Intermolecular+Forces

=Intermolecular Forces- By AA, Bowman, and the 'Dish=

By now we all know that there are strong bonding forces within molecules holding them together. But this unit has called upon us to look beyond what we see within molecules, and extends this knowledge to look at what happens between molecules. As one instructor might say, it is time to look beyond "the land of men" and look to "the land of mice". Personally, we will not comment on this analogy except to say that it is clear now that we are looking at weaker types of forces. The three main intermolecular forces that will be discussed here are dispersion forces, dipole-dipole forces, and hydrogen bonds. To look at these forces, concepts like polarity need to be revisited. So, our group took this challenge of taking this concept, likely the overlying theme of the entire unit, and lay a groundwork which perhaps my be extended upon and applied to in later units. In any respect, the key here is that one alone could not have done this. And so, with that I abandon my isolationist ideals in hope that this will provide a strong basis to build upon, and perhaps not without any entertainment value either. Enjoy! -AA
 * //Overview://**

There can't be any Dipole-Dipole forces between nonpolar atoms or molecules. However there has to be some kind of attractive forces because nonpolar gases can be liquified. This attraction was first proposed by Fritz London, a German-American Physicist, in 1930. He saw that motion of electrons in atoms or molecules can create //instantaneous// dipole moments. For example, in a collection of Helium atoms, the average distribtion of electrons around the nucleus is spherically symmetrical. The atoms are nonpolar and don't have a permanent dipole moment. However, the instantaneous distribution can be different from the average distribution. If the motion of the electrons was stopped at any instant, both would be on the same side of the nucleus. At this instant, the atom would have a instantaneous dipole moment.
 * //Dispersion Forces://**

Therefore, dispersion forces can occur in all molecules due to this instantaneous electron positioning, and every single molecule has this type of intermolecular attraction. This force is also the weakest of all of the intermolecular forces. This is why many of the lighter hydrocarbons, which are nonpolar and have no other types of attractive forces, are in gas form.

The attractive interaction where one atom can cause an adjacent atom to have a similar dipole is called a **London dispersion force**. This is only a signifiant force when molecules are close together. The ease with which charge distribution in a molecule can be distorted by external electric fields is called **polarizability**. The strength of London dispersion forces increases with increasing molecular size. Because of the fact that molecular size and mass normally parallel each other, dispersion forces usually increase in strength with increasing molecular weight.

A Dipole-Dipole force occurs when the positive end of a neutral polar molecule attracts the negative end of a neutral polar molecule. These forces are only effective when the molecules are close together and are generally weaker then ion-dipole forces. In liquids, the polar molecules can move freely and are found in two different orientation, attractive and repulsive. Two molecules that are attracting each other are closer together than the molecules repelling each other. This effect creates a net attraction.
 * //Dipole-Dipole Forces://**
 * In liquids with molecules of equal mass and size, the intermolecular attractions increase with polarity.** Also, **boiling point increases as dipole moments increase.** Molecules need to get close together and in the correct orientation for dipole-dipole forces to operate. **Therefore, molecules with smaller volumes generally have higher dipole-dipole attractive forces.**



Hydrogen bonding is a special type of a dipole-dipole force where there are much larger forces present than normal. These are the strongest intermolecular forces present between molecules. As in the words of one instructor, "In the land of mice, a cat would be king." These are truly the king of all the intermolecular forces.
 * //Hydrogen Bonding://**

Hydrogen bonding occurs when a hydrogen atom bonds to a small electronegative atom, namely nitrogen, oxygen, and fluorine (hence the "FON"). The hydrogen atom bonds to a lone pair of electrons in another molecule, which is very strong due to the high partial negative charges on the other atom. The fact that hydrogen has no inner core of electrons opens it up to attract to strongly electronegative atoms. Hydrogen bonding has much importance in biology.

Hydrogen bonding accounts for many of the properties of water: its surface tension, its high specific heat, its (relatively) high boiling point and melting point, and the density of ice being less than that of liquid.

//**Ion-Dipole Forces**// Another type of intermolecular force is one that occurs primarily in solutions, and this is where its main applications are used. This is ion-dipole bonding. In ion-dipole, an ionic substance will come together with a polar substance. A great example of this is in salt water. In salt water, the Na+ ion will connect with the partial negative (oxygen) end of water, and the Cl- ion will connect with the partial positive (hydrogen) end of water.

//**Sources**// Informational sources: Chemistry, The Central Science (the Book) Wikipedia Pictorial Sources: Click on each individual picture for a link to the site

My comrades in arms, I depart from you with this: Know well the extent of your abilities, and exactly what you can do. Extension beyond this will place you at a point of no return, that is, it will force you to delve too deep into a situation in which you have neither experience nor control over, and cause a downward spiral of your once high-esteemed life. Perhaps we would all like to be self sufficient, but the wise man knows that this is only achievable to an certain extent, and there are times when even the greatest characters need to look past the self for guidance and resources, and even I, one day, will need to ask for help from the people I now see as insignificant, and therefore become the very being of which I currently despise, making my life the biggest hypocrisy of them all. -AA ...What? -NP
 * //Last Words//**