In chemistry the focus is often on the individual molecule its structure, name, physical properties and chemical reactivity. Molecules do not generally exist isolated and alone, however. They are immersed in the environment around them, surrounded by other molecules which may or may not be the same chemical. Either way, the molecules interact, exerting attractive forces upon one another. (This is fortunate, since if they all repelled, people, buildings, and every other object would literally fall to pieces.)

All intermolecular forces (forces between molecules) are attractive, Coulombic forces. The Coulomb is a measure of charge, and you are probably familiar with the phrase "opposite charges attract". They do indeed, and this Coulombic force is the underlying principle for each of the forces below.

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Hydrogen Bonding

A hydrogen bond is not an actual bond. It is, however, a strong attractive force, with enough strength to hold two molecules together. The two halves of your DNA are held together by hydrogen bonds. Many of water's unique properties (relatively high boiling point and surface tension) are a result of its ability to hydrogen bond with itself.

A hydrogen bond is formed by the attraction between a negatively charged lone pair of electrons on one molecule and a hydrogen atom that is attached to an electronegative element (like O, N, or S) on another molecule. The hydrogen atom has a partial positive charge because the electronegative element draws the electrons away from the hydrogen, leaving the proton's positive charge unbalanced.

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Dipole-Dipole Forces

A dipole is anything that has two oppositely charged ends. This creates two poles, a positive and a negative, thus the name di- (two) pole. The hydrogen atom discussed in hydrogen bonding is an example of half of a dipole. The atom (O, N, S...) that it is attached to is the other half. Whatever atom the electron pair is on is also a dipole, with the lone pair representing the negative end. Thus hydrogen bonding is a form of dipole-dipole forces. Hydrogen bonding is far stronger than the rest, however, so it gets its own name.

A dipole results between two bonded atoms when they have significantly different electronegativities. The higher electronegativity atom becomes slightly negative, the lower, positive. An entire molecule becomes a dipole when the dipoles inside it are unbalanced. For example, carbon dioxide has two carbon-oxygen bonds, each of

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