Intermolecular Forces
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Phase Changes
 
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Summary

What intermolecular forces exist between nonpolar molecules?

In a non polar molecule, electron density is evenly distributed and no partial charges exist. You must remember however that electrons are not static, they are constantly in motion. On average, electron density is evenly distributed throughout the molecule.

Click on molecule below to see dynamic nature of electrons (electron density represented by pink area).



As you can see, even though the electrons are moving, on average the electron density is evenly distributed throughout the molecule.

If two chlorine molecules are in close proximity, the electron density of one will effect the other. Imagine that at a particular instant, the electron density in one chlorine molecule is shifted slightly to one side, leaving that side with a slight negative charge and the opposite side with a slight positive charge. A nearby molecule will feel this charge and its electrons will be influenced accordingly:

Click on molecules below to see how a dipole could be induced. Click on mouse to reset.




This results in intermolecular attractions called London forces. London forces occur in all molecules. The strength of London forces depend on how readily electrons can be polarized. Electrons that are tighly held by nuclear attraction are more difficult to polarize.

Which of the following atoms will be easier to polarize?

Mg Ba

Atoms will be easier to polarize if the electrons do not feel a strong attraction to the nucleus. Which of these atoms will feel a weaker attraction to its nucleus?

Good! The outer electrons in barium are further from the positive nucleus than those of magnesium and thus are not as tightly held by the nuclear charge. Thus barium is easier to polarize than magnesium.

In general, larger molecules with more electrons are easier to polarize resulting in stronger London forces. This explains the difference in melting points of chlorine and iodine.









Which of the following molecules will have the higher boiling point?

CH4 CH3CH2CH2CH3 CH3CH2CH2CH2CH2CH2CH2CH3

Larger molecules are easier to polarize than smaller molecules and molecules that are easier to polarize will have stronger London forces.

Good! Octane is the largest of the three molecules and will have the strongest London forces. The boiling point of octane is 126ºC while the boiling point of butane and methane are -0.5ºC and -162ºC respectively.

Intermolecular Forces