7/25/2019 Pblock class 12th notes for iitjee http://slidepdf.com/reader/full/pblock-class-12th-notes-for-iitjee 1/61 Objectives After studying this Unit, you will b e able to • appreciate general trends in the chemistry of elements of groups 15,16,17 and 18; • learn the preparation, properties and uses of dinitrogen and phosphorus and some of their important compounds; • describe the preparation, properties and uses of dioygen and o!one and chemistry of some simple oides; • "now allotropic forms of sulphur, chemistry of its important compounds and the structures of its ooacids; • describe the preparation, properties and uses of chlorine and hydrochloric acid; • "now the chemistry of interhalogens and structures of ooacids of halogens; • enumerate the uses of noble gases; • appreciate the importance of these elements and their compounds in our day to day life# Unit 7 The p-Block Elements Diversity in chemistry is the hallmark of p–block elements manifested in their ability to react with the elements of s–, d– and f–blocks as well as with their own. $n %lass &$, you ha'e learnt that the p(bloc" elements are placed in groups 1) to 18 of the periodic table# *heir 'alence shell electronic configuration is ns + np 16 -ecept .e which has 1 s + configuration/# *he properties of p(bloc" elements li"e that of others are greatly influenced by atomic si!es, ionisation enthalpy, electron gain enthalpy and electronegati'ity# *he absence of d ( orbitals in second period and presence of d and0or f orbitals in hea'ier elements -starting from third period onwards/ ha'e significant effects on the properties of elements# $n addition, the presence of all the three types of elements; metals, metalloids and non(metals bring di'ersification in chemistry of these elements# .a'ing learnt the chemistry of elements of roups 1) and 12 of the p(bloc" of periodic table in %lass &$, you will learn the chemistry of the elements of subse3uent groups in this Unit# 7.1 Group 15 Elements 7.1.1 Occurrence roup 15 includes nitrogen, phosphorus, arsenic, antimony and bismuth# As we go down the group, there is a shift from non(metallic to metallic through metalloidic character# 4itrogen and phosphorus are non(metals, arsenic and antimony metalloids and bismuth is a typical metal# olecular nitrogen comprises 78 by 'olume of the atmosphere# $n the earths crust, it occurs as sodium nitrate, 4a4) -called %hile saltpetre/ and potassium nitrate -$ndian saltpetre/# $t is found in the form of proteins in plants and animals# 9hosphorus occurs in minerals
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ObjectivesAfter studying this Unit, you will be able to
• appreciate general trends in the chemistry
of elements of groups
15,16,17 and 18;
• learn the preparation, properties and uses
of dinitrogen and phosphorus and some of
their important compounds;
• describe the preparation, properties and
uses of dioygen and o!one and chemistry
of some simple oides;
• "now allotropic forms of sulphur,
chemistry of its important compounds and
the structures of its ooacids;
• describe the preparation, properties and
uses of chlorine and hydrochloric acid;
• "now the chemistry of interhalogens and
structures of ooacids of halogens;
• enumerate the uses of noble gases;
• appreciate the importance of these
elements and their compounds in our day
to day life#
Unit7
The p-Block
Elements
Diversity in chemistry is the hallmark of p–block elements manifested in their
ability to react with the elements of s–, d– and f–blocks as well as with their
own.
$n %lass &$, you ha'e learnt that the p(bloc" elements are placed
in groups 1) to 18 of the periodic table# *heir 'alence shell
electronic configuration is ns+np16 -ecept .e which has 1 s+
configuration/# *he properties of p(bloc" elements li"e that of
others are greatly influenced by atomic si!es, ionisation enthalpy,
electron gain enthalpy and electronegati'ity# *he absence of d (
orbitals in second period and presence of d and0or f orbitals in
hea'ier elements -starting from third period onwards/ ha'e
significant effects on the properties of elements# $n addition, the
presence of all the three types of elements; metals, metalloids and
non(metals bring di'ersification in chemistry of these elements#
.a'ing learnt the chemistry of elements of roups 1) and 12of the p(bloc" of periodic table in %lass &$, you will learn the
chemistry of the elements of subse3uent groups in this Unit#
7.1 Group 15
Elements
7.1.1 Occurrence
roup 15 includes nitrogen, phosphorus, arsenic, antimony and bismuth# As we godown the group, there is a shift from non(metallic to metallic through metalloidic
character# 4itrogen and phosphorus are non(metals, arsenic and antimony metalloids
and bismuth is a typical metal#
olecular nitrogen comprises 78 by 'olume of the atmosphere# $n the earths crust,
it occurs as sodium nitrate, 4a4) -called %hile saltpetre/ and potassium nitrate
-$ndian saltpetre/# $t is found in the form of proteins in plants and animals#9hosphorus occurs in minerals
of the apatite family, %a:-92/6# %a&+ -& <, %l or ./ -e#g#, fluorapatite %a :
-92/6# %a<+/ which are the main components of phosphate roc"s# 9hosphorus is an
essential constituent of animal and plant matter# $t is present in bones as well as inli'ing cells# 9hosphoproteins are present in mil" and eggs# Arsenic, antimony and
bismuth are found mainly as sulphide minerals#
*he important atomic and physical properties of this group elements along with
their electronic configurations are gi'en in *able 7#1#
Table 7.1: Atomic and Physical Properties of Group 15 Elements
Atomic number
Atomic mass0g mol 1
=lectronic configuration$onisation
- i H 0-">
=lectronegati'ity
%o'alent radius0pma
$onic radius0pm
elting
?oiling
@ensity0g cm )-+:8
a E
III single bond (E = element!
b E
"– !
c E
"#!
d $hite phosphor%s!
e &rey α 'form at ".) atm!
f *%blimation temperat%re!
g +t )" !
h&rey α 'form! - olec%lar / 0.
*rends of some of the atomic, physical and chemical properties of the group are
discussed below#
7.1.2 Electronic *he 'alence shell electronic configuration of these elements is ns+np)# onfi!uration *he s
orbital in these elements is completely filled and p orbitals are
half(filled, ma"ing their electronic configuration etra stable#
7.1." Atomic and %o'alent and ionic -in a particular state/ radii increase in si!e #onic $adii down the group#
*here is a considerable increase in co'alent radius
from 4 to 9# .owe'er, from As to ?i only a small increase in co'alent radius is
obser'ed# *his is due to the presence of completely filled d and0or f orbitals in
hea'ier members#
7.1.% #onisation $onisation enthalpy decreases down the group due to gradual increase Enthalpy in atomic
si!e# ?ecause of the etra stable half(filled p orbitals electronic
configuration and smaller si!e, the ionisation enthalpy of the group 15 elements is
much greater than that of group 12 elements in the corresponding periods# *he order
of successi'e ionisation enthalpies, as epected is i.1 B i.+ B i.) -*able 7#1/#
7.1.5 *he electronegati'ity 'alue, in general, decreases down the group with
Electrone!ati&ity increasing atomic si!e# .owe'er, amongst the hea'ier elements, the
difference is not that much pronounced#
7.1.' Physical All the elements of this group are polyatomic# @initrogen is a diatomic gas Properties while allothers are solids# etallic character increases down the group#
7.1.7 hemical
Properties
4itrogen and phosphorus are non(metals, arsenic and antimony metalloids and
bismuth is a metal# *his is due to decrease in ionisation enthalpy and increase in
atomic si!e# *he boiling points, in general, increase from top to bottom in the group
but the melting point increases upto arsenic and then decreases upto bismuth# =cept
nitrogen, all the elements show allotropy#
Oxidation states and trends in chemical reactivity
*he common oidation states of these elements are ), C) and C5# *he tendency to
ehibit ) oidation state decreases down the group due to increase in si!e and
metallic character# $n fact last member of the group, bismuth hardly forms any
compound in ) oidation state# *he stability of C5 oidation state decreases downthe group# *he only well characterised ?i -D/ compound is ?i<5# *he stability of C5
oidation state decreases and that of C) state increases -due to inert pair effect/ down
the group# 4itrogen ehibits C 1, C +, C 2 oidation states also when it reacts with
oygen# 9hosphorus also shows C1 and C2 oidation states in some ooacids#
$n the case of nitrogen, all oidation states from C1 to C2 tend to disproportionate
in acid solution# <or eample,
).4+ → .4) C .+ C +4
Eimilarly, in case of phosphorus nearly all intermediate oidation states
disproportionate into C5 and ) both in al"ali and acid# .owe'er C) oidation state
in case of arsenic, antimony and bismuth become increasingly stable with respect todisproportionation#
4itrogen is restricted to a maimum co'alency of 2 since only four -one s and
three p/ orbitals are a'ailable for bonding# *he hea'ier elements ha'e 'acant d orbitals in the outermost shell which can be used for bonding -co'alency/ and hence,
epand their co'alence as in 9<6 #
Anomalous properties of nitrogen
4itrogen differs from the rest of the members of this group due to its smaller si!e,
high electronegati'ity, high ionisation enthalpy and non(a'ailability of d orbitals#
4itrogen has uni3ue ability to form p ( p multiple bonds with itself and with other
elements ha'ing small si!e and high electronegati'ity -e#g#, %, /# .ea'ier elements
of this group do not form pπ ( pπ bonds as their atomic orbitals are so large and
diffuse that they cannot ha'e effecti'e o'erlapping# *hus, nitrogen eists as adiatomic molecule with a triple bond -one s and two p/ between the two atoms#
%onse3uently, its bond enthalpy -:21#2 "> mol 1/ is 'ery high# n the contrary,
phosphorus, arsenic and antimony form single bonds as 99, AsAs and EbEb while bismuth forms metallic bonds in elemental state# .owe'er, the single 44 bond is
wea"er than the single 99 bond because of high interelectronic repulsion of the non(
bonding electrons, owing to the small bond length# As a result the catenation
$n ooacids phosphorus is tetrahedrally surrounded by other atoms# All these
acids contain one 9 and at least one 9. bond# *he ooacids in which
phosphorus has lower oidation state -less than C5/ contain, in addition to 9 and
9. bonds, either 99 -e#g#, in .29+6/ or 9. -e#g#, in .)9+/ bonds but not both#
*hese acids in C) oidation state of phosphorus tend to disproportionate to higher and lower oidation states# <or eample, orthophophorous acid -or phosphorous acid/
on heating disproportionates to gi'e orthophosphoric acid -or phosphoric acid/ and
phosphine#
2.) 9) →).) 92 + 9.)
*he acids which contain 9. bond ha'e strong reducing properties# *hus,
hypophosphorous acid is a good reducing agent as it contains two 9. bonds and
reduces, for eample, Ag4) to metallic sil'er#
2 Ag4) C +.+ C .)9+ → 2Ag C 2.4) C .)92
*hese 9. bonds are not ionisable to gi'e .
C
and do not play any role in basicity# nly those . atoms which are attached with oygen in 9. form are
ionisable and cause the basicity# *hus, .)9) and .)92 are dibasic and tribasic,
respecti'ely as the structure of .)9) has two 9. bonds and .)92 three#
Example 7.*
Solution
.ow do you account for the reducing beha'iour of .)9+ on the
basis of its structure M
$n .)9+, two . atoms are bonded directly to 9 atom whichimparts reducing character to the acid#
Intext Questions7.11 Lhat is the basicity of
.)92M
7.12 Lhat happens when
.)9) is heatedM
7.1+ Group 1$ygen, sulphur, selenium, tellurium and polonium constitute
roup
Elements
16 of the periodic table# *his is sometimes "nown as group of
chalcogens# *he name is deri'ed from the ree" word for brass and
points to the association of sulphur and its congeners with copper#
ost copper minerals contain either oygen or sulphur andfre3uently the other members of the group#
7.1.1 Occurrence ygen is the most abundant of all the elements on earth# ygen forms about 26#6 by
mass of earths crust# @ry air contains +O#:26 oygen by 'olume#
.owe'er, the abundance of sulphur in the earths crust is only O#O)(O#1#
%ombined sulphur eists primarily as sulphates such as gyps%m %aE2#+.+, epsom
salt gE2#7.+, baryte ?aE2 and sulphides such as galena 9bE, :inc blende KnE,
copper pyrites %u<eE+# *races of sulphur occur as hydrogen sulphide in 'olcanoes#
rganic materials such as eggs, proteins, garlic, onion, mustard, hair and wool
Eelenium and tellurium are also found as metal selenides and tellurides in
sulphide ores# 9olonium occurs in nature as a decay product of thorium and uranium
minerals#
*he important atomic and physical properties of roup16 along with electronic
configuration are gi'en in *able 7#6# Eome of the atomic, physical and chemical properties and their trends are discussed below#
7.1.2 Electronic *he elements of roup16 ha'e si electrons in the outermost shell and onfi!uration ha'e
ns+np2 general electronic configuration#
7.1." Atomic @ue to increase in the number of shells, atomic and ionic radii increase and #onic from top to
bottom in the group# *he si!e of oygen atom is, howe'er,
$adii eceptionally small#
7.1.% #onisation $onisation enthalpy decreases down the group# $t is due to increase in Enthalpy si!e#
.owe'er, the elements of this group ha'e lower ionisation enthalpy
'alues compared to those of roup15 in the corresponding periods# *his is due to the
fact that roup 15 elements ha'e etra stable half(filled p orbitals electronic
configurations#
7.1.5 Electron ?ecause of the compact nature of oygen atom, it has less negati'e
Gain electron gain enthalpy than sulphur# .owe'er, from sulphur onwards
Enthalpy the 'alue again becomes less negati'e upto polonium#
7.1.' 4et to fluorine, oygen has the highest electronegati'ity 'alue amongst
Electrone!ati&ity the elements# Lithin the group, electronegati'ity decreases with an
increase in atomic number# *his implies that the metallic character
increases from oygen to polonium#
=lements of roup 16 generally show lower 'alue of first ionisation enthalpy compared to
the corresponding periods of group 15# LhyM
@ue to etra stable half(filled p orbitals electronic configurations of roup 15 elements,
larger amount of energy is re3uired to remo'e electrons compared to roup 16 elements#
Example 7.1+
Solution
7.1.7 Physical Eome of the physical properties of roup 16 elements are gi'en in Properties *able 7#6#
ygen and sulphur are non(metals, selenium and tellurium metalloids, whereas polonium is a metal#
9olonium is radioacti'e and is short li'ed -.alf(life 1)#8 days/# All these elements ehibit allotropy#
*he melting and boiling points increase with an increase in atomic number down thegroup# *he large difference between the melting and boiling points of oygen and
sulphur may be eplained on the basis of their atomicity; oygen eists as diatomic
molecule -+/ whereas sulphur eists as polyatomic molecule -E8/#
7.1./ hemicalOxidation states and trends in chemical reactivityProperties
*he elements of roup 16 ehibit a number of oidation states -*able
7#6/# *he stability of (+ oidation state decreases down the group#
9olonium hardly shows + oidation state# Eince electronegati'ity of
oygen is 'ery high, it shows only negati'e oidation state as + ecept
Table 7.': -ome Physical Properties of Group 1' Elements
Property
Atomic number
Atomic mass0g mol 1
=lectronic configuration
%o'alent radius0-pm/a
$onic radius, =+ 0pm
=lectron gain enthalpy,
0 eg H "> mol 1
$onisation enthalpy - i H 1/
0"> mol 1
=lectronegati'ity
@ensity 0g cm ) -+:8 I/
elting point0I
?oiling point0I
idation states0
a*ingle bond!
b +ppro2imate val%e!
c +t the melting point!
d 1hombic s%lph%r!
e He2agonal grey!
f onoclinic form, );" .
92ygen shows o2idation states of #0 and #< in o2ygen fl%orides 93 0 and 90 3 0 respectively.
in the case of <+ where its oidation state is C +# ther elements of the group
ehibit C +, C 2, C 6 oidation states but C 2 and C 6 are more common# Eulphur,
selenium and tellurium usually show C 2 oidation state in their compounds with
oygen and C 6 with fluorine# *he stability of C 6 oidation state decreases down thegroup and stability of C 2 oidation state increase -inert pair effect/# ?onding in C2
and C6 oidation states are primarily co'alent#
Anomalous "ehaviour of oxygen
*he anomalous beha'iour of oygen, li"e other members of p(bloc" present in
second period is due to its small si!e and high electronegati'ity# ne typical eample
of effects of small si!e and high electronegati'ity is the presence of strong hydrogen
bonding in .+ which is not found in .+E#
*he absence of d orbitals in oygen limits its co'alency to four and in practice,
rarely eceeds two# n the other hand, in case of other elements of the group, the'alence shells can be epanded and co'alence eceeds four#
-i/ 1eactivity with hydrogenG All the elements of roup 16 form hydrides of the
type .+= -= E, Ee, *e, 9o/# Eome properties of hydrides are gi'en in *able
7#7# *heir acidic character increases from . + to .+*e# *he increase in acidic
character can be eplained in terms of decrease in bond -.=/ dissociationenthalpy down the group# wing to the decrease in bond -.=/ dissociation
enthalpy down the group, the thermal stability of hydrides also decreases from
.+ to .+9o# All the hydrides ecept water possess reducing property and this
Table 7.7: Properties of *ydrides of Group 1' Elements
Property
m#p0I
b#p0I
.= distance0pm
.=. angle -J/
f H 0"> mol 1
diss H (H–E0"> mol 1
@issociation constanta
a +%eo%s sol%tion, 0>
-ii/ 1eactivity with o2ygenG All these elements form oides of the = + and =)
types where = E, Ee, *e or 9o# !one - )/ and sulphur dioide -E+/ are
gases while selenium dioide -Ee+/ is solid# Feducing property of dioidedecreases from E+ to *e+; E+ is reducing while *e+ is an oidising agent#
?esides =+ type, sulphur, selenium and tellurium also form = ) type oides
-E), Ee), *e)/# ?oth types of oides are acidic in nature#
-iii/ 1eactivity towards the halogensG =lements of roup 16 form a large number of
halides of the type, =&6, =&2 and =&+ where = is an element of the group and
& is a halogen# *he stability of the halides decreases in the order < H %l H ?r
H $ # Amongst heahalides, heafluorides are the only stable halides# All
heafluorides are gaseous in nature# *hey ha'e octahedral structure# Eulphur
heafluoride, E<6 is eceptionally stable for steric reasons#
Amongst tetrafluorides, E<2 is a gas, Ee<2 a li3uid and *e<2 a solid# *hesefluorides ha'e sp)d hybridisation and thus, ha'e trigonal bipyramidal structures in
which one of the e3uatorial positions is occupied by a lone pair of electrons# *hisgeometry is also regarded as see'saw geometry#
All elements ecept selenium form dichlorides and dibromides# *hese dihalides
are formed by sp) hybridisation and thus, ha'e tetrahedral structure# *he well "nown
monohalides are dimeric in nature# =amples are E+<+, E+%l+, E+?r +, Ee+%l+ and
Ee+?r +# *hese dimeric halides undergo disproportionation as gi'en belowG
+Ee+%l+ → Ee%l2 C )Ee
.+E is less acidic than .+*e# LhyM
@ue to the decrease in bond -=./ dissociation
enthalpy down the group, acidic character increases#
Example 7.11Solution
Intext Questions7.1" Nist the important sources of sulphur#
7.1% Lrite the order of thermal stability of the hydrides of roup 16 elements#
Lhen a slow dry stream of oygen is passed through a silent electrical discharge,
con'ersion of oygen to o!one -1O/ occurs# *he product is "nown as o!onised
oygen#
)+ → +) .V -+:8 I/ C12+ "> mol 1
Eince the formation of o!one from oygen is an endothermic process, it is
necessary to use a silent electrical discharge in its preparation to pre'ent its
decomposition#
$f concentrations of o!one greater than 1O per cent are re3uired, a battery of
o!onisers can be used, and pure o!one -b#p# )85 I/ can be condensed in a 'essel
surrounded by li3uid oygen#
Properties
9ure o!one is a pale blue gas, dar" blue li3uid and 'iolet(blac" solid# !one has a
characteristic smell and in small concentrations it is harmless# .owe'er, if the
concentration rises abo'e about 1OO parts per million, breathing becomesuncomfortable resulting in headache and nausea#
!one is thermodynamically unstable with respect to oygen since its
decomposition into oygen results in the liberation of heat - . is negati'e/ and an
increase in entropy - E is positi'e/# *hese two effects reinforce each other, resulting
in large negati'e ibbs energy change - / for its con'ersion into oygen# $t is not
really surprising, therefore, high concentrations of o!one can be dangerously
eplosi'e#
@ue to the ease with which it liberates atoms of nascent oygen -) →+ C /, itacts as a powerful oidising agent# <or e#g#, it oidises lead sulphide to lead sulphateand iodide ions to iodine#
9bE-s/ C 2)-g/ → 9bE2-s/ C 2+-g/
+$ -a3/ C .+-l/ C )-g/ →+. -a3/ C $+-s/ C +-g/
Lhen o!one reacts with an ecess of potassium iodide solution buffered with a
borate buffer -p. :#+/, iodine is liberated which can be titrated against a standard
solution of sodium thiosulphate# *his is a 3uantitati'e method for estimating ) gas#
=periments ha'e shown that nitrogen oides -particularly nitric oide/ combine
'ery rapidly with o!one and there is, thus, the possibility that nitrogen oides emitted
from the ehaust systems of supersonic Set aeroplanes might be slowly depleting the
concentration of the o!one layer in the upper atmosphere#
4 (g ) + ) (g ) → 4+ (g ) + + (g)
Another threat to this o!one layer is probably posed by the use of freons which
are used in aerosol sprays and as refrigerants#
*he two oygen(oygen bond lengths in the o!one molecule are identical-1+8 pm/ and the molecule is angular as epected with a bond angle of
about 117o# $t is a resonance
hybrid of two main formsG
Uses: $t is used as a germicide, disinfectant and for sterilising water# $t is also used for
bleaching oils, i'ory, flour, starch, etc# $t acts as an oidising agent in the manufacture of
%oncentrated sulphuric acid is a strong dehydrating agent# any wet gases can
be dried by passing them through sulphuric acid, pro'ided the gases do not react with
the acid# Eulphuric acid remo'es water from organic compounds; it is e'ident by its
charring action on carbohydrates#
%1+.++11 ????→.+E2 1+% C 11.+
.ot concentrated sulphuric acid is a moderately strong oidising agent# $n thisrespect, it is intermediate between phosphoric and nitric acids# ?oth metals and non(
metals are oidised by concentrated sulphuric acid, which is reduced to E+#
%u C + .+E2-conc#/ →%uE2 C E+ C +.+ )E C
+.+E
2-conc#/ →)E
+ C +.
+
% C +.+E2-conc#/ →%+ C + E+ C + .+
Uses: Eulphuric acid is a 'ery important industrial chemical# A nations industrial
strength can be Sudged by the 3uantity of sulphuric acid it produces and consumes# $t is
needed for the manufacture of hundreds of other compounds and also in many industrial
processes# *he bul" of sulphuric acid produced is used in the manufacture of fertilisers
-e#g#, ammonium sulphate, superphosphate/# ther uses are inG
-a/ petroleum refining -b/ manufacture of pigments, paints and dyestuff intermediates -c/
detergent industry -d/ metallurgical applications -e#g#, cleansing metals before enameling,
electroplating and gal'anising
-e/ storage batteries -f/ in the manufacture of nitrocellulose products and -g/ as a
laboratory reagent#
Lhat happens when
-i/ %oncentrated .+E2 is added to calcium fluoride
-ii/ E) is passed through waterM
-i/ $t forms hydrogen fluoride %a<+ + . + E 2 →%aE 2 +
*he trends of some of the atomic, physical and chemical properties are discussed
below#
7.1/.2 Electronic All these elements ha'e se'en electrons in their outermost shell onfi!uration -ns+np5/
which is one electron short of the net noble gas#
7.1/." Atomic*he halogens ha'e the smallest atomic radii in their respecti'e periods
and #onic due to maimum effecti'e nuclear charge# *he atomic radius of fluorine $adii li"e the
other elements of second period is etremely small# Atomic and ionic radii increase from fluorine to
iodine due to increasing number
of 3uantum shells#
7.1/.% #onisation *hey ha'e little tendency to lose electron# *hus they ha'e 'ery high Enthalpy ionisation
enthalpy# @ue to increase in atomic si!e, ionisation enthalpy
decreases down the group#
7.1/.5 Electron.alogens ha'e maimum negati'e electron gain enthalpy in the
Gain corresponding periods# *his is due to the fact that the atoms of theseEnthalpy elements ha'e only one electron less than stable noble gas configurations#
=lectron gain enthalpy of the elements of the group becomes less negati'e
down the group# .owe'er, the negati'e electron gain enthalpy of fluorine
is less than that of chlorine# $t is due to small si!e of fluorine atom# As
a result, there are strong interelectronic repulsions in the relati'ely
small + p orbitals of fluorine and thus, the incoming electron does not
eperience much attraction#
7.1/.' *hey ha'e 'ery high electronegati'ity# *he electronegati'ity decreases
Electrone!ati&ity down the group# <luorine is the most electronegati'e element in the
periodic table#
.alogens ha'e maimum negati'e electron gain enthalpy in the Example 7.14 respecti'e
periods of the periodic table# LhyM
.alogens ha'e the smallest si!e in their respecti'e periods and therefore Solution high
effecti'e nuclear charge# As a conse3uence, they readily accept
one electron to ac3uire noble gas electronic configuration#
7.1/.7 Physical .alogens display smooth 'ariations in their physical properties# <luorine Properties and
chlorine are gases, bromine is a li3uid and iodine is a solid# *heir melting and boiling points steadily
increase with atomic number# All
halogens are coloured# *his is due to absorption of radiations in 'isible region whichresults in the ecitation of outer electrons to higher energy le'el# ?y absorbing
different 3uanta of radiation, they display different colours# <or eample, < +, has
yellow, %l+ , greenish yellow, ?r +, red and $+, 'iolet colour# <luorine and chlorine
react with water# ?romine and iodine are only sparingly soluble in water but are
soluble in 'arious organic sol'ents such as chloroform, carbon tetrachloride, carbon
disulphide and hydrocarbons to gi'e coloured solutions#
ne curious anomaly we notice from *able 7#8 is the smaller enthalpy of
dissociation of <+ compared to that of %l+ whereas &(& bond dissociation enthalpiesfrom chlorine onwards show the epected
*he relati'e oidising power of halogens can further be illustrated by their
reactions with water# <luorine oidises water to oygen whereas chlorine and bromine react with water to form corresponding hydrohalic and hypohalous acids#
*he reaction of iodine with water is non(spontaneous# $n fact, $ can be oidised by
oygen in acidic medium; Sust the re'erse of the reaction obser'ed with fluorine#
+<+ (g) + +.+ ( l ) →2.+ (a3) + 2<
− (a3) + + (g)
&+ ( g) + .+ ( l ) →.& ( a3) + .& ( a3)
( where & %l or ?r )
2$− ( a3) + 2.
+ ( a3) + + ( g) →+$+ ( s) + +.+ ( l )
Anomalous "ehaviour of fluorine
Ni"e other elements of p(bloc" present in second period of the periodic table, fluorine
is anomalous in many properties# <or eample, ionisation enthalpy, electronegati'ity,
enthalpy of bond dissociation and electrode potentials are all higher for fluorine than
epected from the trends set by other halogens# Also, ionic and co'alent radii, m#p#
and b#p# and electron gain enthalpy are 3uite lower than epected# *he anomalous
beha'iour of fluorine is due to its small si!e, highest electronegati'ity, low <(< bond
dissociation enthalpy, and non a'ailability of d orbitals in 'alence shell#
ost of the reactions of fluorine are eothermic -due to the small and strong
bond formed by it with other elements/# $t forms only one ooacid while other
halogens form a number of ooacids# .ydrogen fluoride is a li3uid -b#p# +:) I/ due
to strong hydrogen bonding# ther hydrogen halides are gases#
-i/ 1eactivity towards hydrogenG *hey all react with hydrogen to gi'e hydrogen
halides but affinity for hydrogen decreases from fluorine to iodine# *hey
dissol'e in water to form hydrohalic acids# Eome of the properties of hydrogen
halides are gi'en in *able 7#:# *he acidic strength of these acids 'aries in the
orderG .< B .%l B .?r B .$# *he stability of these halides decreases down the
group due to decrease in bond -.&/ dissociation enthalpy in the orderG .< H
.%l H .?r H .$#
Table 7.: Properties of *ydro!en *alides
Property
elting point0I
?oiling point0I
?ond length -. &/0pm
diss H V0"> mol
1
p a
-ii/ 1eactivity towards o2ygenG .alogens form many oides with oygen but mostof them are unstable# <luorine forms two oides <+ and +<+# .owe'er, only
are essentially oygen fluorides because of the higher electronegati'ity of
fluorine than oygen# ?oth are strong fluorinating agents# +<+ oidises
plutonium to 9u<6 and the reaction is used in remo'ing plutonium as 9u<6 fromspent nuclear fuel#
%hlorine, bromine and iodine form oides in which the oidation states of these halogens range from C1 to C7# A combination of "inetic and
thermodynamic factors lead to the generally decreasing order of stability of
oides formed by halogens, $ H %l H ?r# *he higher oides of halogens tend to
be more stable than the lower ones#
%hlorine oides, %l+, %l+, %l+6 and %l+7 are highly reacti'e oidising
agents and tend to eplode# %l+ is used as a bleaching agent for paper pulpand tetiles and in water treatment#
*he bromine oides, ?r +, ?r+ , ?r) are the least stable halogen oides
-middle row anomally/ and eist only at low temperatures# *hey are 'ery
powerful oidising agents#
*he iodine oides, $+2 , $+5, $+7 are insoluble solids and decompose onheating# $+5 is a 'ery good oidising agent and is used in the estimation of carbon monoide#
-iii/ 1eactivity towards metalsG .alogens react with metals to form metal halides#
<or e#g#, bromine reacts with magnesium to gi'e magnesium bromide#
g (s) + ?r + (l) →g?r + (s)
Example7.1$
Solution
*he ionic character of the halides decreases in the order <H %l H ?r H $ where is a mono'alent metal# $f a metal
ehibits more than one oidation state, the halides in higher
oidation state will be more co'alent than the one in lower
oidation state# <or e#g#, En%l2, 9b%l2, Eb%l5 and U<6 are more
co'alent than En%l+, 9b%l+, Eb%l) and U<2 respecti'ely#
-i'/ 1eactivity of halogens towards other halogensG .alogens
combine amongst themsel'es to form a number of compounds
"nown as interhalogens of the types &&′ , &&)
′, &&5
′ and
&&7′ where & is a larger si!e halogen and &
′ is smaller si!e
halogen#
<luorine ehibits only 1 oidation state whereas other halogens
ehibit C 1, C ), C 5 and C 7 oidation states also# =plain#
<luorine is the most electronegati'e element and cannot ehibit any
positi'e oidation state# ther halogens ha'e d orbitals and
therefore, can epand their octets and show C 1, C ), C 5 and C 7
oidation states also#
Intext Questions
7.2' %onsidering the parameters such as bond dissociation enthalpy, electron gain
enthalpy and hydration enthalpy, compare the oidising power of <+ and %l+#
7.27 i'e two eamples to show the anomalous beha'iour of fluorine#
7.2/ Eea is the greatest source of some halogens# %omment#
*he composition of bleaching powder is %a-%l/+#%a%l+#%a-./+#+.+# %hlorine
reacts with hydrocarbons and gi'es substitution products with saturated hydrocarbons
and addition products with unsaturated
hydrocarbons# <or eample,%.2 C %l+
ethane%+.2 C %l+
=thene
%hlorine water on standing loses its yellow colour due to the formation of .%l
and .%l# .ypochlorous acid -.%l/ so formed, gi'es nascent oygen which is
responsible for oidising and bleaching properties of chlorine#
-i/ $t oidises ferrous to ferric, sulphite to sulphate, sulphur dioide to sulphuric
acid and iodine to iodic acid#
+<eE2 C .+E2 C %l+ →<e+-E2/) C +.%l 4a+E) C %l+ C .+ → 4a+E2 C +.%l E+ C
+.+ C %l+ →.+E2 C +.%l
$+ C 6.+ C 5%l+ → +.$) C 1O.%l
-ii/ $t is a powerful bleaching agent; bleaching action is due to oidation# %l+ C .+
→+.%l C
%oloured substance C →%olourless substance
$t bleaches 'egetable or organic matter in the presence of moisture# ?leaching
effect of chlorine is permanent#
Uses: $t is used -i/ for bleaching woodpulp -re3uired for the manufacture of paper and
rayon/, bleaching cotton and tetiles, -ii/ in the etraction of gold and platinum -iii/ in themanufacture of dyes, drugs and organic compounds such as %%l2, %.%l), @@*, refrigerants,
etc# -i'/ in sterilising drin"ing water and -'/ preparation of poisonous gases such as phosgene
-%%l+/, tear gas -%%l) 4+/, mustard gas -%l%.+%.+E%.+%.+%l/#
Example7.17
Solution
Lrite the balanced chemical e3uation for the reaction of %l+ with hot and
concentrated 4a.# $s this reaction a disproportionation reactionM >ustify#
)%l+ C 64a. → 54a%l C 4a%l) C ).+
es, chlorine from !ero oidation state is changed to 1 and C5 oidation states#
Intext Questions
7.2 i'e the reason for bleaching action of %l+#
7." 4ame two poisonous gases which can be prepared from chlorine gas#7.2+ /,rogen &lorie
$n laboratory, it is prepared by heating sodium chloride with concentrated sulphuric
acid#
4a%l C .+E2 ⎯⎯⎯⎯→
2+OI
4a.E2 C .%l 4a.E2 C 4a%l ⎯⎯⎯⎯→
8+)I 4a+E2 C .%l
.%l gas can be dried by passing through concentrated sulphuric acid#
Properties
$t is a colourless and pungent smelling gas# $t is easily li3uefied to a colourless li3uid
-b#p#18: I/ and free!es to a white crystalline solid -f#p# 15: I/# $t is etremely
soluble in water and ionises as belowG
.%l (g) + .+ ( l ) →.) + (a3) + %l
− (a3) a = 1O
7
$ts a3ueous solution is called hydrochloric acid# .igh 'alue of dissociation
constant - a/ indicates that it is a strong acid in water# $t reacts with 4.) and gi'es
white fumes of 4.2%l#
4.) C .%l → 4.2%l
Lhen three parts of concentrated .%l and one part of concentrated .4 ) aremied, a3ua regia is formed which is used for dissol'ing noble metals, e#g#, gold, platinum#
Au + 2.+ + 4)
− + 2%l
− →Au%l
−2 + 4 + +.+
)9t + 16.+ + 24)
− + 18%l
− →)9t%l
+6
− + 24 + 8.+
.ydrochloric acid decomposes salts of wea"er acids, e#g#, carbonates,
hydrogencarbonates, sulphites, etc#
4a+%) C +.%l →+4a%l C .+ C %+
4a.%) C .%l → 4a%l C .+ C %+
4a+E) C +.%l → +4a%l C .+ C E+
Uses: $t is used -i/ in the manufacture of chlorine, 4. 2%l and glucose -from corn starch/,
-ii/ for etracting glue from bones and purifying bone blac", -iii/ in medicine and as a
laboratory reagent#
Lhen .%l reacts with finely powdered iron, it forms ferrous chloride and not
ferric chloride# LhyM
$ts reaction with iron produces .+# <e + +.%l→ <e%l + + .+
Niberation of hydrogen pre'ents the formation of ferric chloride#
Example 7.1)
Solution
7.21 Oxoa#is
o!
/alogens
@ue to high electronegati'ity and small si!e, fluorine forms only one
ooacid, .< "nown as fluoric -$/ acid or hypofluorous acid# *he other
halogens form se'eral ooacids# ost of them cannot be isolated in pure
state# *hey are stable only in a3ueous solutions or in the form of their salts#
*he ooacids of halogens are gi'en in *able 7#1O and their structures are
Eome properties of interhalogen compounds are gi'en in *able 7#11#
Table 7.11: -ome Properties of #nterhalo!en ompounds
Type ormula
&&′ 1 %l<
?r<
$<a
?r%l b
$%l
$?r
&&′ ) %l<)
?r<)
$<)
$%l)c
&&′ 5 $<5
?r<5
%l<5
&&′ 7 $<7
a@ery %nstable!
b5he p%re solid is known at room temperat%re!
c Dimerises as 7l–bridged dimer (I 07l )
*hese are all co'alent molecules and are diamagnetic in nature# *hey are 'olatile
solids or li3uids ecept %$< which is a gas at +:8 I# *heir physical properties are
intermediate between those of constituent halogens ecept that their m#p# and b#p# are
a little higher than epected#
*heir chemical reactions can be compared with the indi'idual halogens# $n
general, interhalogen compounds are more reacti'e than halogens -ecept fluorine/#
*his is because &&′ bond in interhalogens is wea"er than && bond in halogensecept << bond# All these undergo hydrolysis gi'ing halide ion deri'ed from the
smaller halogen and a hypohalite - when &&′ /, halite - when &&′ )/, halate -when
&&′ 5/ and perhalate -when &&′ 7/ anion deri'ed from the larger halogen#
&&B + . + →.&
B + .&
*heir molecular structures are 'ery interesting which can be eplained on the basis of DE=9F theory -=ample 7#1:/# *he &&) compounds ha'e the bent V*
shape, &&5 compounds s3uare pyramidal and $<7 has pentagonal bipyramidalstructures -*able 7#11/#
*he important atomic and physical properties of the roup 18 elements along
with their electronic configurations are gi'en in *able 7#1+# *he trends in some of the
atomic, physical and chemical properties of the group are discussed here#
Table 7.12: Atomic and Physical Properties of Group 1/ Elements
Propery
Atomic number
Atomic mass0 g mol 1
=lectronic configuration
Atomic radius0pm
$onisation enthalpy
0">mol(1
=lectron gain enthalpy
0">mol(1
@ensity -at E*9/0gcm )
elting point0I
?oiling point0I
Atmospheric content
- by 'olume/
radioactive
7.2".2 Electronic All noble gases ha'e general electronic configuration ns+np6 ecept onfi!uration
helium which has 1s+ -*able 7#1+/# any of the properties of noble gases including their inacti'e nature
are ascribed to their closed
shell structures#
7.2"." #onisation @ue to stable electronic configuration these gases ehibit 'ery high Enthalpy ionisationenthalpy# .owe'er, it decreases down the group with increase
in atomic si!e#
7.2".% AtomicAtomic radii increase down the group with increase in atomic
$adii number#
7.2".5 Electron Eince noble gases ha'e stable electronic configurations, they ha'e no Gain tendency to accept
the electron and therefore, ha'e large positi'e 'alues Enthalpy of electron gain enthalpy#
Physical Properties
All the noble gases are monoatomic# *hey are colourless, odourless and tasteless#
*hey are sparingly soluble in water# *hey ha'e 'ery low melting and boiling points because the only type of interatomic interaction in these elements is wea" dispersion
forces# .elium has the lowest boiling point -2#+ I/ of any "nown substance# $t has an
unusual property of diffusing through most commonly used laboratory materials such
as rubber, glass or plastics#
4oble gases ha'e 'ery low boiling points# LhyM
4oble gases being monoatomic ha'e no interatomic forces ecept wea" dispersion forces
and therefore, they are li3uefied at 'ery low temperatures# .ence, they ha'e low boiling
-ii/ *hey ha'e high ionisation enthalpy and more positi'e electron gain enthalpy#
*he reacti'ity of noble gases has been in'estigated occasionally, e'er since their
disco'ery, but all attempts to force them to react to form the compounds, were
unsuccessful for 3uite a few years# $n arch 1:6+, 4eil ?artlett, then at theUni'ersity of ?ritish %olumbia, obser'ed the reaction of a noble gas# <irst, he
prepared a red compound which is formulated as +C9t<6
# .e, then realised that the
first ionisation enthalpy of molecular oygen -1175 ">mol 1/ was almost identical
with that of enon -117O "> mol 1/# .e made efforts to prepare same type of
compound with &e and was successful in preparing another red colour compound
&eC
9t<6
by miing 9t<6 and enon# After this disco'ery, a number of enoncompounds mainly with most electronegati'e elements li"e fluorine and oygen,
ha'e been synthesised#
*he compounds of "rypton are fewer# nly the difluoride -Ir<+/ has beenstudied in detail# %ompounds of radon ha'e not been isolated but only identified
-e#g#, Fn<+/ by radiotracer techni3ue# 4o true compounds of Ar, 4e or .e are yet"nown#
$a& ,enon%fluorine compounds
&enon forms three binary fluorides, &e<+, &e<2 and &e<6 by the direct reactionof elements under appropriate eperimental conditions#
&e -g/ C <+ -g/
-enon in ecess/
&e -g/ C +<+ -g/
-1G5 ratio/
&e -g/ C )<+ -g/
-1G+O ratio/
&e<6 can also be prepared by the interaction of &e<2 and +<+ at 12)I# &e<2 +
+<+ →&e<6 + +
&e<+, &e<2 and &e<6 are colourless crystalline solids and sublime readily at +:8
I# *hey are powerful fluorinating agents# *hey are readily hydrolysed e'en by tracesof water# <or eample, &e<+ is hydrolysed to gi'e &e, .< and +#
+&e<+ -s/ C +.+-l/ → +&e -g/ C 2 .<-a3/ C +-g/
*he structures of the three enon fluorides can be deduced from DE=9F and
these are shown in <ig# 7#:# &e<+ and &e< 2 ha'e linear and s3uare planar structures
respecti'ely# &e<6 has se'en electron pairs -6 bonding pairs and one lone pair/ andwould, thus, ha'e a distorted octahedral structure as found eperimentally in the gas phase#
&enon fluorides react with fluoride ion acceptors to form cationic species and
Summar,roups 1) to 18 of the periodic table consist of p(bloc9 elements with their 'alence shell electronic
configuration ns+np16# roups 1) and 12 were dealt with in %lass &$# $n this Unit remaining groups of the p(bloc" ha'e been discussed#
Group 15 consists of fi'e elements namely, 4, 9, As, Eb and ?i which ha'e general electronic
configuration ns+np)# 4itrogen differs from other elements of this group due to small si!e, formation of
p ) p multiple bonds with itself and with highly electronegati'e atom li"e or % and non(
a&ailability of d orbitals to epand its 'alence shell# =lements of group 15 show gradation in
properties# *hey react with oygen, hydrogen and halogens# *hey ehibit two important oidationstates, C ) and C 5 but C) oidation is fa'oured by hea'ier elements due to Vinert pair effect#
@initrogen can be prepared in laboratory as well as on industrial scale# $t forms oides in 'arious
oidation states as 4+, 4, 4+), 4+, 4+2 and 4+5# *hese oides ha'e resonatin! structures
and ha'e multiple bonds# Ammonia can be prepared on large scale by *abers process# .4) is an
important industrial chemical# $t is a strong monobasic acid and is a powerful oidising agent# etals
and non(metals react with .4) under different conditions to gi'e 4 or 4+#
9hosphorus eists as 92 in elemental form# $t eists in se'eral allotropic forms# $t forms hydride,
9.) which is a highly poisonous gas# $t forms two types of halides as 9& ) and 9&5# 9%l) is prepared by
the reaction of white phosphorus with dry chlorine while 9%l5 is prepared by the reaction of phosphorus
with E+%l+# 9hosphorus forms a number of ooacids# @epending upon the number of 9. groups,
their basicity 'aries# *he ooacids which ha'e 9. bonds are good reducing agents#
The Group 1' elements ha'e general electronic configuration ns+np2# *hey show maimum
oidation state, C6# radation in physical and chemical properties is obser'ed in the group 16 elements#
$n laboratory, dioygen is prepared by heating I%l) in presence of n+# $t forms a number of oides
with metals# Allotropic form of oygen is ) which is a highly oidising agent# Eulphur forms a number
of allotropes# f these, α and β forms of sulphur are the most important# Eulphur combines with
oygen to gi'e oides such as E+ and E)# E+ is prepared by the direct union of sulphur withoygen# E+ is used in the manufacture of .+E2# Eulphur forms a number of ooacids# Amongst them,
most important is .+E2# $t is prepared by contact process# $t is a dehydrating and oidising agent# $t is
used in the manufacture of se'eral compounds#
Group 17 of the periodic table consists of the following elements <, %l, ?r, $ and At#*heseelements are etremely reacti'e and as such they are found in the combined state only# *he common
oidation state of these elements is 1# .owe'er, highest oidation state can be C7# *hey show regular gradation in physical and chemical properties# *hey form oides, hydrogen halides, interhalogen
compounds and ooacids# %hlorine is con'eniently obtained by the reaction of .%l with In 2# .%l
is prepared by heating 4a%l with concentrated .+E2# .alogens combine with one another to form
interhalo!en compounds of the type & &1n -n 1, ), 5, 7/ where & 1 is lighter than &# A number of
ooacids of halogens are "nown# $n the structures of these ooacids, halogen is the central atom whichis bonded in each case with one . bond as &.# $n some cases & O bonds are also found#
Group 1/ of the periodic table consists of noble !ases# *hey ha'e ns+ np
6 'alence shell electronic
configuration ecept .e which has 1 s+# All the gases ecept Fn occur in atmosphere# Fn is obtained as
the decay product of ++6Fa#@ue to complete octet of outermost shell, they ha'e less tendency to form compounds# *he best
characterised compounds are those of enon with fluorine and oygen only under certain conditions#
*hese gases ha'e se'eral uses# Argon is used to pro'ide inert atmosphere, helium is used in filling
balloons for meteorological obser'ations, neon is used in discharge tubes and fluorescent bulbs#