ 🡪 HBO₂🡪 B₂O₃
• Boric acid is used as antiseptic.

Diborane-Preparation and Properties

• Diborane is prepared by reducing boron trifluoride with LiAlH₄
• 4BF₃+3LiAlH₄ 🡪 2B₂H₆+3LiF+3AlF₃
• It is also prepared oxidizing sodium borohydride with iodine or reducing BF3 with sodium hydride.
• 2NaBH₄+I₂ 🡪 B₂H₆+2NaI +H₂
• 2BF₃+6NaH 🡪 B₂H₆+6NaF
• Diborane easily burns in oxygen to produce boric oxide and water.
• B₂H₆+3O₂ 🡪 B₂O₃+3H₂O

Diborane-Preparation and Properties

• The structure of diborane contains two hydrogen bridge atoms.
• Two boron atoms along with four terminal hydrogen lies on a plan and two hydrogen bridges lie above and below the plane.
• This bond can be easily cleaved by electron donors like carbon monoxide to give borane adducts.
• B₂H₆+2CO 🡪 2BH₃.CO

Diborane-Preparation and Properties

• Reacting diborane with ammonia gives an adduct with formula B₂H₆.2NH₃ which on further heating gives borazine.

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• Borazine is called as inorganic benzene due to structural resemblance with benzene.
• Hydrides like LiAlH₄ and NaBH₄ are used as reducing agents in organic chemistry.

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General Properties of Carbon Family

• Carbon is seventeenth most abundant element.
• Besides in native state the carbon compounds which are widely known as organic compounds, are a major part of all living organisms.
• General electronic configuration of carbon family elements is ns²np²
• The atomic size or covalent radius increases down the group. But the increase from Si to Pb is very small due to presence of (n-1)d electrons which reduces the atomic size due to poor shielding effect.

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General Properties of Carbon Family

• Ionization energy and electronegativity decreases in the group. But the difference is small from Ge to Sn and Sn to Pb due to change in the inner core electronic configuration.
• Carbon, silicon are nonmetals. Germanium is metalloid, tin and lead are metals.
• The elements show +2 and +4 oxidation state. Due to high IE the compounds with +4 oxidation state are covalent in nature.
• Down the group +2 oxidation state becomes more stable due to inert pair effect.

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General Properties of Carbon Family

• They react with oxygen to form monoxides (MO) and dioxides (MO₂). The oxides of top elements in the group are acidic (CO₂, SiO₂, GeO₂) while the bottom oxides are basic (SnO₂, PbO₂)
• They will not react with water. But tin decomposes water to give hydrogen.
• They form halides with formula MX₄ . All the halides are covalent in nature.
• The stability of lower oxidation halides with formula MX₂ is higher for the elements in the bottom of the group due to inert pair effect.

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General Properties of Carbon Family

Allotropes of Carbon-Diamond

• Diamond, graphite and fullerene are crystalline forms of carbon.
• In diamond each carbon is sp³ hybridized and connected with four other carbon atoms in tetrahedral shape.
• The bond length is 154pm and the bond extends in three-dimension space.
• Due to rigid bond formed between carbon atoms, diamond is the hardest substance.

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Allotropes of Carbon-Graphite

• In graphite each carbon is sp² hybridized and connected with three other carbon atoms in trigonal shape.
• The bond length is 141pm due to partial double bond formed between carbon atoms.
• The layers of graphite are connected through Vander Walls force with a distance of 340pm.
• Graphite is thermodynamically stable form of carbon

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Allotropes of Carbon-Fullerene

• Fullerenes are allotropes of carbon with cage like structure.
• They are prepared by heating graphite in electric arc in the presence of rare gases
• C-60 fullerene is common form while C-70 and other fullerenes are also known.
• In the structure of C-60, there are 20 six membered carbon rings and 20 five membered carbon rings in such a way that each five membered ring fuses with six membered ring and vice versa.

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Allotropes of Carbon-Fullerene

• In fullerene each carbon atom is sp2 hybridized and the remaining electron is delocalized to give aromatic character.
• The bond lengths in fullerene are 143.5pm and 138.3pm for single and double bonds.
• There are other amorphous form of carbon like carbon black, charcoal and coke.

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Important Compounds of Carbon-CO

Important Compounds of Carbon-CO

• Carbon monoxide is colourless, odourless gas insoluble in water.
• It reduces metal oxide into metal and this process is used in metallurgy. For example iron(III)oxide and zinc oxide are reduced to iron and zinc.
• Fe₂O₃+3CO 🡪 2Fe+3CO₂
• ZnO+CO 🡪 Zn+CO₂
• With transition metals it forms metal carbonyls in which the carbon monoxide function as electron donor and transition metals are electron acceptors.
• Ni+4CO🡪 [Ni(CO)₄]

Important Compounds of Carbon-CO

• Carbon monoxide forms carboxy haemoglobin complex with haemoglobin.
• This complex is 300 times stable than oxygen-haemoglobin complex.
• So, RBCs are not available for binding with oxygen and hence carbon monoxide is more poisonous.

Important Compounds of Carbon-CO₂

• Carbon dioxide is formed due to complete combustion of carbon and other fossil fuels.
• C+O₂ 🡪 CO₂
• In the laboratory it is prepared by treating calcium carbonate with dil. HCl
• CaCO₃+2HCl 🡪 CaCl₂+H₂O+CO₂
• Industrially carbon dioxide is prepared by heating limestone
• CaCO₃ 🡪 CaO+CO₂

Important Compounds of Carbon-CO₂

• Carbon dioxide is colourless, odourless gas. It is sparingly soluble in water to give carbonic acid (H₂CO₃)
• Carbonic acid dissociates in water in two steps
• H₂CO_3(aq) + H₂O_(l) 🡪 HCO₃^–(aq) + H₃O⁺(aq)
• HCO₃^–_(aq)+H₂O_(l)🡪 CO₃^2– _(aq)+H₃O⁺_(aq)
• The H₂CO₃ and HCO₃^– buffer system helps to maintain the pH of the blood in the range of 7.26 to 7.42

Important Compounds of Carbon-CO₂

Important Compounds of Silicon-Silica

• Silicon dioxide or silica is naturally available in various crystallographic forms like quartz, cristobalite and tridymite.
• In silicon dioxide each silicon atom is connected with four oxygen atoms and each oxygen atom is connected with another silicon atom as shown in the structure.
• Due to strong bond between Si and O, silica is rigid.
• But it reacts with NaOH and HF.
• 2NaOH+SiO₂ 🡪 Na₂SiO₃+H₂O
• SiO₂+4HF 🡪 SiF₄+2H₂O

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Important Compounds of Silicon-Silicones

• Alkyl or aryl substituted silicone chlorides with general formula R₂SiCl₂ on hydrolysis with KOH followed by polymerization gives a long chain polymer of silicones.
• The chain length can be controlled by adding R₃SiCl as chain terminator.

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Important Compounds of Silicon- Silicones

• Due to presence of water repelling alkyl groups, the silicones are used in water proofing fabrics.
• They are highly stable and corrosion resistant and hence used as coating material for containers.
• They are also used in sealants and insulators.

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Important Compounds of Silicon- Silicates

• Feldspar, mica, zeolites are silicates with basic structural unit of SiO₄^2-
• There are chain, ring, sheet and three-dimensional silicates based on the number of oxygen atoms connected in basic SiO₄^2- unit.
• Glass and cement are examples of man-made silicates.

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Important Compounds of Silicon-Zeolites

• In silicates structures few silicon atoms will be replaced by Al³⁺ ions to give aluminosilicates or zeolites.
• In this the excess negative charge is balanced by other metal ions like K⁺, Na⁺
• Zeolites are used in petrochemical industries for cracking.
• ZSM-5 is a zeolite used to directly convert alcohol to gasoline.
• Zeolites are also used to soften the hard water.

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THE END