Forces in Solids and
Liquids
Types


Strong forces – ionic bonds between ions
(metal and non-metal atoms), covalent
bonds (C and Si structures), metallic bonds
(metal atoms only)
Weak forces – between MOLECULES
(non-metal atoms only)
The nature of intermolecular forces
Intermolecular forces are the forces that
hold MOLECULES together in a
MOLECULAR SOLID or LIQUID.
 The forces are WEAK in comparison to
covalent, metallic and ionic bonds – this
explains why molecular solids have LOW
melting and boiling points

The three types

Hydrogen bonding
Occurs only between molecules that contain a F, O
or N bonded to a H

Permanent dipole-permanent dipole
Occurs between molecules which are polar

Temporary dipole-temporary dipole
Occurs between all molecules. Is significant for
non-polar molecules and large molecules
Questions:
For each compound state the type of force holding the
particles together. If molecular, draw its structure and
state the most significant type of WIF
Cobalt
Methanol
Tetrafluoromethane
Hydrogen sulfide
Magnesium fluoride
Phosphine (PH3)
Dichloromethane
Hydrogen peroxide
Hydrogen chloride
Nickel
Graphite
Ammonia
Helium
Carbon dioxide
Copper sulfate
Answers:
Cobalt
Metallic bond
Methanol
Hydrogen bonding
Tetrafluoromethane
Temporary dipole
Hydrogen sulfide
Permanent dipole
Magnesium fluoride
Ionic bond
Phosphine (PH3)
Permanent dipole
Dichloromethane
Permanent dipole
Hydrogen peroxide
Hydrogen bonding
Hydrogen chloride
Permanent dipole
Nickel
Metallic bond
Graphite
Covalent bonds
Ammonia
Hydrogen bonding
Helium
Temporary dipole
Carbon dioxide
Temporary dipole
Copper sulfate
Ionic bond
Question:
Bromine melts at -7°C, and sodium bromide
melts at 755°C. Explain why there is a
big difference in their melting points.
Answer:
Bromine is a molecular solid with molecules composed of
2 covalently bonded bromine atoms. The molecules
are held together by temporary dipoles as Br2 is a
non-polar molecule. It is non-polar because the both
Br atoms have identical electronegativity. Temporary
dipoles are relatively weak, therefore little energy is
required to break them and the melting point is low.
NaBr is an ionic solid composed of Na+ and Br- ions in a
3D lattice. The ionic bonds holding the ions together
are strong, requiring a lot of energy to break and
therefore the melting point is high.
Question: (complete and hand in for
marking)
Rank the following compounds from highest
to lowest melting point. Explain your
reasoning:
Water
diamond
carbon dioxide
sulfur dioxide sulfur trioxide
iron
Answer:
Highest to lowest:
Diamond - strong covalent bonds between C atoms
Iron – strong metallic bonds between Fe atoms
Water – WIF – hydrogen bonds between H2O molecules
(contains O bonded to H)
Sulfur dioxide – WIF – permanent dipoles between SO2
molecules (polar)
Sulfur trioxide – WIF – temporary dipoles between SO3
molecules (non-polar)
Carbon dioxide - WIF – temporary dipoles between CO2
molecules (non-polar and fewer electrons than SO3)