Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

THE p-BLOCK ELEMENTS

315

UNIT 11

THE p -BLOCK ELEMENTS

The variation in properties of the p-block elements due to the
influence of d and f electrons in the inner core of the heavier
elements makes their chemistry interesting
After studying this unit, you will be
able to
•

appreciate the general trends in the
chemistry of p-block elements;

•

describe the trends in physical and
chemical properties of group 13 and
14 elements;

•

explain anomalous behaviour of
boron and carbon;

•

describe allotropic forms of carbon;

•

know the chemistry of some
important compounds of boron,
carbon and silicon;

•

list the important uses of group 13
and 14 elements and their
compounds
...
As we know that the number of p orbitals is three
and, therefore, the maximum number of electrons that can
be accommodated in a set of p orbitals is six
...
Boron, carbon, nitrogen,
oxygen, fluorine and helium head the groups
...

The inner core of the electronic configuration may,
however, differ
...
) as well as chemical
properties
...
The maximum
oxidation state shown by a p-block element is equal to the
total number of valence electrons (i
...
, the sum of the sand p-electrons)
...
In
addition to this so called group oxidation state, p-block
elements may show other oxidation states which normally,
but not necessarily, differ from the total number of valence
electrons by unit of two
...
1
...
However, the oxidation state two unit less than the
group oxidation state becomes progressively more stable
for the heavier elements in each group
...


2019-20

316

CHEMISTRY

Table 11
...

It is interesting to note that the non-metals
and metalloids exist only in the p-block of the
periodic table
...
In fact the
heaviest element in each p-block group is the
most metallic in nature
...

In general, non-metals have higher ionisation
enthalpies and higher electronegativities than
the metals
...
The compounds formed by highly
reactive non-metals with highly reactive metals
are generally ionic because of large differences
in their electronegativities
...
The change of non-metallic
to metallic character can be best illustrated by
the nature of oxides they form
...


16

17

18

The first member of p-block differs from the
remaining members of their corresponding
group in two major respects
...

Thus, the lightest p-block elements show the
same kind of differences as the lightest s-block
elements, lithium and beryllium
...
The
second period elements of p-groups starting
from boron are restricted to a maximum
covalence of four (using 2s and three 2p
orbitals)
...

Using these d-orbitals the third period
elements can expand their covalence above
four
...
The
presence of these d-orbitals influences the
chemistry of the heavier elements in a number
of other ways
...
The first member of a group differs
from the heavier members in its ability to form
pπ - pπ multiple bonds to itself ( e
...
, C=C, C≡C,

2019-20

THE p-BLOCK ELEMENTS

317

N≡N) and to other second row elements (e
...
,
C=O, C=N, C≡N, N=O)
...
The heavier elements do form
π bonds but this involves d orbitals (dπ – pπ
or dπ –dπ )
...

However, the coordination number in species
of heavier elements may be higher than for
the first element in the same oxidation state
...
In
this unit we will study the chemistry of group
13 and 14 elements of the periodic table
...
1 GROUP 13 ELEMENTS: THE BORON
FAMILY
This group elements show a wide variation in
properties
...

Boron is a fairly rare element, mainly
occurs as orthoboric acid, (H3BO3), borax,
Na2B4O7·10H2O, and kernite, Na2B4O7·4H2O
...
The
abundance of boron in earth crust is less than
0
...
There are two isotopic
10
11
forms of boron B (19%) and B (81%)
...
3% by mass) after oxygen (45
...
7%)
...
2H2O and cryolite,
Na 3 AlF 6 are the important minerals of
aluminium
...
Gallium, indium and thallium are less
abundant elements in nature
...
So far it has been prepared
in small amount and half life of its most stable

isotope is 20 seconds
...

Nihonium is a synthetically prepared
radioactive element
...

11
...
1 Electronic Configuration
The outer electronic configuration of these
2
1
elements is ns np
...

Thus, the electronic structures of these
elements are more complex than for the first
two groups of elements discussed in unit 10
...

11
...
2 Atomic Radii
On moving down the group, for each successive
member one extra shell of electrons is added
and, therefore, atomic radius is expected to
increase
...

Atomic radius of Ga is less than that of Al
...
The
presence of additional 10 d-electrons offer
only poor screening effect (Unit 2) for the outer
electrons from the increased nuclear charge in
gallium
...

11
...
3 Ionization Enthalpy
The ionisation enthalpy values as expected
from the general trends do not decrease
smoothly down the group
...
The
observed discontinuity in the ionisation
enthalpy values between Al and Ga, and
between In and Tl are due to inability of d- and
f-electrons ,which have low screening effect, to
compensate the increase in nuclear charge
...
The sum of the
first three ionisation enthalpies for each of the

2019-20

318

CHEMISTRY

11
...
6 Chemical Properties

elements is very high
...


Oxidation state and trends in chemical
reactivity
Due to small size of boron, the sum of its first
three ionization enthalpies is very high
...
But as we move from
B to Al, the sum of the first three ionisation
enthalpies of Al considerably decreases, and
3+
is therefore able to form Al ions
...

However, down the group, due to poor
shielding effect of intervening d and f orbitals,
the increased effective nuclear charge holds ns
electrons tightly (responsible for inert pair
effect) and thereby, restricting their
participation in bonding
...
In fact in Ga, In and Tl, both +1 and
+3 oxidation states are observed
...
In
thallium +1 oxidation state is predominant

11
...
4 Electronegativity
Down the group, electronegativity first
decreases from B to Al and then increases
marginally (Table 11
...
This is because of the
discrepancies in atomic size of the elements
...
1
...
It is extremely
hard and black coloured solid
...
Due to very strong crystalline
lattice, boron has unusually high melting point
...

It is worthwhile to note that gallium with
unusually low melting point (303 K), could
exist in liquid state during summer
...

Density of the elements increases down the
group from boron to thallium
...
2 Atomic and Physical Properties of Group 13 Elements
Property

Boron
B

Atomic number

Aluminium
Al

5
–1

Atomic mass(g mol )

13

10
...
5

62
...
0

88
...
0

1
...
6

1
...
8

2
...
70

5
...
31

11
...
66

–0
...
34

+1
...
79(acid)
–1
...
18

–0
...
38

135

(88)

Electronegativityc

114
...
72
2

Electronic
Configuration

Ionization
enthalpy
(kJ mol–1)

31

26
...
55
c

6-coordination, Pauling scale,

2019-20

THE p-BLOCK ELEMENTS

319

whereas the +3 oxidation state is highly
oxidising in character
...

In trivalent state, the number of electrons
around the central atom in a molecule
of the compounds of these elements
(e
...
, boron in BF3) will be only six
...
The tendency to behave as Lewis
acid decreases with the increase in the size
down the group
...


Solution
Standard electrode potential values for two
half cell reactions suggest that aluminium
3+
has high tendency to make Al (aq) ions,
3+
whereas Tl is not only unstable in
solution but is a powerful oxidising agent
+
also
...
Aluminium being able to form
+3 ions easily, is more electropositive than
thallium
...

Aluminium forms a very thin oxide layer on
the surface which protects the metal from
further attack
...
With dinitrogen at high
temperature they form nitrides
...
For
example, the trichlorides on hyrolysis in water
−
form tetrahedral  M ( OH )4  species; the
3
hybridisation state of element M is sp
...

In this complex ion, the 3d orbitals of Al are
involved and the hybridisation state of Al is
sp3d2
...
1

Standard electrode potential values, E
3+
3+
for Al /Al is –1
...
26 V
...


(E = element)
The nature of these oxides varies down the
group
...

Aluminium and gallium oxides are amphoteric
and those of indium and thallium are basic in
their properties
...

Aluminium dissolves in dilute HCl and
liberates dihydrogen
...

Aluminium also reacts with aqueous alkali
and liberates dihydrogen
...

(X = F, Cl, Br, I)
2E(s) + 3 X2 (g) → 2EX3 (s)
Problem 11
...
Give
reason
...

It is a white crystalline solid of formula
Na 2 B 4 O 7⋅ 10H 2 O
...
2 IMPORTANT
TRENDS
AND
ANOMALOUS PROPERTIES OF
BORON
Certain important trends can be observed
in the chemical behaviour of group
13 elements
...

–
Species like tetrahedral [M(OH) 4] and
3+
octahedral [M(H2O)6] , except in boron, exist
in aqueous medium
...
Boron
trifluoride easily reacts with Lewis bases such
as NH3 to complete octet around boron
...
Since the
d orbitals are available with Al and other
elements, the maximum covalence can be
expected beyond 4
...
g
...
g
...
The metal
species completes its octet by accepting
electrons from halogen in these halogen
bridged molecules
...
3
3–
Boron is unable to form BF6 ion
...

Solution
Due to non-availability of d orbitals, boron
is unable to expand its octet
...
3 SOME IMPORTANT COMPOUNDS OF
BORON
Some useful compounds of boron are borax,
orthoboric acid and diborane
...

11
...
1 Borax

Solution
Anhydrous aluminium chloride is
partially hydrolysed with atmospheric
moisture to liberate HCl gas
...


F3 B + :NH 3

the maximum covalence of boron cannot
exceed 4
...
8H2O
...

Na2B4O7 + 7H2O → 2NaOH + 4H3BO3
Orthoboric acid
On heating, borax first loses water
molecules and swells up
...

∆
∆
Na2B4O7
...
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
...
3
...
It is sparingly soluble
in water but highly soluble in hot water
...

Na2B4O7 + 2HCl + 5H2O → 2NaCl + 4B(OH)3
It is also formed by the hydrolysis (reaction
with water or dilute acid) of most boron
compounds (halides, hydrides, etc
...
11
...


2NaBH4 + I2 → B2H6 + 2NaI + H2
Diborane is produced on an industrial scale
by the reaction of BF3 with sodium hydride
...
It is
not a protonic acid but acts as a Lewis acid
by accepting electrons from a hydroxyl ion:
–
+
B(OH)3 + 2HOH → [B(OH)4] + H3O
On heating, orthoboric acid above 370K
forms metaboric acid, HBO2 which on further
heating yields boric oxide, B2O3
...
p
...
Diborane catches fire
spontaneously upon exposure to air
...


H3BO3 ∆→ HBO2 ∆→ B2O3

B2 H6 +3O2 → B2 O3 + 3H2 O;
∆c H

Fig
...
1 Structure of boric acid; the dotted lines
represent hydrogen bonds



= −1976 kJ mol−1

Most of the higher boranes are also
spontaneously flammable in air
...

B2H6(g) + 6H2O(l) → 2B(OH)3(aq) + 6H2(g)
Diborane undergoes cleavage reactions
with Lewis bases(L) to give borane adducts,
BH3⋅L
B2H6 + 2 NMe3 → 2BH3⋅NMe3
B2H6 + 2 CO → 2BH3⋅CO
Reaction of ammonia with diborane gives
initially B2H6
...

–

3B2H6 +6NH3 → 3[BH2 (NH3 )2 ]+ [BH4 ]
Heat

→2B3 N3 H6 +12H2

Problem 11
...
11
...
The four terminal hydrogen atoms
and the two boron atoms lie in one plane
...
The four terminal
B-H bonds are regular two centre-two electron
bonds while the two bridge (B-H-B) bonds are
different and can be described in terms of three

Solution
+
Because it is not able to release H ions
–
on its own
...

11
...
3 Diborane, B2H6
The simplest boron hydride known, is
diborane
...

4BF3 + 3 LiAlH4 → 2B2H6 + 3LiF + 3AlF3
A convenient laboratory method for the
preparation of diborane involves the oxidation
of sodium borohydride with iodine
...
11
...
11
...

Boron also forms a series of hydridoborates;
–
the most important one is the tetrahedral [BH4]
ion
...
Lithium and sodium tetrahydridoborates, also known as borohydrides,
are prepared by the reaction of metal hydrides
with B2H6 in diethyl ether
...
11
...
Each B atom
uses sp3 hybrids for bonding
...
The terminal
B-H bonds are normal 2-centre-2electron bonds but the two bridge
bonds are 3-centre-2-electron bonds
...


Both LiBH 4 and NaBH 4 are used as
reducing agents in organic synthesis
...

11
...
Boron fibres are used in making
bullet-proof vest and light composite material
10
for aircraft
...
The main
industrial application of borax and boric acid
is in the manufacture of heat resistant glasses
(e
...
, Pyrex), glass-wool and fibreglass
...
An aqueous solution of

orthoboric acid is generally used as a mild
antiseptic
...
It has a high
electrical and thermal conductivity
...
Aluminium is used extensively in
industry and everyday life
...
Aluminium and its
alloys can be given shapes of pipe, tubes,
rods, wires, plates or foils and, therefore, find
uses in packing, utensil making,
construction, aeroplane and transportation
industry
...

11
...
Carbon
is the seventeenth most abundant element by
mass in the earth’s crust
...
In elemental state it is available
as coal, graphite and diamond; however, in
combined state it is present as metal
carbonates, hydrocarbons and carbon dioxide
gas (0
...
One can emphatically say
that carbon is the most versatile element in the
world
...
Organic chemistry is devoted to
carbon containing compounds
...

Naturally occurring carbon contains two stable
12
13
isotopes: C and C
...
It is a radioactive
isotope with half-life 5770 years and used for
radiocarbon dating
...
7 % by mass) most abundant element on
the earth’s crust and is present in nature in
the form of silica and silicates
...
Germanium exists only in traces
...
5
...
This is
due to the presence of completely filled d and f
orbitals in heavier members
...
Flerovium is synthetically
prepared radioactive element
Ultrapure form of germanium and silicon
are used to make transistors and
semiconductor devices
...
It has atomic
-1
number 114, atomic mass 289 gmol and
14
10
2
2
electronic configuration [Rn] 5f 6d 7s 7p
...

Its half life is short and its chemistry has not
been established yet
...
3
...
5
...
The influence of inner
core electrons is visible here also
...

Small decrease in ∆iH from Si to Ge to Sn and
slight increase in ∆iH from Sn to Pb is the
consequence of poor shielding effect of
intervening d and f orbitals and increase in size
of the atom
...
5
...
The inner core of the
electronic configuration of elements in this
group also differs
...
5
...
The electronegativity values for
elements from Si to Pb are almost the same
...
3 Atomic and Physical Properties of Group 14 Elements
Element
Carbon
C
6

Property
Atomic Number
–1

Atomic mass (g mol )

12
...
09

2

[He]2s 2p

Electronic
configuration

2

2

72
...
71
2

[Ar]3d 4s 4p

2

10

207
...
5

1
...
8

1
...
9

e

2
...
32

7
...
34

4373

1693

1218

505

600

3550

3123

2896

2024

50

–5

3
...
22; β-form (stable at room temperature)

2019-20

10
e

2

2

[Xe]4f 5d 6s 6p

4+

Covalent radius/pm

a

Silicon
Si
14

–5

2 × 10

for diamond; for graphite, density is

324

CHEMISTRY

11
...
5 Physical Properties
All members of group14 are solids
...
Melting points and boiling points
of group 14 elements are much higher than those
of corresponding elements of group 13
...
5
...
The common oxidation states
exhibited by these elements are +4 and +2
...

Since the sum of the first four ionization
enthalpies is very high, compounds in +4
oxidation state are generally covalent in nature
...
It is due to the inability of ns
electrons of valence shell to participate in
bonding
...
Carbon
and silicon mostly show +4 oxidation state
...
Tin
forms compounds in both oxidation states (Sn
in +2 state is a reducing agent)
...
In tetravalent
state the number of electrons around the
central atom in a molecule (e
...
, carbon in CCl4)
is eight
...
Although
carbon cannot exceed its covalence more than
4, other elements of the group can do so
...

Due to this, their halides undergo hydrolysis
and have tendency to form complexes by
accepting electron pairs from donor species
...

(i) Reactivity towards oxygen
All members when heated in oxygen form
oxides
...
e
...
SiO only exists at high
temperature
...
The dioxides
— CO2, SiO2 and GeO2 are acidic, whereas
SnO2 and PbO2 are amphoteric in nature
...

Problem 11
...

Solution
(i) carbon (ii) lead
(iii) silicon and germanium
(ii) Reactivity towards water
Carbon, silicon and germanium are not
affected by water
...

∆
Sn + 2H2 O 
→
SnO2 + 2H2

Lead is unaffected by water, probably
because of a protective oxide film formation
...
Except
carbon, all other members react directly with
halogen under suitable condition to make
halides
...

The central metal atom in these halides
3
undergoes sp hybridisation and the molecule
is tetrahedral in shape
...
PbI4 does
not exist because Pb—I bond initially formed
during the reaction does not release enough
2
energy to unpair 6s electrons and excite one
of them to higher orbital to have four unpaired
electrons around lead atom
...
Stability of dihalides increases down the
group
...
Except CCl4,
other tetrachlorides are easily hydrolysed

2019-20

THE p-BLOCK ELEMENTS

325

by water because the central atom can
accommodate the lone pair of electrons from
oxygen atom of water molecule in d orbital
...
It undergoes hydrolysis
by initially accepting lone pair of electrons
from water molecule in d orbitals of Si, finally
leading to the formation of Si(OH)4 as shown
below :

Carbon also has unique ability to form
pπ– pπ multiple bonds with itself and with other
atoms of small size and high electronegativity
...
Heavier elements
do not form pπ– pπ bonds because their atomic
orbitals are too large and diffuse to have
effective overlapping
...
This property is called
catenation
...
Down the group the size increases
and electronegativity decreases, and, thereby,
tendency to show catenation decreases
...
The order of catenation is C > > Si >
Ge ≈ Sn
...

Bond

Problem 11
...

Give possible reasons
...

(ii) interaction between lone pair of
4+
chloride ion and Si is not very strong
...

11
...
6 IMPORTANT
TRENDS
AND
ANOMALOUS BEHAVIOUR OF
CARBON
Like first member of other groups, carbon
also differs from rest of the members of its
group
...

In carbon, only s and p orbitals are
available for bonding and, therefore, it can
accommodate only four pairs of electrons
around it
...


Carbon exhibits many allotropic forms; both
crystalline as well as amorphous
...
In 1985, third form of carbon
known as fullerenes was discovered by
H
...
Kroto, E
...
F
...
For this
discovery they were awarded the Nobel Prize
in 1996
...
7
...
In diamond each
3
carbon atom undergoes sp hybridisation and
linked to four other carbon atoms by using
hybridised orbitals in tetrahedral fashion
...
The structure
extends in space and produces a rigid threedimensional network of carbon atoms
...
11
...
4 The structure of graphite

structure (Fig
...
3) directional covalent bonds
are present throughout the lattice
...
It is used as an
abrasive for sharpening hard tools, in making
dyes and in the manufacture of tungsten
filaments for electric light bulbs
...
Graphite cleaves easily between the
layers and, therefore, it is very soft and slippery
...


Problem 11
...
Why ?
Solution
Diamond has a three-dimensional
network involving strong C—C bonds,
which are very difficult to break and, in
turn has high melting point
...
7
...
11
...

Layers are held by van der Waals forces and
distance between two layers is 340 pm
...
C—C bond length within the
layer is 141
...
Each carbon atom in
2
hexagonal ring undergoes sp hybridisation
and makes three sigma bonds with three
neighbouring carbon atoms
...
The electrons are delocalised
over the whole sheet
...
7
...
The sooty material
n
formed by condensation of vapourised C small
molecules consists of mainly C60 with smaller
quantity of C 70 and traces of fullerenes
consisting of even number of carbon atoms up
to 350 or above
...

Fullerenes are cage like molecules
...
11
...

It contains twenty six- membered rings and
twelve five-membered rings
...
All the carbon atoms are
2
equal and they undergo sp hybridisation
...
The remaining
electron at each carbon is delocalised in

2019-20

THE p-BLOCK ELEMENTS

327

molecular orbitals, which in turn give aromatic
character to molecule
...
5 pm and 138
...
Spherical fullerenes are also called
bucky balls in short
...

Carbon black is used as black pigment in
black ink and as filler in automobile tyres
...
Diamond is a precious
stone and used in jewellery
...

11
...

11
...
1 Carbon Monoxide
Direct oxidation of C in limited supply of
oxygen or air yields carbon monoxide
...
11
...


It is very important to know that graphite
is thermodynamically most stable allotrope of

carbon and, therefore, ∆f H of graphite is taken

as zero
...
90 and 38
...

Other forms of elemental carbon like carbon
black, coke, and charcoal are all impure forms
of graphite or fullerenes
...
Charcoal and coke are obtained
by heating wood or coal respectively at high
temperatures in the absence of air
...
7
...

The composites are used in products such as
tennis rackets, fishing rods, aircrafts and
canoes
...
Crucibles made from graphite are
inert to dilute acids and alkalies
...
H SO→ H2 O + CO
2

4

On commercial scale it is prepared by the
passage of steam over hot coke
...

473−1273K
C ( s ) + H2 O ( g ) 
→ CO ( g ) + H2 ( g )

Water gas
When air is used instead of steam, a mixture
of CO and N2 is produced, which is called
producer gas
...
Carbon monoxide
in water gas or producer gas can undergo
further combustion forming carbon dioxide
with the liberation of heat
...
It
is a powerful reducing agent and reduces
almost all metal oxides other than those of the
alkali and alkaline earth metals, aluminium
and a few transition metals
...

∆
Fe2 O3 ( s ) + 3CO ( g ) 

→ 2Fe ( s ) + 3CO2 ( g )
∆
ZnO ( s ) + CO ( g ) 

→ Zn ( s ) + CO2 ( g )

In CO molecule, there are one sigma and
two π bonds between carbon and oxygen,
:C ≡ O:
...
The highly poisonous
nature of CO arises because of its ability to
form a complex with haemoglobin, which
is about 300 times more stable than the
oxygen-haemoglobin complex
...

11
...
2 Carbon Dioxide
It is prepared by complete combustion of
carbon and carbon containing fuels in excess
of air
...

CaCO3(s) + 2HCl (aq) → CaCl2 (aq) + CO2 (g) +
H2O(l)
On commercial scale it is obtained by
heating limestone
...
Its low
solubility in water makes it of immense biochemical and geo-chemical importance
...
26 to 7
...

Being acidic in nature, it combines with alkalies
to form metal carbonates
...
03 % by volume in the

atmosphere, is removed from it by the process
known as photosynthesis
...
The
overall chemical change can be expressed as:
hν
6CO2 +12H2O →
C6 H12O6 + 6O2
Chlorophyll

+ 6H2O
By this process plants make food for
themselves as well as for animals and human
beings
...
But the
increase in combustion of fossil fuels and
decomposition of limestone for cement
manufacture in recent years seem to increase
the CO2 content of the atmosphere
...

Carbon dioxide can be obtained as a solid
in the form of dry ice by allowing the liquified
CO2 to expand rapidly
...

Gaseous CO2 is extensively used to carbonate
soft drinks
...
A
substantial amount of CO 2 is used to
manufacture urea
...
Two sp hybridised orbitals
of carbon atom overlap with two p orbitals of
oxygen atoms to make two sigma bonds while
other two electrons of carbon atom are involved
in pπ– pπ bonding with oxygen atom
...
The resonance structures are shown
below:

Resonance structures of carbon dioxide
11
...
3 Silicon Dioxide, SiO2
95% of the earth’s crust is made up of silica
and silicates
...
Quartz, cristobalite and tridymite are
some of the crystalline forms of silica, and they
are interconvertable at suitable temperature
...
Each oxygen atom in turn
covalently bonded to another silicon atoms as
shown in diagram (Fig 11
...
Each corner is
shared with another tetrahedron
...


substituted chlorosilane of formula MeSiCl3,
Me2SiCl2, Me3SiCl with small amount of Me4Si
are formed
...


Fig
...
6 Three dimensional structure of SiO2

Silica in its normal form is almost nonreactive because of very high Si — O bond
enthalpy
...
However, it is
attacked by HF and NaOH
...

Silica gel is used as a drying agent and as a support
for chromatographic materials and catalysts
...


The chain length of the polymer can be
controlled by adding (CH3)3SiCl which blocks
the ends as shown below :

11
...
4 Silicones
They are a group of organosilicon polymers,
which have (R2SiO) as a repeating unit
...
When methyl chloride reacts with
silicon in the presence of copper as a catalyst
at a temperature 573K various types of methyl

2019-20

330

CHEMISTRY

Silicones being surrounded by non-polar
alkyl groups are water repelling in nature
...
They have wide
applications
...
Being biocompatible they are also used
in surgical and cosmetic plants
...
8
What are silicones ?

(a)
Fig
...
7

Solution
Simple silicones consist of

(b)
4–

(a) Tetrahedral structure of SiO 4
4–
anion; (b) Representation of SiO4 unit

neutralised by positively charged metal ions
...


chains in which alkyl or phenyl groups
occupy the remaining bonding positions
on each silicon
...


Two important man-made silicates are
glass and cement
...
8
...
Some of the examples are feldspar,
zeolites, mica and asbestos
...
11
...
In
silicates either the discrete unit is present or
a number of such units are joined together
via corners by sharing 1,2,3 or 4 oxygen
atoms per silicate units
...

Negative charge on silicate structure is

11
...
6 Zeolites
If aluminium atoms replace few silicon atoms
in three-dimensional network of silicon dioxide,
overall structure known as aluminosilicate,
acquires a negative charge
...

Examples are feldspar and zeolites
...
g
...

Hydrated zeolites are used as ion exchangers
in softening of “hard” water
...
There are six groups of p-block elements in the periodic
2
1–6
table numbering from 13 to 18
...
Differences in the inner core of their electronic configuration greatly
influence their physical and chemical properties
...
In addition to the group oxidation
state, these elements show other oxidation states differing from the total number of valence
electrons by unit of two
...
The combined effect of size and availability of d orbitals considerably

2019-20

THE p-BLOCK ELEMENTS

331

influences the ability of these elements to form π-bonds
...
Absence of d orbital in
second period elements limits their maximum covalence to 4 while heavier ones can
exceed this limit
...
The availability of 3
2
1
valence electrons (2s 2p ) for covalent bond formation using four orbitals (2s, 2px, 2py and
2pz) leads to the so called electron deficiency in boron compounds
...

Boron forms covalent molecular compounds with dihydrogen as boranes, the simplest of
which is diborane, B2H6
...

The important compounds of boron with dioxygen are boric acid and borax
...
Borax is a white crystalline solid of formula Na2[B4O5(OH)4]·8H2O
...

Aluminium exhibits +3 oxidation state
...
This is a consequence of the so called
inert pair effect
...
It shows the property of catenation, the ability to form chains or
rings, not only with C–C single bonds but also with multiple bonds (C=C or C≡C)
...
Carbon provides one of the
best examples of allotropy
...
The members of the carbon family mainly exhibit +4 and +2 oxidation
states; compouds in +4 oxidation states are generally covalent in nature
...
Lead in +2 state is stable
whereas in +4 oxidation state it is a strong oxidising agent
...
It forms two important oxides: CO and CO2
...
Carbon monoxide having lone pair of electrons on C
forms metal carbonyls
...
Carbon dioxide as such is not
toxic
...
This,
in turn, raises the temperature of the atmosphere and causes serious complications
...


EXERCISES
11
...


11
...
3

Why does boron triflouride behave as a Lewis acid ?

11
...
How will they behave with
water ? Justify
...
5

Is boric acid a protic acid ? Explain
...
6

Explain what happens when boric acid is heated
...
7

Describe the shapes of BF3 and BH4–
...


11
...


2019-20

332

CHEMISTRY

11
...


11
...


11
...
12

Explain the difference in properties of diamond and graphite on the basis
of their structures
...
13

Rationalise the given statements and give chemical reactions :

2–

–

2–

•

Lead(II) chloride reacts with Cl2 to give PbCl4
...


•

Lead is known not to form an iodide, PbI4
...
14

Suggest reasons why the B–F bond lengths in BF 3 (130 pm) and BF4
(143 pm) differ
...
15

If B–Cl bond has a dipole moment, explain why BCl3 molecule has zero
dipole moment
...
16

Aluminium trifluoride is insoluble in anhydrous HF but dissolves on
addition of NaF
...
Give reasons
...
17

Suggest a reason as to why CO is poisonous
...
18

How is excessive content of CO2 responsible for global warming ?

11
...


11
...
21

Explain the following reactions
(a) Silicon is heated with methyl chloride at high temperature in the
presence of copper;
(b) Silicon dioxide is treated with hydrogen fluoride;
(c) CO is heated with ZnO;
(d) Hydrated alumina is treated with aqueous NaOH solution
...
22

Give reasons :
Conc
...

A mixture of dilute NaOH and aluminium pieces is used to open
drain
...

(iv) Diamond is used as an abrasive
...

(vi) Aluminium utensils should not be kept in water overnight
...


(i)
(ii)

11
...
24

How would you explain the lower atomic radius of Ga as compared to Al ?

11
...
What is the impact of structure on physical
properties of two allotropes?

2019-20

THE p-BLOCK ELEMENTS

11
...


11
...
Support this statement by giving some
evidences
...
28

When metal X is treated with sodium hydroxide, a white precipitate (A) is
obtained, which is soluble in excess of NaOH to give soluble complex (B)
...
The compound
(A) when heated strongly gives (D), which is used to extract metal
...
Write suitable equations to support their identities
...
29

What do you understand by (a) inert pair effect
(c) catenation?

11
...


(b) allotropy

(i)

Its aqueous solution is alkaline to litmus
...


and

(iii) When conc
...

Write equations for all the above reactions and identify X, Y and Z
...
31

Write balanced equations for:
(i) BF3 + LiH

→

(ii) B2H6 + H2O →
(iii) NaH + B2H6 →
∆
(iv) H3BO3 
→
(v) Al + NaOH →

(vi) B2H6

+ NH3 →

11
...
Give one method for industrial preparation and one for laboratory
preparation of CO and CO2 each
...
33

An aqueous solution of borax is
(a) neutral
(b) amphoteric
(c) basic
(d) acidic
11
...
35 The type of hybridisation of boron in diborane is
(c) sp3
(d) dsp2
(a) sp
(b) sp2
11
...
37 Elements of group 14
(a) exhibit oxidation state of +4 only
(b) exhibit oxidation state of +2 and +4
(c) form M2– and M4+ ions
(d) form M2+ and M4+ ions
11
...


2019-20



---