Chapter 11 Intermolecular forces, liquids, and solids
11.1 A molecular comparison of gases, liquids and solids
The fundamental difference between states of matter is the distance between
particles.
The state a substance is in at a particular temperature and pressure depends on
two antagonistic entities:
11.2 Intermolecular forces
The attractions between molecules are not nearly as strong as the intramolecular
attractions that hold compounds together.
These intermolecular forces as a group are referred to as van der Waals forces.
Three types:
Ion-dipole forces
A 4th type of force, ion-dipole interactions,
are an important force in solutions of ions.
Dipole-Dipole Interactions
Molecules that have permanent dipoles are attracted to each
other.
London Dispersion Forces
The electrons in the 1s orbital of helium
repel each other and, therefore, tend to
stay far away from each other.
For an instant the helium atom is polar, with an excess of electrons on the left side
and a shortage on the right side.
Another helium nearby, then, would have a dipole induced in it, as the electrons on
the left side of He atom 2 repel the electrons in the cloud on He atom 1.
These forces are present in all molecules, whether they are polar or nonpolar.
Factors affecting London forces
The shape of the molecule affects the
strength of dispersion forces:
If two molecules are of comparable size and shape, dipole-dipole interactions will
likely be the dominating force with respect to intermolecular attractions.
Hydrogen bonding
The nonpolar series (SnH4 to CH4) follow the
expected trend.
The dipole-dipole interactions experienced when H is bonded to N, O, or F are
unusually strong.
Hydrogen bonding arises in part from the high electronegativity of N, O and F.
Intermolecular forces: a summary
11.8 Bonding in solids
The physical properties of crystalline solids depend on:
Molecular Solids
Consist of atoms or molecules held together by intermolecular forces.
Efficient packing of molecules is important
(since they are not regular spheres).
Covalent-Network Solids
Consist of atoms held together, in large networks or chains, with covalent bonds.
Strong covalent bonds connect the atoms,
e.g. quartz (SiO2) or diamond (C):
Strong covalent bonds connect the atoms within
the layers of graphite:
Ionic Solids
Consist of ions held together by ionic bonds (i.e. by electrostatic forces of attraction).
The larger the charges (Q1, Q2) and the smaller the distance (d) between ions, the
stronger the ionic bond.
Metallic Solids
Consist entirely of metal atoms; not covalently bonded.