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Interactions, Attractive forces where we think there might be none
Some vocabulary and understanding
Here is a chart illustrating different forces between and within molecules
Intra: the forces that keep a molecule together, describes interactions in the molecules and the atoms
that comprise them
Inter: Interactions between molecules or atoms (in the case of noble gases) don’t limit this definition to
simply molecules.
Intermolecular and Intramolecular Forces
Force Model Basis of
Attraction
Energy
(kJ/mol)
Example
Intramolecular
Ionic Opposite charges 4000 – 400 NaCl
Covalent Nuclei – shared e-
pair
1100 – 150 H - H
Metallic Metal cations and
delocalizedelectrons
1000 – 75 Au
Intermolecular
Ion-dipole Ion and polar
molecule
600 – 40 Na+ &
H2O
Dipole-dipole Partial charges of polar molcules
25 – 5 HCl &HCl
Hydrogen bond H bonded to N, O,
or F, and another
N, O, or F
40 – 10 H2O &
NH3
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London
dispersion
Induced dipoles of
polarizable
molecules
40 – 0.05 Xe & Xe
Transient dipoles: occur when a dipole of one atom or molecule approaches another neutral or non-
polar atom and molecule and creates a momentary dipole usually these propagate throughout a liquid
or solid.
Polarizabilty: the ability to distort electron cloud how easy it is
I will discuss what affects this at the end
First!
Let’s establish electrons don’t adhere to perfection, a spherical shape or perfect distribution,
From Wikipedia:
In atoms with multiple electrons, the energy of an electron depends not only on the intrinsic properties of
its orbital, but also on its interactions with the other electrons. These interactions depend on the detail of
its spatial probability distribution, and so the energy levels of orbitals depend not only on n but also on .
So electrons movement is influenced by multiple factors.
So why do these atoms that have a full valence shell and are nonpolar in nature have attractive forces
and electrostatic interactions?
Well on average these full valence shells have a spherical electron distribution but remember this is an
average, electrons aren’t moving in a perfectly spherical path as we know from our previous
understanding in other chemistry courses. These electrons are consistently in motion and it is possible
for the concentration of electrons or electron density to be on one side of the atom, the attractive
forces between non polar molecules comes from this momentary or transient dipole. On one end of the
atom there is a concentration of electrons (weak force and momentary but it is still there) the
concentration of electrons on the one side of the atom at this particular moment repels the electrons of
a neighboring atom and attracts the positively charge nucleus towards the negative end of the first atom
which has that transient dipole. But now to deal with our second atom, as the nucleus of atom 2 is draw
towards the area of atom 1 that has a higher electron density, the electrons of atom 2 are repelled to
the other end of atom 2 and again we have a transient dipole! Now this effect propagates through the
entire liquid or solid! Crazy right! Well now let’s address some important factors that affect the extent of
these attractive forces from the presence of transient dipoles!
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Molecular weight
-Heavier atoms will have larger dipoles than lighter smaller ones
More electrons, larger extent of London dispersion forces
-As the number of electrons around an atom increases ( heavier atoms when neutral protons=electrons)
the nucleus is more shielded from the other electrons in lower orbitals closer to the nucleus the valence
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electrons there force are less attracted to the nucleus as they get farther away from it and therefore
they can form larger transient dipoles.
Example
Helium with amu 4 and 2 electrons around its nucleus will form weaker London dispersion
forces or van derwalls interactions compared to argon
Surface Area
Increasing surface area increases the magnitude of London dispersion forces. (As Dr. H said maybe
molecular weight)