Intermolecular forces- not within molecules, rather between molecules
Dipole-dipole attractions – electrons are not evenly distributed, partial charges (signified by greek letter Delta)
Hydrogen bonds (type of dipole-dipole attraction) – extremely strong, must contain N, O, or F (most electronegative)- otherwise they would be regular dipole-dipole. Greatest tendency to stick together, highest boiling points and melting points.
- Outer structure of DNA has covalent bonds. Inner structure (stairstep) are hydrogen bonds. Important to allow transcription/replication without pulling entire structure apart, but will hold together under normal circumstances.
- Water has a higher boiling point than similar fluids because of the hydrogen bonds. It is more likely to be found in a condensed state because it is more difficult to break apart the bonds to cause a state change into a gas.
London Dispersion Forces
- Instantaneous dipoles- dipoles that exist for only an instant due to the electron’s random movement, can cause a neighboring molecule to also change for an instant. Often shown with two deltas (partial partial positive, partial partial negative). As we increase molecular weight, we increase the dispersion forces. Ie the larger the molecular weight of a molecule, the more prevalent the dispersion forces. We can see this on the periodic table. As molecular weight increases, we increase the tendency for the particles to stick together. (both the same, as in the case of diatomic gases, like oxygen, nitrogen, etc.)
For a video lecture on intermolecular forces, click here.
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