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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.

dispersion

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

Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
Damian
yes that's correct
Professor
I think
Professor
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
if virus is killing to make ARTIFICIAL DNA OF GRAPHENE FOR KILLED THE VIRUS .THIS IS OUR ASSUMPTION
Anam
analytical skills graphene is prepared to kill any type viruses .
Anam
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
Hafiz
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
sciencedirect big data base
Ernesto
Introduction about quantum dots in nanotechnology
Praveena Reply
hi
Loga
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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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