Elements in which the last electron enters in the any one of the three p - orbital of
their outermost shell s – p-block ele ments
•
Gen. electronic configuration of outer shell is ns
2
np
1-6
The inner core of e-config.may differ which greatly influences their physical & to
some extent chemical properties.
GROUP 13 : The boron group
Outer Electronic Configuration:-ns
2
np
1
• group members: boron (B), aluminum (Al), gallium (Ga), indium (In)&
thallium (Tl) . All, except boron, are metals.
• Boron show diagonal relationship with Silicon; both are semiconductors
metalloids & forms covalent compounds.
• Boron compounds are electron deficient, they are lack of an octet of electrons
about the B atom .
• diborane B2H6
, is simplest boron hydride
• Structure: three-center two-electron: the H atoms are simultaneously bonded to
two B atoms the B-H bridging bond lengths are greater than B-H terminal.
• - Boron oxide is acidic (it reacts readily with water to f orm boric acid)
• aluminium compounds:aluminium oxide is amphoteric
• aluminum hali des, e.g., AlCl
3
is dimer, an important catalyst in organic
chemistry have anincomplete octet, acts as Lewic acid by acc epting lone pairs
from Lewic bases, forming adduct
• aluminum hydride, e.g., LiAlH4
, a reducing agen
. Atomic and ionic radii
• The atomic and ionic radii of group 13 elements are compared to
corresponding elements of group 2. From left to right in the period, the
magnitude of nuclear charge increases but the electrons are added to, the same
shell. These electrons do not screen each other, theref ore, the electrons
experience greater nuclear charge.
• In other words, effective nuclear charge increases and thus, size decreases.
Therefore, the elements of this group have smaller size than the corresponding
elements of second group.
• On moving down the group both atomic and ionic radii are expected to
increase due to the addition of new shell s. However, the observed atomic
radius of Al (143 pm) is slightly more than that of Ga (l35 pm).
Ionization energies
The first ionization energies of group 13 elements are less than the corresponding
me mbers of the alkaline earths.
The sharp decrease in I.E. from B to Al is due to increase in size. In case of Ga, there
are ten d-electrons in its inner electronic configuration.
The very high value of 3
rd
I. E. of thallium indicates that +3 O.N. state is not stable,
rather +1 is more stable f or thallium .
Electropositive (or metallic) character
the elements of group 13 are less electropositive as compared to ele ments of group 2.
On moving down the group the electropositive (metallic) character increases because
ionization energy decreases. For e.g., Boron is a non -metal white the other elements
are typical metals.
Oxidation states
The common oxidation states of group 13 elements are +3 and + l .The stability of
the + 1 oxidation state increases in the sequence Al <Ga< In <Tl, Due to Inert pair
effect.
Hydrides
• None of the group 13 ele ments reacts directly with hydrogen. However, a no.
of hydrides of these elements have been prepared by indirect methods. The
boron hydrides are called boranes& classified in two s eries: (a) BnHn+4
called nidoboranes (b) BnHn+6
called arachnoboranes
• INUDUSTRIAL PREPERATION :-2BF
3
(g) + 6LiH(s) → B2H6
(g) + 6LiF(s)
• Laboratory method:
(i) By the reaction of iodine with sodium borohydride in a high boiling
solvent.
2NaBH4
+ I
2 → B2H6
+ 2NaI + H2
(ii) By reduction of BCl
3
with LiAlH4
4BCl
3
+ 3LiAlH4 → 2 B2H6
+ 3AlCl
3
+ 3 LiCl
Some important characteristics of boranes:
i) Lower boranes are colourless gases while higher boranes are volatile liquids
or solids.
ii) They undergo spontaneous combustion in air due to strong affinity of boron
for oxygen.
B2H6
+ 3O2 → B2O3
+ 3H2O + Heat
iii) Boranes react with alkali metal hydrides in diethyl ether to form
borohydride complexes.
B2H6
+ 2MH →2M
+
[BH4
]
-
(M= Li or Na)
Metal borohydride
• (iv) Diborane reacts with ammonia to give borazine at 450 K.
B2H6
+ 6NH3 → 3B3N3H6
+ 12H2
• Borazine has a cyclic structure similar to benzene and thus is called inorganic
benzene
• The other ele ments of this group form only a few stable hydrides. The thermal
stability decreases as we move down the group.
• AlH3
is a colourless soli d polymerized via Al - H - Al bridging units. These
hydrides are weak Lewis acids and readily form adducts with strong Lewis
base (B:) to give compounds of the type MH3
(M = Al or Ga). They also form
complex-tetrahydrido anions, [MH4]-. The most important tetrahydrido
compound is Li[AlH4
]
ether
4LiH + AlCl
3 ―――→ LiAlH4
+ 3LiCl
Dimeric structure of aluminium chloride
– Boron halides do not form dimers because the size of boron
is so small that it is unable to coordinate four large -sized
halide ions.
Anomalous properties of boron
1. Boron is a non-metal & bad conductor of electricity whereas aluminium is a metal
& good conductor. B is hard but Al is a soft metal.
2. Boron exists in two forms-crystalline and amorphous. But Al does not exist in
different f orms.
3. The melting and boiling point of boron are much higher than that of Al .
4. Boron forms only covalent compounds whereas Al forms even some ionic
compounds.
5. The hydroxides and oxides of boron are acidic in nature whereas those of
aluminium are amphoteric.
6. The trihalides of boron exist as monomers. On the other hand, aluminium halides
exist as dimers .
7. The hydrides of boron are quite stable whil e those of aluminium are unstable
Group 14 Ele ments:-The Carbon Family
Group 14 includes carbon (C), silicon (Si), germanium (Ge), tin (Sn) and lead (Pb).
General electronic configuration of carbon fa mily is ns
2
np
2
.
Covalent radius:-Covalent radius expected to increase from Cto Si,
From Si to Pb small increase is found.
Ionization Enthalpy:-The first ionization enthalpies of group 14 elements are higher
than those of the corresponding group 13 elements.
Electronegativi ty:-Group 14 ele ments are smaller in size as compared to group 13
elements that‘s why this group ele ments are slightly more electronegative than group
13
Chemical properties:-Carbon and silicon mostly show +4 oxidation state. Germanium forms stable
compounds in +4 state and only few compounds in +2 state.
Tin f orms compounds in both oxidation states. Lead compounds in +2 state are stable
and in +4 state are strong oxidizing agents.
Exception:-Pb
4
and SnF
4
are ionic in nature.
Except CCl
4
other tetrachlorides are easily hydrolysed by water.
Allotropes of Carbon:-The three types of all otropes are –
1-Diamond 2-Graphite 3-Full erence
Dia mond:-In diamond each carbon atom under goes SP
3
hybridisation. Each carbon is
tetrahedrally linked to four other carbon atoms.
Graphite:-In graphite, carbon is SP
2
-hybridized graphite has a two-dimensional sheet
like structure consisting of a number of hexagonal rings fused together.
Graphite conducts electricity along the sheet. It is very soft and Slippery
Fullerence was discovered collectively by three scientists namely R.E
Smalley,R.F Curl and H.W Kroto
1. Why is boron used in nuclear reactions?
Ans:-Because Boron can absorb neutrons.
2. By giving a balanced equation show how B(OH)
3
behaves as an acid in water.
Ans:-B(OH)
3
+2H2O [B(OH)4
]
-+H3O
+
3.Name the ele ment of group 14 which exhibits maximum tendency for
catenation?
Ans:-Carbon
4. What is the basic building unit of all silicates?
Ans:-SiO4
4-is the basic unit of all silicates.
5. What happens when NaBH4
reacts with iodine?
6. What happens when boric acid is heated
Ans:-4H3BO3
4HBO2 H2B4O7
.
7. What is producer gas?
Ans:-Producer gas is a mixture of CO and N2
in the ratio of 2:1.
8.Write the state of hybridization of ‗B‘ in BF
3
.
ANS:-Hybridisation of ‗B‘ in BF
3
is Sp
2
.
9.Mention the state of hybridization in B in BH4
-.
Ans:-Sp
3
.
10. Which oxide of carbon is regarded as anhydride of carbonic acid.
Ans:-CO2
is regarded as a hydride of carbonic acid .
What happens when
(i) Quick lime is heated with coke?
(ii) Carbon monoxide reacts with Cl
2
Ans:- (i) Cao +3C → CaC2
+CO
(iii) CO +Cl
2→ COCl
2
Give reason
(i) C and Si are always tetravalent but Ge,Sn,Pb show divalency.
(ii) Gallium has higher ionization enthalpy than Al. Explain.
Ans:-(i) Ge, Sn, Pb show divalency due to inert pair effect, Pb
2+
is more stable than
Pb
4+
.
(ii) Due to poor shielding effect of d-electrons in Ga effect ive nuclear
charge increases as compared to Al thus the I.E is higher than Al.
If B-Cl bond has a dipole moment, Explain why BCl
3 molecule has zero dipolemo ment.
Ans:- B-Cl bond has dipole mo ment because of polarity.In BCl
3
since the molecule issymmetrical thus the polarities cancel out.
What do you understand by-(a) Inert pair effect:-The pair of electron in the valence shell does not take part in
bond formation it is called inert pair effect.
(b) Allotropy:-It is the property of the element by which an element can exists in
two f orms which have same chemical properties but different physical properties due
to their structures.
Give reason for the f ollowing observations:-(a) The tendency for catenation decreases down the group in Group 14.
(b) The decreasing stability of +3 oxidations state with increasing atomic
number in group 13.
(c) PbO2
is a stronger oxidizing agent than SnO2.
(d) Molten aluminium bromide is a poor conductor of electricity.
Ans:- (i)(a) It is due to decrease in bond dissociation energy which is due to increase
in atomic size.
C-C > Si-Si >Ge-Ge>Sn-Sn>Pb-Pb.
(b) It is due to inert pair effect.
(c) PbO2
is stronger oxidizing agent than SnO2
because Pb
2+
is more
stable than Pb
4+
whereas Sn
4+
is more stable than Sn
2+
.
compound.
(d) Molten AlBr
3
is poor conductor of electricity because it is covalent.
their outermost shell s – p-block ele ments
•
Gen. electronic configuration of outer shell is ns
2
np
1-6
The inner core of e-config.may differ which greatly influences their physical & to
some extent chemical properties.
GROUP 13 : The boron group
Outer Electronic Configuration:-ns
2
np
1
• group members: boron (B), aluminum (Al), gallium (Ga), indium (In)&
thallium (Tl) . All, except boron, are metals.
• Boron show diagonal relationship with Silicon; both are semiconductors
metalloids & forms covalent compounds.
• Boron compounds are electron deficient, they are lack of an octet of electrons
about the B atom .
• diborane B2H6
, is simplest boron hydride
• Structure: three-center two-electron: the H atoms are simultaneously bonded to
two B atoms the B-H bridging bond lengths are greater than B-H terminal.
• - Boron oxide is acidic (it reacts readily with water to f orm boric acid)
• aluminium compounds:aluminium oxide is amphoteric
• aluminum hali des, e.g., AlCl
3
is dimer, an important catalyst in organic
chemistry have anincomplete octet, acts as Lewic acid by acc epting lone pairs
from Lewic bases, forming adduct
• aluminum hydride, e.g., LiAlH4
, a reducing agen
. Atomic and ionic radii
• The atomic and ionic radii of group 13 elements are compared to
corresponding elements of group 2. From left to right in the period, the
magnitude of nuclear charge increases but the electrons are added to, the same
shell. These electrons do not screen each other, theref ore, the electrons
experience greater nuclear charge.
• In other words, effective nuclear charge increases and thus, size decreases.
Therefore, the elements of this group have smaller size than the corresponding
elements of second group.
• On moving down the group both atomic and ionic radii are expected to
increase due to the addition of new shell s. However, the observed atomic
radius of Al (143 pm) is slightly more than that of Ga (l35 pm).
Ionization energies
The first ionization energies of group 13 elements are less than the corresponding
me mbers of the alkaline earths.
The sharp decrease in I.E. from B to Al is due to increase in size. In case of Ga, there
are ten d-electrons in its inner electronic configuration.
The very high value of 3
rd
I. E. of thallium indicates that +3 O.N. state is not stable,
rather +1 is more stable f or thallium .
Electropositive (or metallic) character
the elements of group 13 are less electropositive as compared to ele ments of group 2.
On moving down the group the electropositive (metallic) character increases because
ionization energy decreases. For e.g., Boron is a non -metal white the other elements
are typical metals.
Oxidation states
The common oxidation states of group 13 elements are +3 and + l .The stability of
the + 1 oxidation state increases in the sequence Al <Ga< In <Tl, Due to Inert pair
effect.
Hydrides
• None of the group 13 ele ments reacts directly with hydrogen. However, a no.
of hydrides of these elements have been prepared by indirect methods. The
boron hydrides are called boranes& classified in two s eries: (a) BnHn+4
called nidoboranes (b) BnHn+6
called arachnoboranes
• INUDUSTRIAL PREPERATION :-2BF
3
(g) + 6LiH(s) → B2H6
(g) + 6LiF(s)
• Laboratory method:
(i) By the reaction of iodine with sodium borohydride in a high boiling
solvent.
2NaBH4
+ I
2 → B2H6
+ 2NaI + H2
(ii) By reduction of BCl
3
with LiAlH4
4BCl
3
+ 3LiAlH4 → 2 B2H6
+ 3AlCl
3
+ 3 LiCl
Some important characteristics of boranes:
i) Lower boranes are colourless gases while higher boranes are volatile liquids
or solids.
ii) They undergo spontaneous combustion in air due to strong affinity of boron
for oxygen.
B2H6
+ 3O2 → B2O3
+ 3H2O + Heat
iii) Boranes react with alkali metal hydrides in diethyl ether to form
borohydride complexes.
B2H6
+ 2MH →2M
+
[BH4
]
-
(M= Li or Na)
Metal borohydride
• (iv) Diborane reacts with ammonia to give borazine at 450 K.
B2H6
+ 6NH3 → 3B3N3H6
+ 12H2
• Borazine has a cyclic structure similar to benzene and thus is called inorganic
benzene
• The other ele ments of this group form only a few stable hydrides. The thermal
stability decreases as we move down the group.
• AlH3
is a colourless soli d polymerized via Al - H - Al bridging units. These
hydrides are weak Lewis acids and readily form adducts with strong Lewis
base (B:) to give compounds of the type MH3
(M = Al or Ga). They also form
complex-tetrahydrido anions, [MH4]-. The most important tetrahydrido
compound is Li[AlH4
]
ether
4LiH + AlCl
3 ―――→ LiAlH4
+ 3LiCl
Dimeric structure of aluminium chloride
– Boron halides do not form dimers because the size of boron
is so small that it is unable to coordinate four large -sized
halide ions.
Anomalous properties of boron
1. Boron is a non-metal & bad conductor of electricity whereas aluminium is a metal
& good conductor. B is hard but Al is a soft metal.
2. Boron exists in two forms-crystalline and amorphous. But Al does not exist in
different f orms.
3. The melting and boiling point of boron are much higher than that of Al .
4. Boron forms only covalent compounds whereas Al forms even some ionic
compounds.
5. The hydroxides and oxides of boron are acidic in nature whereas those of
aluminium are amphoteric.
6. The trihalides of boron exist as monomers. On the other hand, aluminium halides
exist as dimers .
7. The hydrides of boron are quite stable whil e those of aluminium are unstable
Group 14 Ele ments:-The Carbon Family
Group 14 includes carbon (C), silicon (Si), germanium (Ge), tin (Sn) and lead (Pb).
General electronic configuration of carbon fa mily is ns
2
np
2
.
Covalent radius:-Covalent radius expected to increase from Cto Si,
From Si to Pb small increase is found.
Ionization Enthalpy:-The first ionization enthalpies of group 14 elements are higher
than those of the corresponding group 13 elements.
Electronegativi ty:-Group 14 ele ments are smaller in size as compared to group 13
elements that‘s why this group ele ments are slightly more electronegative than group
13
Chemical properties:-Carbon and silicon mostly show +4 oxidation state. Germanium forms stable
compounds in +4 state and only few compounds in +2 state.
Tin f orms compounds in both oxidation states. Lead compounds in +2 state are stable
and in +4 state are strong oxidizing agents.
Exception:-Pb
4
and SnF
4
are ionic in nature.
Except CCl
4
other tetrachlorides are easily hydrolysed by water.
Allotropes of Carbon:-The three types of all otropes are –
1-Diamond 2-Graphite 3-Full erence
Dia mond:-In diamond each carbon atom under goes SP
3
hybridisation. Each carbon is
tetrahedrally linked to four other carbon atoms.
Graphite:-In graphite, carbon is SP
2
-hybridized graphite has a two-dimensional sheet
like structure consisting of a number of hexagonal rings fused together.
Graphite conducts electricity along the sheet. It is very soft and Slippery
Fullerence was discovered collectively by three scientists namely R.E
Smalley,R.F Curl and H.W Kroto
1. Why is boron used in nuclear reactions?
Ans:-Because Boron can absorb neutrons.
2. By giving a balanced equation show how B(OH)
3
behaves as an acid in water.
Ans:-B(OH)
3
+2H2O [B(OH)4
]
-+H3O
+
3.Name the ele ment of group 14 which exhibits maximum tendency for
catenation?
Ans:-Carbon
4. What is the basic building unit of all silicates?
Ans:-SiO4
4-is the basic unit of all silicates.
5. What happens when NaBH4
reacts with iodine?
6. What happens when boric acid is heated
Ans:-4H3BO3
4HBO2 H2B4O7
.
7. What is producer gas?
Ans:-Producer gas is a mixture of CO and N2
in the ratio of 2:1.
8.Write the state of hybridization of ‗B‘ in BF
3
.
ANS:-Hybridisation of ‗B‘ in BF
3
is Sp
2
.
9.Mention the state of hybridization in B in BH4
-.
Ans:-Sp
3
.
10. Which oxide of carbon is regarded as anhydride of carbonic acid.
Ans:-CO2
is regarded as a hydride of carbonic acid .
What happens when
(i) Quick lime is heated with coke?
(ii) Carbon monoxide reacts with Cl
2
Ans:- (i) Cao +3C → CaC2
+CO
(iii) CO +Cl
2→ COCl
2
Give reason
(i) C and Si are always tetravalent but Ge,Sn,Pb show divalency.
(ii) Gallium has higher ionization enthalpy than Al. Explain.
Ans:-(i) Ge, Sn, Pb show divalency due to inert pair effect, Pb
2+
is more stable than
Pb
4+
.
(ii) Due to poor shielding effect of d-electrons in Ga effect ive nuclear
charge increases as compared to Al thus the I.E is higher than Al.
If B-Cl bond has a dipole moment, Explain why BCl
3 molecule has zero dipolemo ment.
Ans:- B-Cl bond has dipole mo ment because of polarity.In BCl
3
since the molecule issymmetrical thus the polarities cancel out.
What do you understand by-(a) Inert pair effect:-The pair of electron in the valence shell does not take part in
bond formation it is called inert pair effect.
(b) Allotropy:-It is the property of the element by which an element can exists in
two f orms which have same chemical properties but different physical properties due
to their structures.
Give reason for the f ollowing observations:-(a) The tendency for catenation decreases down the group in Group 14.
(b) The decreasing stability of +3 oxidations state with increasing atomic
number in group 13.
(c) PbO2
is a stronger oxidizing agent than SnO2.
(d) Molten aluminium bromide is a poor conductor of electricity.
Ans:- (i)(a) It is due to decrease in bond dissociation energy which is due to increase
in atomic size.
C-C > Si-Si >Ge-Ge>Sn-Sn>Pb-Pb.
(b) It is due to inert pair effect.
(c) PbO2
is stronger oxidizing agent than SnO2
because Pb
2+
is more
stable than Pb
4+
whereas Sn
4+
is more stable than Sn
2+
.
compound.
(d) Molten AlBr
3
is poor conductor of electricity because it is covalent.
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