19
Polar molecules and polar bonds
19.1 What is a polar molecule?
Hydrogen chloride is a good example of a polar molecule. Imagine a hydrogen atom and a chlorine atom
coming close together and forming a bond. If you were
one of the two electrons in the bond you would feel two
attractions; one for the 17 protons in the chlorine
nucleus, and one for the single proton belonging to the
hydrogen atom. In spite of the shielding of the chlorine
nucleus, the chlorine tends to win the competition. You
would be drawn towards the chlorine and away from
the hydrogen nucleus. Remember, though, that the
electrons in a molecule are far from being stationary.
Each electron in the bond would still have a chance of
being found near the hydrogen, but a higher probability
of being nearer the chlorine. Figure l9.l gives an
impression of the spread of the electron charge cloud in
hydrogen chloride.
6+
6-
o -
p.
Water, H2O
Ho*
ii+H
6N
Ammonia,NH3
\
-"/-l
H
I
H
Hx-
d+
E t h a n o l , C 2 H 5 0H
6+ 5H-lF
.
'HF
H y d r o g e nf l u o r i d e ,
6H-o*-O- -Hd*
H
:C\r/
|
I
u-Y
I'
I
IJ
H
LI
I
Figure19.1 Thesigmaorbitalin hydrogenchlorideis 'fatter'
at the chlorineend; the electronsspendmore of their time
nearerto the chlorinethanthehydrogen
P r o p a n o n e , ( C H 3 ) 2 CO
H--C
6lt.
\ 6.+C : O
"
H
/
H-\C,
'!
Figure 19.2 Examples
of polarmolecules
The result of this uneven distributionof chargeis that
one end of the moleculehasa slightpositivecharge,and
the other a slight negativecharge.Theseslight charges
are shown by the symbols0+ and 6- respectively(6 is
the Greekletter 'delta').
Hydrogenchlorideis calleda polarmolecule
becauseof
its uneven distribution of charge.Notice that hydrogen
chloride li a molecule; that is, it is predominantly
covalent.Its small 6+ and 6- chargesdo notmake it an
ionic substance.
Many moleculesare polar, someof which are shown
in Figure 19.2.
19.2 Polar bonds and
electronegativities
One way of predictingwhether a moleculeis polar is to
Electronegativity is a
use values oI electronegativity.
measurewe use to tell us how well an element attracts
electronsin a bond. The elementsfluorine and chlorine
tend to attract electronsto themselvesvery strongly.
havehigh valuesof electronegativitf On
These.elements
Polarmolecules
andpolarbonds
107
Table 19.1.Electronegativity values for some
elements*
Table 19.2.Predictions from electronegativity
values
(a) Pauling scale
Electronegativity
values
H
2.1
L
i
B
e
B
C
r.0
r.5
2.0
2.t
Na
0.9
Mg
r.2
Al
I .5
Si
I .8
K
C
0.8
a
1. 0
Rb
0.8
Sr
I .0
G
a
| .6
G
In
l .5
e
1.8
Sn
r.8
N
1.0
P
2.t
As
2.0
Sb
|.9
O
).5
S
2.5
Se
2.4
Te
'2.1
4.4
CI
1.0
Br
2.8
I
2.5
(b) Allred-Rochowscale
H
2.1
L
1.0
i
B
1.5
Na
1.0
Mg
t.3
K
C
0.9
a
1.1
Rb
0.9
Sr
t.0
C
2.5
N
3.1
o
2.0
Al
1.5
Si
1.8
p
S
2.1
2.4
Se
2.5
Te
2.O
e
G
B
a
1.8
In
1.5
G
e
2.0
AS
Sn
L7
5D
2.2
I.8
3.5
F
4.1
c1
2.9
Br
2.8
2.2
nNoticethat electronegativityincreasesacrossa Period,and
decreases
down a Group (just like ionisationenergy)
the other hand, metals like sodium or potassium very
rarely form negative ions. On the contrary they tend to
lose an electron to make positive ions. These elements
have low electronegativities. You will find values for
s o m e e l e m e n t si n T a b l e 1 9 . 1 .
The first set were calculated by Linus Pauling in the
l9l0s. On his scale,elements are given electronegativities calculated from values of their bond energies.
Another scaleof electronegativitieswas invented by A.
L. Ailred and E. G. Rochow in 1958.They used a method
that estimated the shielding effect of the various
electronsin an atom. This allowed them to work out the
effective nuclear charge that an electron some short distance away would feel. Having calculatedthe attractive
force on the electron, they converted their result to give
numbers roughly in the range I to 4. They chose this
scale so that it would give similar figures to Pauling's
scale.
You can see parts of both scalesin Table 19.1. Pauling's scaleis the most widely used, and we shall use his
figures in the rest of the book.
Electronegativity values have to be used carefully
becausethey are not direct properties of elements. We
cannot measure an electronegativity we can only use
one of the scalesand see how well predictions made
from it fit with the results of experiment (Table 19.2).In
particular, the differencesbetween values are often correlated with the degree of ionic character in a bond.
108
PhysicalChemistry
Examples
Prediction
Non-polarcovalentbond H2,Cl2,A2
Identical
HCl,No, Bclr
Slightlydifferent Polarcovalentbond*
RbF,NaCl,Mgo
Ionic bond formed
Very different
*The presenceof polar bondsdoesnot guaranteethat a
is polar
molecule
As we have seen, the bond in hydrogen chloride has
an uneven distribution of chargeat each end. That is, the
bond is polar. Becausehydrogen chloride is a molecule
with only one bond, the molecule must be polar as
well. This fits with the difierence of 0.9 in their electronegativity values, and correspondsto around 20% ionic
character. The values for sodium and chlorine are 2.1
units apart, which works out at nearly 650/o ionic
character.
19.3 Polar molecules and dipole
moments
Electronegativities are not properties of atoms like their
massesor spectra: there are no machines that we can use
to measure electronegativities. The different scalesthat
have been used from time to time have proved useful in
some ways; for example, in explaining why hydrogen
chloride is polar. But if you think about what you have
read in the previous section, one or two things might
worry you. In particular, you have not been given any
experimental evidence that shows that hydrogen
chloride really is polar. Fortunately such evidence does
exist. It comes from measuring the dipole moment of the
molecule.
Suppose a negative and a positive charge are kept
apart from one another as shown in Figure 19.1. This
arrangement defines an electric dipole moment, or just
dipole moment for short. The size of the dipole moment
is given by multiplying the size of the charge by the
distance apart, i.e.
w:qxr
Dipole moment
+q
f------>
-q
r
Dioole
TT:".1
Figure 19.3 Two charges, +q and *q, separated by a distance r possess a dipole moment. (The symbol for a dipole
moment is +-)
O,$.
\U,K
\.,
C
Moleculeswith no
dioole moment
19.3.Table of dipole moments*
+
+
+
v,\
On\,
Structure
Molecule
Dipole
momentlD
T
?
+
Hydrogen fluclride
f*
6 +g - p
Hydrogen chloride
+-*
d +g - 6 1 6 -
6-
1.91
T
Chargingthe plates
has little effect
1.08
+*
Hydrogen bromide
6 +g - 6 , 6 -
Hydrogen iodide
6 +g -
0.80
+*
sry
Moleculeswith
a dipole moment
@ @
@ @
@ @
@ @
0.42
16.
l
l+
Water
r) o|
s- * . , , / - \ , , r * |
-n
H' +
l+
Ammonia
i-* ,n, / ' i - - , , . ux- * I
|
|
l+
l+
l+
rr 6-
l+
lI +
6-
Ethanol
u.-a*.r,-orn n* J
' t.68
H.-^r.l
H
It is possibleto measurethe sizeof a dipole moment.
Supposeyou were to set up two setsof metal platesas
shown in Figure 19.4. Between one set of plates you
placemoleculeswith a dipolemoment, and betweenthe
other pair you put moleculeswith no dipole moment.
Now imagine chargingup eachpairbf platessothat one
becamenegativeand the other positive.Thepositiveend
of the dipoleswill be attractedto the negativeplate,and
the negativeend towardsthe positiveplate.Thesemoleculestend to line up in the electricfield between the
plates.The field of the dipolesis in the oppositedirection
to the field betweenthe plates.By using somesensitive
electronics,the changein the field can bi measured.On
'the
other hand, the molecules without the dipole
moment
'randomlywill not line up. They remain more or less
oriented so the electricfield will changevery
little.
You will find that, for the simplermolecules,the sizes
of the dipolemomentsare (approximately)proportional
to the differencebetween the electronegativities
of the
atoms (seeTable 19.3).For the more complicatedmoleculesthe matter is not so straightforward.The reasonis
that the dipole moments of the different bonds can
reinforceone another,or in somecasescanceleachother
out. An example is tetrafluoromethane,CFn (Figure
19.5).The electronegativity
of carbonis 2.5, while that
of fluorine is 4.0. This suggeststhat a carbon-fluorine
bond shouldbe polar,with the carbonbeingpositiveand
the fluorine negative.However, even though CFnhas
four polar bonds, the molecule as a whole is not polar.
1.47
T
6+
li
Figure19.4 Thetendencyof moleculeswitha dipotemoment
to line up in an electricfield can be usedto measuretheir
dipole moments.(ln practice they never lineup pertecilysurroundingmoleculesjostlesomeof themout of position)
I
H
l+
Chargingthe plates
causes the molecules
to line up in the
same direction
1.85
..)
H
l
H
Proparibne
1.30
H
s
"
Chlorobenzene
6+
1,2-Dichlorobenzene
!
y
-cl
r t
rai
v
t.57
fi-cru-
2.25
'-'r
/
*v
6- c l
1.3-Dichlorobenzene
A
"'
t ( ) l
1.72
aSZ---.,ucl
I
1,4-Dichlorobenzene
16r
0.00
Y
cl
Polarmolecules
andpolarbonds
109
Resultant
- Cont'
Table lg.t'
Dipole
momentlD
Structure
llolecule
6+
NH,
I
t.rt
'-A)
l ( ) l
PhenYlamine
tlt /
1, 2 - D i c h l o r o b e n z e n e
II
.A
t[ )l
1
have dipole
Figure 19.6 Two of the dichlorobenzenes
moments:the third does not
5,
,v
5F
,i\
F
d-
Figure 19.5 Atthough tetrafluoromethanehas four polar
binds, themolecuti as a wholeis not polar' Thisoftenhap'
penswithhightysymmetricmolecules
The reasonlies in the symmetricalarrangementof the
Uo"at, and their dipoie moments' The four dipole
momentscancelout.
Further examplesare the dichlorobenzeneslisted in
Table19.1.The waysthe dipolescombineare shown in
Figure19.6.
"In
somemoleculesthe presenceof highly electronegative atoms can affectUonassome distanceaway in the
molecule.Trichloromethane,CHClr,is an example'Her-e
pull
;h;-;h;.; highly electronegativechlorine atoms
carbon;
central
the
from
awiy
them,
.t".tto"t to*"utdt
this in turn causeselectronsto be drawn towards the
carbonfrom the hydrogen-carbonbond (Figurel9'7)'
il tltit way the nyArog*enatom has a greaterdegreeof
Chemistry
Physical
ci
o" -
1.02
6F
6-t
'
I '
r,
;d+
.
cl. \
\\
6-^,*-_--C-H
L'I
ll
*Dipole moments are usually stated in debyes (D)' One
debye equals 3.14x l0-ro C m
110
tcl
r^l
d-ct)c_Ho*
clo" -
Diooles cancel:
no resultant moment
1, 4 - D i c h l o r o b e n z e n e
I
Trichloromethane
j
i,:
VI I
t.4i
NO,
*_--i
o -^'
Llr
i
;i
a
6-
.
1, 3 - D i c h l o r o b e n z e n e
tLl
,v i
Nitrobenzene
{
'i
- o H
rA,
Resurtant
ffi*,,o,"
v\.,
\/
8+
Phenol
lf
ffi;"
u\2
I
t
dipore
6+
Chlorinedrawselectrons
towards itself
draw
Fioure 19.7 The chlorine atoms in trichtoromethane
'"if"iiiriinductive
negative
a
show
They
io*airdstn"*'
effect
'ordinary'
positivechargethan we would expectin an
iirUo"-nyat5ge" bond. The electron withdrawing
said to exert a negativeinductiveeffect.
.niori". uio,,'i
"t.
itr.r" ut. groups of atoms that can exert a positive
inductive eltect, but we shall ignore them until later
(unit I l2).
wouldyou
19.1 Whichof the followingmolecules
boron
HrS;
polar:
sulphide,
hydrogen
or"Oictto be
ethane'
CClo;
tetra-chloromethane,
eitr;
iricnroriOe,
CO?
CrHu;carbonmonoxide,
1g.2 Lookat the diagramsof the two moleculesin
f'rgureig.a. woutOi-hemoleculeshave a dipole
moment?
cl
cl
cl
I
H
cls-1,2-Dichloroethene
(,t
H
trans-l'2-Dichloroethene
Figure19.8 Twoformsof 1,2-dichloroethene
19.4 Polarisability
Imagine putting a Iarge negative ion near to a small
positive ion. Becausethe outer electronsof the large ion
are far from the nucleus, they are not held very tightly.
They will be attracted towards the positive ion. The
/-E-\
+ion(;,)
Far apart: no influenceon each oiher
Closertogether:the negativeion is polarisedby the positiveion
Evencloser:thechargecloudof the negative
ionmergeswiththat
of the positiveion;coValency
results
Figure 19.9 A small positiveion close to a larger negative ion
will polarisethe negative ion
Table 19.4. Fajans' rules
An ionic compound will have a high degree of covalency if:
The positive ion is small and highly charged.and
The negative ion is large (highly polarisable)
An alternative version is;
Covalency is promoted by a small cation and a large anion*
pound may be almost entirely covalent.For example,the
beryllium ion, Be2*, is so small that it manages to
polarise any negative ion to such a degree that its compounds are predominantly.covalent.
19.3 A largenegative
ionisto someextentsimilar
to
a largeatomor molecule.
Because
of theirsize,each
of themwill be highlypolarisable.
lf you prefer,you
could say that they were 'soft' or 'squashy'.The
meaning
isthesame:bothwillbe easilydistorted
by a
nearbyelectricfield.Try to givea reasonwhy highly
polarisable
atomsor molecules
havea tendency
to
line up in an electricfield,eventhough,theymight
haveno permanent
dipolemoment.
19.4 Hydroxide ions react with iodomethane
according
to the equation
OH-+CH.l--+CH.OH+lln somebooksit is saidthatthe iodineatomattracts
electrons
towardsit, leavingthe carbonatomslightly
positively
charged.The positivechargeis claimedto
be the reasonwhy the hydroxideion is attractedto
the CH.l.Whatdo youthinkof thisreasoning?
19.5 lf a streamof waterfalls closeto a charged
rod,the streamis attractedto the rod (Figure19.10).
(i) Explainwhy the streambends.
(ii) Doesthisexperiment
provethatWatermolecules
are polar?
lt
tl r**.,
"A cation is a positive ion; an anion is a negative ion
chargecloud of the negativeion will be distorted.We say
that the negative ion has been polarisedby ttre positive
ion (Figure 19.9);or that the largenegativeion is highly
polarisable.If
we were to put a much smallernegativeion
closeto the positive ion, it would not be polarisedso
much. Thisis becausethe electronswould be held tightly
by the nucleus and the charge cloud could not be so
easilydistorted.The smallernegativeion is only slightly
polarisable.
You should be able to understand that if a highly
polarisablenegativeion is placednear a small positive
ion, then the charge cloud on the negative ion may
spreadover the regionof influenceof the positiveion. A
degreeof covalencyresults.
The ideasthat we havejust met lie behind a setof rules
known as Fajans'rules.The rules,which are developed
from a different set that Fajanspublishedin 1924, are
summarisedin Table 19.4. In extreme cases,the com-
IV
\
Figure19.10A streamof waterwill bend towardsa
chargedrod
Polarmolecules
and polar bonds
tlr
20
Intermolecular forces
20.1 Where are intermolecular
forces found?
The noble gas helium will only turn to a liquid at the
very low temperature of 4I(. The question is, why doesit
liquefy at all? For some reason, ar 4K helium atoms stick
together, while at higher temperatures they do not stick
together. Helium may be an extreme casein terms o{ the
low temperature at which it liquefles,but other elements
or covalent substances follow a similar pattern. For
example, iodine exists as a black solid up to 456K
(183'C). The solid contains a regular pattern of iodine
molecules, I, (Figure 20.1). Above 456K the crystal
breaks up and the molecules escape from the crystal,
giving a purple vapour.
F
P
@
@ ffi
,ffi
\
\ @ .
N S @ N
The forces that hold helium atoms, or covalent substanceslike iodine, together in a liquid or solid are called
intermolecularforces. If the forces are between atomt
rather than molecules, we can speak about interatomic
forces. Another name given to the forces is van der Waals
forces after the Dutch chemist who first investigated the
origin of the forces. An important point is that intermolecular (or interatomic) forces are not meant to apply to
the purely electrostatic forces that exist between
oppositely charged ions. Likewise they do not include
the forces that hold atoms together through covalent
bonds, or hydrogen bonds (which we shall discussin the
following unit).
20.2 What causesintermolecular
forces?
To find one answer to this question we shall begin with
the single atom of Figure 20.2. On averagethe negative
charge of the electrons in an atom is spread evenly
around the nucleus. However, over very short periodsof
time (e.9.millionths of a second)the charge is not completely evenly distributed. At times more electrons will
be found on one side of the atom rather than the other
( s e eF i g u r e 2 0 . 2 ) . I f y o u I o o k b a c k t o U n i t l 9 o n p o l a r
molecules, you should recognisethat the separation of
positive and negative charge produces a dipole. Unlike
P , @ffi,ffi
lnstantaneous
dipole moment
#
f f i @
ffi
laYer
ToP
eh-@
layer
Bottom
Figure 20.1 The crystal structure of iodine. The crystal contains planes of l, molecules, with alternate planes staggered
as shown in the diagram. One peculiarity of the crystal is that
the distance betuveenthe layers is about 427 pm while the
distance between molecules within the same plane is much
shorter, 356pm
On averagethe negativecharge
of the electronsis spread
evenry
For brief periodsthe
electronsare more on
one side oJ an atom
than another.This
causes a short-lived
dipole moment
Figure 20.2 The origin of temporary dipole moments
Intermolecular
forces
lll
\
\
/
,-
1 l--
*2>
?,ffJ8t---r
preserit
-
-- : _=_A
@-r-:-}
{- ---:;7
:--ry
/-
2=:\
/- --:-:-\\f _ _ @ _ _ _ _\l_ _ z
\ -
- - ' I
---/
\ -\___/
\
\
\
\
Figure 20.3The dipoleon
neighbouringatom
one atomcan induce a'dipoleon a
permanent dipole
hvtlrogen chloride, which has a
only temporary'
moment-is
the atom's dipole
"-lior,
"io-"""t, imagine that you were watching another atom
would see
oortine.totJto the atom with the dipole' You
away
it't"-.tJ.t.otls in the passing atom being repelled
situThe
dipole'
the
temporary
iio- tf r. negative encl ol
the
moment
a
For
20'3'
in
Figure
utlo" it repiesented
new
a
induced
has
atom
first
the
on
dipole
,"-po.ury
aipo'f o.r ih. i..ottd atom. These two dipoles will briefly
utiru.t" one another. The force of attraction was first caliutut"a by the German physicist Fritz London' For this
reason the force of attraction between two temporary
ciipoles is known as a London force. Another name often
ur.,l it to call the force a dispersionforce' London' or
dispersion, forces are just one type of intermolecular
{orce.
20.3 Dispersion forces and
polarisability
If our two atoms pass close to one another and are not
travelling too quickly, the dispersion force may be strong
enough io *ui" them stick together. The way to make
,ur. ih" atoms do not merely collide with one another
and bounce off without sticking is to reduce their kinefic
energies. The easiest way to do this- is..to lower the
temp"erature.You should realisethat in helium the forces
of aitraction are so weak that the temperature has to be
very low before the helium will liquefy. For iodine the
molecules stick at a much higher temperature' This tells
us that the dispersionforcesin iodine are much stronger
than in helium. The next problem is to explain why this
is so. Our work in Unit 13 tells us that the electrons
nearer the outside of a large atom are lessstrongly attracted by the nucleus. The electrons in a smaller atom are'
by comparison, more strongly held by the nucleus'
tLerefoie, the outer electrons in a large atom will be
attracted more easily by a neighbouring charge than the
electronsin a smaller itom. Figure 20.4 shows that the
electron cloud about a large atom is more easilydistorted
than the electron cloud around a srnall atom'
The shorthand way of describingthe different degrees
I l4
Smallatom:
littledistortion,
i.e. small atom is not
very polarisable
This dipoleis
inducedbY
the first dipole
PhysicalChemistrY
o
I
I
\
Large atom:
much distortion,
i,e. large atom
is highly
polarisable
polarise
Figure 20.4 An atom with a dipole moment will
atom
a
small
than
easily
more
much
atom
talge
of distortion is to say that large atoms are more polarisable than smaller aioms. Altirnatively. we say that a
large atom has a larger polarisabilitythan a smaller atom'
Ii is possible (but difficult) to calculate the polarisability of utt uto-' It turns out that the dispersion force
U"t*."tt two atoms depends on the polarisability of each
of them. In other wordi, the higher the polarisability, the
stronger the dispersion force. In this casewhat holds true
io, ut"o-t tendsio hold true for molecules' The larger the
atoms in a molecule, the larger is the polarisability of the
molecule - hence the temperature of 456I( at which
iodine vaporises.
20.4 Intermolecular forces are also
produced bY Permanent diPoles
You have discovered that dispersion forces rely on an
atom or molecule having a temporary dipole moment
and this dipole inducing another dipole on a neighbouring atom or molecule. Clearly, dipole moments are
iniportant sourceso{ intermolecular forces' What better'
ifrett, tftatt a molecule having its own permanent dipole
moment? You may remember that such molecules are
.utteA potur moleiules. Indeed, a polar molecule will
nor-uily have higher melting and.boiling points than a
non-polar molecule of similar molecular mass'
An example of this is shown in Figure 20'5' Propanone, (CHr)rC-O, is an organic compound with a
ielative molecular mass of 58. It also happens to have a
dipole moment. Compare-its-boilingpoint of
p"r-ut..tt
'lz'C
*ith thit of butane, C4Hr0,which has the same
molecular mass, no permanent dipole moment and a
boiling point of - IoC.
2o.5 Some words of warning
It is important not to be too dogmatic when making
p..ai.tii,nt about the relative melting or boiling points of
molecules. For example' 1,2-dichlorobenzeneis polar'
but 1,4-dichlorobenzeneis not (Figure 20 '6) ' (They have
the same molecular mass.)
H
H=c
I
l ' C : Ot
/ a* 6-
s
H
H
H
#=.
I
\/
c
--a
- -H
-c
,_\a
u
/ \
|
H
H
H
Propanone
has a dipolemoment.
Mr(proPanone)=58
b.p. 57"C
Bu t a n e
has no dipolemoment.
M'(butane)=58
b.p. -1'c
H
I
H
H
I
Figure 20.5 Thedifferencein boitingpointof propanoneand
butaneis a resultof the dipolemomentin propanone
&"ff
1 . 2 - D i c h l o r o b e n z e n1e, 3 - D i c h l o r o b e n z e n e
eculehas a melting point of -18"C whereasthe nonpolar form has a melting point oI 5)' C, i.e. some 70'C
higher.Thenon-polarmoleculestillhastwo polarbonds.
Observationslike this show that it is not just the dipole
moment that counts. Especially,in solidsthe way the
moleculespack together to make a crystal can have a
marked-influenceon melting points. Although 1,4dichlorobenzeneis non-polar, it still has two polar
bonds,It is the interactionbetweenthe polar bonds on
the'upset'in the orderof
differentmoleculesthat causes
the melting points.
Similarly, we must be careful not to ignore other
important effectsthat may be at work. For example,
many polar moleculescan make hydrogen bonds and
the strengthsof thesebondsmay overcomeother effects
solely due to induced or permanent dipole moments.
Similarly,as moleculesbecomelarger and heavier,it is
lesslikely that they will be gases,and more likely thaf
they will be liquids or solids.Partly this is becauseof the
influence of the large numbelof intermolecularforces
that can arise;for example,between the long chain of
atoms in a polymer. However, heavy molecules are
harder to move no matter whether the intermolecular
forces are weak or strong; so without a great deal of
energythey will not fly around loose in a gas.A summary of the points made in this unit is given in Table
20.1.
Table,2o.l. Summary of intermolecular forces
No dipole
Intermolecular forcesdepend on dipole moments
Thereare three types:'
temporary dipole-induced dipole
permanent dipole-induced dipole
permanent dipole-permanent dipole
cl
:
' :.
1,4-Dichlorobenzene
Figure 20.6 All of the dichlorobenzenes
have polar bonds
betweencarbonand chlorineatoms.Onlytwo of themhave
dipolemoments
The larger, or heavier, the atom or molecule the greater the
polarisability and the stronger the intermolecular forces
As expected,the boiling point of the former r higher
than that of the latter (but only by 5"C). However,their
melting points are in the reverse order. The polar mol-
Intermolecular forcesare sometimescalled van der Waals
forces
20.1 Explainthe followingorderof boilingpoints:
Helium Neon Argon Xenon
Relativeatomic
mass
BoilingpoinvK
4.0
4
20.2 39.9
27
87
131.3
165
2O.2 Whichof the halogenswouldyou expectto
fluorine,chlorine,
have the highest polarisability:
bromine,or iodine?Brieflyexplainyouranswer.
2O.3 lf two atoms,A and B, are a distancer apart,
the dispersion
energybetweenthemvariesas -1116.
Intermolecular forcesbetween non-polar moleculesare
calledLondon or dispersionforces
For two oppositelychargedions a distancer aparl,
energyvariesas -11r. Writea comthe interaction
plotgraphsof -1116and -1lr to the
puterprogram.to
samescale.First,explainwhatthe negativesigntells
Secondly,when
us aboutthe energyof interaction.
you haveplottedthe graphs,explainwhy the disperforce,whereas
sionforceis knownas a short-range
force.
the forcebetweentwo ionsis a longer-range
20.4 Afierlookingin a databook,a studentnoticed
that the water moleculehas a permanentdipole
moment.The studentalso rememberedthat, conmass,waterhasan
sideringitsfairlysmallmolecular
unusuallyhigh boilingpoint.The studentwrotethat
Intermolecular
forces
ll5