Bonding, Molecular Shape & Structure

Page
8

Very polar bond in H-F.

The other hydrogen halides don’t form hydrogen bonds, since H-X bond is less polar. As well as that, their lone pairs are at higher energy levels. That makes the lone pairs bigger, and so they don't carry such an intensely concentrated negative charge for the hydrogens to be attracted to.

Slide 47

Hydrogen Bonding & Water

Slide 48

One of the most remarkable consequences of H-bonding is found in the lower density of ice in comparison to liquid water, so ice floats on water. In most substances the molecules in the solid are more densely packed than in the liquid. A given mass of ice occupies a greater volume than that of liquid water. This is because of an ordered open H-bonding arrangement in the solid (ice) in comparison to continual forming & breaking H-bonds as a liquid.

Slide 49

Weaker Intermolecular Forces

Ion-Dipole Forces

An ion-dipole force is an attractive force that results from the electrostatic attraction between an ion and a neutral molecule that has a dipole.

Most commonly found in solutions. Especially important for solutions of ionic compounds in polar liquids.

A positive ion (cation) attracts the partially negative end of a neutral polar molecule.

A negative ion (anion) attracts the partially positive end of a neutral polar molecule.

Ion-dipole attractions become stronger as either the charge on the ion increases, or as the magnitude of the dipole of the polar molecule increases.

Slide 50

Dipole-dipole Attractive Forces

A dipole-dipole force exists between neutral polar molecules

Polar molecules attract one another when the partial positive charge on one molecule is near the partial negative charge on the other molecule

The polar molecules must be in close proximity for the dipole-dipole forces to be significant

Dipole-dipole forces are characteristically weaker than ion-dipole forces

Dipole-dipole forces increase with an increase in the polarity of the molecule

Slide 51

Boiling points increase for polar molecules of similar mass, but increasing dipole:

Slide 52

London Dispersion Forces –

significant only when molecules are close to each other

Prof. Fritz London

Due to electron repulsion, a temporary dipole on one atom can induce a similar dipole on a neighboring atom

Slide 53