THE p -BLOCK ELEMENTS

electronic configuration is ns2np1-6

(except for He)

B.K.SHARMA

Session Objectives

Group 13 elements1. properties of group 13

elements

2. Halides of group 13 elements

3. Hydrides of group 13 elements

4. Oxides of group 13 elements

5. Extraction of aluminum

6. Compounds and uses of aluminum

7. Physical properties of group 14 elements

Group 14 elements1. properties of group 14 elements

2. Extraction and properties of silicon

3. Extraction of tin

4. Extraction of lead

5. Silicates and silicones

6. Glass

7. General properties of group 14 elements

8. Oxoacids of phosphorous

ATOMIC RADIUS

Elements of group 13• Increases down the group• Atomic radius (pm)

B(88)<Al(143)>Ga(135)<In(167)<Tl(170)

• Ionic radius M+3pm• B(27)<Al(53.5)>Ga(62)<In(80)<Tl

(88.5)• Ionic radius M+pm• Ga(120)<In(140)<Tl(150)• (Higher nuclear charge and poor

shielding by 3d-electron)

• Increases down the group C<Si<Ge<Sn<Pb

Covalent radius M(iv)pmC(77)<Si(118)<Ge(122)<Sn(140)<Pb(146)Ionic radius M+4pmSi(40)<Ge(53)<Sn(69)<Pb(78)

Elements of group 14

IONISATION ENERGY

Elements of group 13

• decreases down the group

• B>Tl>Ga≈Al>In

• decreases down the group

• C>Si>Ge>Sn<Pb

Elements of group 14

ELECTRONEGATIVITY

Elements of group 13

B(2.0)>Al(1.5)>Ga(1.6)>Tl(1.7)>In(1.8)

• Al Is most metallic and best reducing agent in the Boron family

• decreases down the group

• C(2.5)>Si≈Ge≈Sn(1.8)<Pb(1.9)

Elements of group 14

INERT PAIR EFFECT

• The reluctance of ns2 pair in the bond formation increases down the group

INERT PAIR EFFECT & OXIDATION STATES

Elements of group 13

B<Tl<Ga<Al<In (+1)• B>Tl>Ga>Al>In (+3)

• C<Si<Ge<Sn<Pb(+2)• C>Si>Ge>Sn<Pb(+4)

Elements of group 14

MELTING POINTS

Elements of group 13

B(2453K)>Al(933K)>Ga(303K)<In(430K)<Tl(576K)

• `Ga MELTS IN HAND OF VERY POOR PACKING OF Ga2 MOLECULES

• C(4373)>Si(1693)>Ge(1218)>Sn(505)<Pb(600)

Elements of group 14

DENSITY

Elements of group 13

B(2.34)<Al(2.7)<Ga(5.9)<In(7.31)<Tl(11.85)• Unit In gcm-3

• C(diamond3.51)<Si(2.34)<Ge(5.32)<Sn(7.26)<Pb(11.34)

• Unit In gcm-3

Elements of group 14

REACTIVITY TOWARDS AIRElements of group 13

B is unreactive in crystalline form. Al forms a very thin oxide layer on the surface which protects the metal from further attack. Amorphous B and Al metal on heating in air form B2O3 and Al2O3 respectively. With N2 at high temperature they form nitrides. B2O3(a),Al2O3(am),Ga2O3(am),In2O3(b), Tl2O3(b)

They form mainly two types of oxides,i.e., MO and MO2. SiO only exists at high temperature. Oxides in higher oxidation states of elements are generally more acidic than those in lower oxidation states.

Dioxides CO2, SiO2 and GeO2 (acidic), SnO2 and PbO2 (amphoteric) monoxides, CO, GeO is distinctly acidic, SnO and PbO amphoteric.

Elements of group 14

REACTIVITY TOWARDS WATERElements of group 13

Carbon, silicon and germanium are not affected by water. Tin decompose steam to form dioxide and dihydrogen gas.

Lead is unaffected by water probably because of a protective oxide film formation.

Elements of group 14

Except B Rest of elements of group 13 react with water at high.

REACTIVITY TOWARDS HALOGENSElements of group 13

• These elements react with halogens to form trihalides (except Tl I3).

(MX2 and MX4) Except C, other members react with X2. MX4 covalent ,sp3 , tetrahedral in shape.Exceptions are SnF4 and PbF4, ionic. PbI4 does not exist. Ge to Pb are able to make MX2. Stability of MX2 increases down the group. GeX4 is more stable than GeX2,PbX2 is more stable than PbX4. Except CCl4, other like SiCl4 are easily hydrolysed by water.

Elements of group 14

REACTIVITY TOWARDS ACIDS & ALKALIESElements of group 13

.Pb dissolves in water in the presence of oxygen to produce Pb(OH)2.It is called plumbo-solvency.Pb+H2O+1/2 O2-- Pb(OH)2

Elements of group 14

B does not react with acids and alkalies even at moderate temperature; but Al dissolves in mineral acids and aq alkalies and thus shows amphoteric character.Al dissolves in dilute HCl and liberates H2

concentrated nitric acid renders Al passive by forming a Al2O3layer on the surface.Al also reacts with aqueous alkali and liberates H2

ANOMALOUS PROPERTIES OF BORON

• Reason:- Small Size, High IE,More Electronegativity, Absence of d orbitals.

• (1) In the family only B shows allotropy.• (2) Max. covalency of B is 4(e.g. ) while for

other it is 6 ( ).• (3)Pure B dose not react with water and HCl.

BORAX

it contains the tetranuclear unitsBorax dissolves in water to give an alkaline solution.

On heating, borax first loses water molecules and swells up. On further heating itturns into a transparent liquid, which solidifies into glass like material known as boraxbead.

The metaborates of many transition metals have characteristic colours and,

borax bead test can be used to identify them in the laboratory. For example, when borax is heated in a Bunsen burner flame with CoO on a loop of platinum wire, a blue coloured Co(BO2)2 bead is formed.

Structure of Orthoboric acid H3BO3

Orthoboric acid H3BO3

white crystalline solid, with soapy touch. It is sparingly soluble in water but highly soluble in hot water. It can be prepared by acidifying an aqueous solution of borax.

Boric acid is a weak monobasic acid. It is not a protonic acid but acts as a Lewis acid by accepting electrons from a hydroxyl ion:

On heating, orthoboric acid above 370K forms metaboric acid, HBO2 which on further heating yields boric oxide, B2O3.

Diborane, B2H6(A fuel for super sonic rockets)Boron forms BnHn+4(Nidoboranes),BnHn+6(Arachno boranes)

1.prepared by treating boron trifluoride with LiAlH4 in diethyl ether. 4BF3 + 3 LiAlH4 → 2B2H6 + 3LiF + 3AlF3

2.Lab method 2NaBH4 + I2 → B2H6 + 2NaI + H2

3.Industrial method

PROPERTIES OF DIBORANE

1.colourless, highly toxic gas, b.p.180 K.2.catches fire spontaneously upon exposure to air. It burns in oxygen releasing an enormous amount of energy.

3.Boranes are readily hydrolyzed by water to give boric acid.• B2H6(g) + 6H2O(l) → 2B(OH)3(aq) + 6H2(g)

Diborane undergoes cleavage reactions with Lewis bases(L)

4. to give borane adducts, BH3 L⋅• B2H6 + 2 NMe3 → 2BH3 NMe⋅ 3

• B2H6 + 2 CO → 2BH3 CO⋅

Reaction of ammonia with diborane

Reaction of ammonia with diborane gives initially B2H6.2NH3 which is formulated as [BH2(NH3)2]+ [BH4]– ; further heating gives borazine, B3N3H6 known as “inorganic benzene” in view of its ring structure with alternate BH and NH groups.

B3N3H3 is isoster of benzene(12 atoms & 42 Electrons)• Borohydrides, are prepared by the reaction of metal hydrides• with B2H6 in diethyl ether.

structure of diborane

• Each B atom uses sp3 hybrids for bonding. one is without an electron shown in broken lines. The terminal B-H bonds are (2c-2e) bonds but the two bridge bonds are (3c-2e) bonds or banana bonds

USES OF BORON AND ALUMINIUMAND THEIR COMPOUNDS

1.B fibres are used in making bullet-proof vest and light composite material for aircraft. 2. B-10 used in nuclear industry as protective shields and control rods. 3.Borax and H3BO3 used as heat resistant glasses (e.g., Pyrex), glass-wool. 4.Borax used as a flux for soldering metals.5.H3BO3 acid is generally used as a mild antiseptic.6.Al forms alloys with Cu, Mn, Mg, Si and Zn.7.The use of Al and its compounds for domestic purposes is now reduced considerably because of their toxic nature.

ANOMALOUS BEHAVIOUR OFCARBON

• It is due to its smaller size, higher electronegativity, higher IE and unavailability of d orbitals.

• C form pπ– pπ multiple bonds with itself and with other atoms of small size and high electronegativity.C=C, C ≡ C, C = O, C = S, and C ≡ N.

• Heavier elements do not form pπ– pπ bonds because their atomic orbitals are too large and diffuse to have effective overlapping.

• Carbon atoms have the tendency to link with one another through covalent bonds to form chains and rings. This property is called catenation. because C—C bonds are very strong.

• C > > Si >Ge ≈ Sn. Pb does not show catenation.• Due to property of catenation and pπ– pπ bond formation, carbon is able to show

allotropic forms.

ALLOTROPES OF CARBON

• Carbon exhibits many allotropic forms; both crystalline as well as amorphous. Diamond and graphite are two well-known crystalline forms of carbon.

• In 1985, third form of carbon known as fullerenes was discovered by H.W.Kroto, E.Smalley and R.F.Curl. For this discovery they were awarded the Nobel Prize in 1996.

Diamond (crystalline lattice)• Each C atom-sp3 hybridization & linked to four other C-atoms

by using tetrahedral fashion. produces a rigid 3- dimensional network of carbon atoms. Diamond is a hardest substance on the earth. It is used as an abrasive for sharpening hard tools, in making dies and in the manufacture of tungsten filaments for electric light bulbs.

GraphiteIt has layered structure & held by vander Waals forces & distance between two layers is 340 pm. Each layer is composed of planar hexagonal rings of carbon atoms. Each C- atom in hexagonal ring undergoes sp2 hybridization and makes 3 sigma bonds with 3 neighboring carbon atoms. Fourth electron forms a π bond. The electrons are delocalized therefore, graphite conducts electricity along the sheet. it is very soft and slippery. used as a dry lubricant in machines running at high temperature, where oil cannot be used as a lubricant.

structure of graphite

Fullerenes• Fullerenes are made by the heating of graphite in an

electric arc in the presence of inert gases such as He or Ar.• The sooty material formed by condensation of vaporized

Cn small molecules consists of mainly C60 with smaller quantity of C70 and traces of fullerenes consisting of even number of carbon atoms up to 350 or above.

• Fullerenes are the only pure form of carbon because they have smooth structure without having ‘dangling’ bonds. Fullerenes are cage like molecules. C60 molecule has a shape like soccer ball and called Buckminsterfullerene.

It contains 20 six- membered rings and twelve five membered rings. A six membered ring is fused with six or five membered rings but a five membered ring can only fuse with six membered rings. All the carbon atoms are equal and they undergo sp2 hybridization. Each carbon atom forms three sigma bonds with other three carbon atoms. The remaining electron at each carbon is delocalized in molecular orbitals, which in turn give aromatic character to molecule. This ball shaped molecule has 60 vertices and each one is occupied by one carbon atom and it also contains both single and double bonds with C–C distances of 143.5 pm and 138.3 pm respectively. Spherical fullerenes are also called Bucky balls in short.

It is very important to know that graphite is thermodynamically most stable allotrope of carbon and, therefore, Δf HV of graphite is taken as zero. Δf HV values of diamond and fullerene, C60 are 1.90 and 38.1 kJ mol–1, respectively. Other forms of elemental carbon like carbon black, coke, and charcoal are all impure forms of graphite or fullerenes. Carbon black is obtained by burning hydrocarbons in a limited supply of air. Charcoal and coke are obtained by heating wood or coal respectively at high temperatures in the absence of air.

structure of C60 ,Buckminsterfullerene

Carbon Monoxide• The mixture of CO and H2 thus produced is known as water

gas or synthesis gas.

• a mixture of CO and N2 is produced, which is called producer gas.

• It highly poisonous because of its ability to form a complex with haemoglobin,

• On small scale pure CO is prepared by dehydration of formic acid with concentrated H2SO4 at 373 K

Carbon Dioxide

photosynthesis

dry ice

• Carbon dioxide can be obtained as a solid in the form of dry ice by allowing the liquified CO2 to expand rapidly. Dry ice is used as a refrigerant for ice-cream and frozen food.

Silicon Dioxide, SiO2

Silicones

• They are a group of organosilicon polymers, which have (R2SiO) as a repeating unit.

Silicates• silicates minerals exist in nature. Some eg. feldspar, zeolites,

mica and asbestos.• The basic structural unit of silicates is SiO4

4–in which silicon atom is bonded to four oxygen atoms in tetrahedron fashion.

Zeolites• If Al atoms replace few silicon atoms in 3-D network of

SiO2, overall structure known as aluminosilicate, acquires a negative charge. Cations such as Na+, K+ or Ca2+ balance the negative charge. e.g. feldspar and zeolites.

• Zeolites are widely used as a catalyst in petrochemical industries for cracking of hydrocarbons and isomerization

• ZSM-5 (A type of zeolite) used to convert alcohols directly into gasoline.

• Hydrated zeolites are used as ion exchangers in softening of “hard” water.