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To spot the difference between these two molecules, we need to rely on our previously developed knowledge of electronegativities, molecular geometries, and molecular polarity. HCl is a polar linear molecular because the Cl atom is much more electronegative than the H atom. Si atoms are actually slightly less electronegative than H atoms, suggesting that SiH 4 should also be polar. But, we also recall from our electron domain model that SiH 4 has a symmetric tetrahedral geometry, like CH 4 . As such, like CH 4 , SiH 4 has no molecular dipole moment. From this comparison, we can conclude that, in comparing two molecules with similar dispersion forces, the molecule with a dipole moment will have stronger intermolecular attractions than the molecule without a dipole moment. This explains why, in each period in Figure 5, the Group IV hydride compound always has the lowest boiling point. Each of these compounds has nonpolar molecules due to their symmetry. By contrast, two HCl molecules will have stronger attractions as the positive end of one HCl will be attracted to the negative end of the other HCl, and vice versa.

We can now conclude that molecules attract one another via dispersion forces and, if the molecules are polar, via dipole-dipole attractions. Understanding these two types of intermolecular attractions works well to explain the major patterns observed in Figure 5.

This does not explain the exceptions, however. Why are the boiling points of NH 3 , H 2 O, and HF so abnormally high? Given that these are small mass molecules, we would not expect them to have larger than average dispersion forces. All three are polar molecules, but there is nothing to suggest that there dipole moments are unusually high. There must be a different type of intermolecular attraction that is unique to these three molecules out of this set of molecules.

We need a pattern to analyze. What do these three molecules have in common with each other that the other molecules in Table 1 do not? N, O, and F are all strongly electronegative atoms and are also amongst the smallest atoms in the periodic table. In the Lewis structures for all three molecules, O, N and F all have non-bonded, lone pairs of electrons. These three properties, taken together in a single molecule, must present a uniquely strong intermolecular attraction.

Chemists account for this strong bonding via a model called “hydrogen bonding.” This is a uniquely strong form of dipole-dipole attraction that only occurs when a molecule contains a hydrogen atom bonded to an N atom, an O atom, or an F atom. Due to the strength of the electronegativity of these atoms, the N-H bond or O-H bond or F-H bond is highly polar, meaning that the H atom is almost a bare, positively charged hydrogen nucleus. This strong positive charge on one molecule is in turn strongly attracted to the negatively charged lone pair electrons on the N, O, or F atom of another molecule. This is illustrated in Figure 7.

Note that size seems to be important in hydrogen bonding as well. HF has a much higher boiling point than HCl, indicating that HF has stronger intermolecular attractions. Even though the Cl atom is strongly electronegative and has lone pair electrons in the HCl molecule, the Cl atom is apparently too large to support the uniquely strong dipole-dipole attraction we call hydrogen bonding.

Questions & Answers

How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
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
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
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
for teaching engĺish at school how nano technology help us
How can I make nanorobot?
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
how can I make nanorobot?
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?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
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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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