10-1-intermolecular-forces_summary

The given passage discusses intermolecular forces, which are the attractions between atoms, molecules, or ions in condensed phases. These forces include dispersion forces, dipole-dipole attractions, and hydrogen bonding. The differences in the properties of solids, liquids, or gases reflect the strengths of the attractive forces between the particles that make up each phase. In solids, particles are tightly packed together and often arranged in a regular pattern, and they vibrate about fixed positions without generally moving in relation to one another. In a liquid, particles are close together with no regular arrangement, moving past each other but remaining in constant contact. In a gas, particles are far apart with no regular arrangement and move independently of one another except when they collide. The strengths of these intermolecular forces (IMFs) and the kinetic energies (KE) of the particles play a significant role in determining the phase in which a substance exists. For example, when gaseous water is cooled sufficiently, the attractive forces between H2O molecules can hold them together to form liquid water. Conversely, if a liquid's temperature becomes sufficiently low, or the pressure on the liquid becomes sufficiently high, the molecules may form a solid because they no longer have enough KE to overcome the IMFs between them. Dispersion forces, also known as London dispersion forces, are present in all condensed phases and are the result of the temporary, instantaneous dipoles that form when an atom or molecule's electrons fluctuate and create asymmetrical distributions. They are relatively weak, but become significant when the molecules are very close. Larger and heavier atoms and molecules exhibit stronger dispersion forces than smaller and lighter ones. Dipole-dipole attractions occur between polar molecules with partial positive and partial negative charges. The effect of a dipole-dipole attraction is apparent when comparing the properties of polar molecules, such as hydrogen chloride (HCl), with nonpolar molecules, such as fluorine (F2). The dipole-dipole attractions between HCl molecules are sufficient to cause them to "stick together" to form a liquid, while the relatively weaker dispersion forces between nonpolar F2 molecules result in their remaining gaseous at the same temperature. Hydrogen bonding is a particularly strong dipole-dipole attraction that occurs when a molecule contains a hydrogen atom bonded to a fluorine, oxygen, or nitrogen atom. Hydrogen bonding has a pronounced effect on the properties of condensed phases. For example, water molecules form multiple hydrogen-bonding interactions with nearby water molecules, which significantly affects its boiling point. The passage also includes examples of geckos, whose feet contain millions of tiny hairs called setae, which branch into many triangular tips called spatulae. Geckos' toes adhere to surfaces due to dispersion forces, and they can stick to surfaces equally well whether they are wet or dry. Finally, the passage includes a link to a video about Kellar Autumn's research that determined that van der Waals forces are responsible for a gecko's ability to cling and climb.