Attractive Forces How Argon and Helium

8/3/2019 Attractive Forces How Argon and Helium

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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



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!

http://en.wikipedia.org/wiki/Energy_level






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



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)

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Attractive Forces How Argon and Helium