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What are Intermolecular Forces? || Chemical Bonds || Chemistry

September 12, 2019

There are two main kinds of inter-molecular
forces – dipole interactions, which is when polar molecules are attracted to each other,
and dispersion forces, which are caused by the motion of electrons. Collectively, these intermolecular forces
are often called van der Waals interactions. Sometimes, especially in Biology, people get
casual in their language, and they call a specific KIND of intermolecular force a van
der Waals interaction. No big deal. We’ll talk about all of these forces individually. First, let’s look at the various kinds of
dipole interactions. Ion-Dipole forces occur when ions are attracted
to a polar molecule. Positive ions are attracted to the negative
end of a polar molecule, and negative ions are attracted to the positive end. For example, when NaCl is in solution in water,
the negative end of the polar water molecules are attracted to the positive Na+ ions. So you can see the water molecules are oriented
so the oxygens, with their partial negative charges, are pointed towards the Na+ ions,
and the Hydrogens are pointed away. Meanwhile, the positive end of the water molecule
is oriented towards the negative Chloride ion. There are also cases of ion-induced dipole
interactions. This is when the PRESENCE of the ion induces
a temporary dipole in another molecule that usually does not have poles. It’s sitting there, being nonpolar, no partial
positive or negative charges, until an ion approaches. The charge of the ion causes a temporary distortion
of the electron cloud of the nonpolar molecule, INDUCING a dipole, which is then attracted
to the ion. For example, as this Chloride ion, Cl -, approaches
hexane (C6H14), a nonpolar molecule, the negative charge on the ion slightly distorts the electron
cloud – pushes the electrons away, just a little. As a result, there is a slight partial positive
charge on this side of the hexane, which is attracted to the negatively charged ion. Dipole-Dipole interactions occur when two
polar molecules are very close to each other, and the positive end of one polar molecule
is near the negative end of the other molecule. The strength of the attraction increases with
increasing polarity. Here, for example, you can see two molecules
of formaldehyde, CH2O. Each of these individual molecules contains
a polar covalent bond. The molecules have positive and negative ends,
or poles. If two formaldehyde molecules come close together,
the partial positive end of one molecule will be attracted to the partial negative end of
the other molecule. A special case of dipole-dipole interaction
is the Hydrogen Bond. This is when the hydrogen atom in a very polar
molecule is attracted to the negatively charged atom in another polar molecule. The hydrogen is most often involved in a bond
with a fluorine, oxygen, or nitrogen. Fluorine, oxygen, and nitrogen are so electronegative,
they are typically involved in extremely polar bonds. These electronegative atoms pull electrons
closer to themselves, resulting in a partial negative charge. The hydrogen has a partial positive charge,
so the two atoms in the different molecules are attracted to each other. Now, here’s a caveat/ an exception/ something
to keep in mind. Not every hydrogen bond is a dipole-dipole
interaction, or even an intermolecular bond. There are cases of hydrogen bonds within a
single molecule. For instance, if you study biochemistry, you’ll
learn about how hydrogen bonds are formed within a single polypeptide chain (that is,
a string of amino acids), as it folds up to form a protein. So that’s a case of hydrogen bonds forming
between different parts of the same molecule. But in chemistry, you’re almost always talking
about intermolecular hydrogen bonds. Now, we’ve done a survey of dipole interactions. Let’s move on to dispersion forces. You can think of the dispersion force as kind
of the default interaction between chemical species. It involves the motion of electrons. Since absolutely every atom and every molecule
and every compound has electrons, every single kind of chemical species is going to experience
dispersion forces. Thinking about the word “dispersion” is
the key to remembering this force. How are the electrons dispersed, or distributed,
in the cloud around the chemical species? Are they absolutely evenly distributed? Yes, they could be, some of the time. But a lot of the time, in fact, almost all
of the time, there will be an uneven distribution of electrons on the very, very surface of
any given molecule. Think of it like patchiness. There will, for a brief moment, be a patch
that is more negative than another patch. One patch will be more positive than another
patch. In the next moment, because the electrons
are constantly in motion, the patch is gone. There will be a different kind of patchiness. Because this is true of all chemical species
– every molecule, every compound, every atom – there will be some degree of attraction
due to this kind of patchiness. For the briefest of moments, the negative
patch on one molecule will line up with a positive patch on another molecule. Then in the next instant, these patches have
disappeared, or rather redistributed. Imagine these bonds blinking on and off at
a dizzying speed. There are ranges of strengths of dispersion
forces. The larger the molecule, the bigger the electron
cloud, the farther the electrons are from nuclei and so they are held less tightly,
and so they can move around more easily and thus more easily create these sorts of patches
of charge. We sometimes call these “instantaneous dipoles.” This kind of interaction is the weakest of
all the intermolecular bonds. Careful you don’t confuse INTRAmolecular
bonds with INTERmolecular bonds. In previous videos, we discussed Intramolecular
Bonds – the strong bonds that hold compounds together: Ionic bonds and Covalent bonds. Intra refers to the bonds WITHIN ONE molecule
or ONE ionic compound. INTER refers to the bonds BETWEEN two different
chemical species. Intra-molecular forces, the forces that hold
one compound together, are much stronger than inter-molecular forces – the forces between
different molecules. Intermolecular forces are all, essentially,
electrostatic interactions – attractions between positive and negative charges – just far weaker
than the strong electrostatic attractions involved in ionic bonds. If you found this video helpful, share it
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  • Reply Socratica September 25, 2017 at 3:23 pm

    This video will help Chemistry Students who are studying Chemical Bonds. This video focuses on Intermolecular Forces (van der Waals Interactions), including Dipole Interactions and Dispersion Forces. If you need a video about INTRAmolecular forces, you can try our videos about Ionic Bonds and Covalent Bonds!
    Ionic Bonds:
    Covalent Bonds:
    Ionic Bonds vs Covalent Bonds:
    Metallic Bonds:

  • Reply Aflah 786 September 25, 2017 at 3:34 pm

    Loved the video. Overall it was equivalent to Khan Academy but better organized. Hope more content from you. Also when are you going to continue the Calculus lectures

  • Reply saar September 25, 2017 at 3:48 pm

    Thank you so much….it was a great explanation….

  • Reply Ata Sancaktar September 25, 2017 at 4:10 pm

    Great explanation.

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  • Reply Jhonatan González September 26, 2017 at 12:19 am

    Nice explanation.

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  • Reply Charles Shanafelt September 27, 2017 at 5:46 am

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  • Reply Sheenu Sabharwal October 2, 2017 at 5:13 pm

    this video is too much helpful really

  • Reply Sheenu Sabharwal October 2, 2017 at 5:13 pm


  • Reply Heisenberg January 10, 2018 at 6:00 pm

    Thank you so much.

  • Reply Ashwani Kapoor March 6, 2018 at 3:50 am

    Nice explaination and examples r also good mam to teach thanks a lot

  • Reply John Bates July 1, 2018 at 4:25 pm

    You refer to intermolecular forces in the video but you refer to them as intramolecular forces when mentioning them – confusing

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  • Reply Dasha T November 8, 2018 at 10:56 pm

    So helpful! Thank you

  • Reply SevenDeMagnus September 3, 2019 at 9:08 pm

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