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First, as we noted before, the hydrides from Group IV are the lowest boiling point compounds in each period. Second, in each group, the boiling point increases as we move down the periodic table. In fact, this second pattern is perhaps the most pronounced trend in the data. From this, we draw our first conclusion about the strengths of intermolecular attractions: for similar types of molecules, the molecules with larger atoms (more mass, more protons) have stronger intermolecular attractions.

Why would this be? The answer is not obvious but does make sense once we know it. Remember that these are all neutral molecules. As such, we might have imagined that there would no positive-negative interactions. However, each molecule consists of a large of number of protons from the nuclei of the atoms, and an equal number of electrons, both core electrons and valence electrons, which are either shared or unshared. When two molecules are close to each other, the positive and negative charges in each of the molecules interact with each other. We might again imagine that the attractions of opposite charges would be exactly offset by the repulsions of like charges. This would be true if the charges were uniformly distributed in the molecules, as we would expect for non-polar molecules. But when the molecules are close enough to each other, the attractions and repulsions cause the charges to rearrange such that the attractions become significantly more favorable.

Such an arrangement of electrons in two adjacent highly simplified molecules is shown in Figure 6. Note that two nonpolar molecules become polarized when they are close to each other, due to the attraction of the negative charges in one molecule to the positive charges in the other molecule, and vice versa. The result is a net attractive force between the two molecules. This type of force is called the “dispersion force,” sometimes also called the “London force” after the discoverer.


What makes the dispersion force larger? The data in Table 1 and Figure 5 tell us: molecules with more positive and negative charges, like SnH 4 , have stronger attractions than molecules with fewer positive and negative charges, like CH 4 . We say that the molecule with more charges is more “polarizable,” meaning it is easier for the molecule to become polarized in the presence of other electrical charges. The more polarizable a molecule is, the stronger the intermolecular forces will be.

Let’s now compare the boiling points of compounds in the same period from Group IV and from Group VII, e.g. SiH 4 versus HCl. The boiling point of HCl is larger. However, if we count the charges in these two molecules, we discover that they have the same number of electrons and the same number of protons. This suggests that the two molecules should be equally polarizable and therefore should have equal dispersion forces and therefore should have equal boiling points. But this is not true. Something else must be contributing to the difference in boiling points than just the dispersion forces.

Questions & Answers

do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
Do somebody tell me a best nano engineering book for beginners?
s. Reply
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
what is the k.e before it land
what is the function of carbon nanotubes?
I'm interested in nanotube
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
what is system testing
what is the application of nanotechnology?
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
anybody can imagine what will be happen after 100 years from now in nano tech world
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
silver nanoparticles could handle the job?
not now but maybe in future only AgNP maybe any other nanomaterials
I'm interested in Nanotube
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Concept development studies in chemistry 2013. OpenStax CNX. Oct 07, 2013 Download for free at http://legacy.cnx.org/content/col11579/1.1
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