previous index next

Here is a video about phases and forces (19min): States of Matter, by Khan Academy, on YouTube

Here is another video about phases and forces (12min): Van Der Waals Forces, by Khan Academy, on YouTube

Here is another video about liquids and forces (11min): Liquids, by Crash Course Chemistry, on YouTube

Dipole-Dipole Forces

Dipole-dipole forces are probably the simplest to understand. You probably already know that in an ionic solid like NaCl, the solid is held together by Coulomb attractions between the oppositely-charges ions. The Na+ and Cl ions alternate so the Coulomb forces are attractive. Dipole-dipole forces work the same way, except that the charges are smaller. A good example is HF (this is also an example of a special type of dipole-dipole force called a hydrogen bonding). In HF, the bond is a very polar covalent bond. That means there is a partial negative (δ—) charge on F and partial positive (δ+) charge on H, and the molecule has a permanent dipole (the electrons always spend more time on F). In the liquid or solid HF, the molecules arrange themselves so that the δ— and δ+ are close together. These partial charges attract each other, and this attraction is what we call dipole-dipole forces. Any molecule with a permanent dipole has dipole-dipole forces that hold the molecules next to each other as a solid or liquid.

previous index next


Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 International License.
Last modified: Sun May 25 12:17:49 KST 2014