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Molecules can be attracted to each other by many forces, liek intermolecular attractions.
Intermolecular attractions are weaker than both ionic and covalent bonds but are
important for determining the physical state a compound.
Van der Waals forces are the two weakest attractions between molecules ,
dipoleinteractions + dispersion forces.
Dipole interactions occur between polar molecules when oppositely charged regions of
the molecules are weakly attracted to each other.
Dispersion forces are caused by the motion of electrons and occur between non-polar
molecules. They are the weakest of all molecular interactions.
The strength of dispersion forces generally increases with the number of electrons in a
molecule.
Halogen diatomic molecules, such as fluorine and chlorine, are gases at ordinary room
temperature a due to their weak dispersion forces and pressure. Bromine is a liquid, and
iodine is a solid at ordinary room temperature and pressure due to their stronger
dispersion forces.
Hydrogen bonds - occur between a hydrogen atom covalently bonded to an
electronegative atom and an unshared electron pair of another
electronegative atom in the same molecule or in a nearby molecule.
In water molecules, the O-H bond is highly polar, oxygen acquiring a slightly
negative charge due to its greater electronegativity, while the hydrogens get a
slightly positive charge.
This leads to dipole-dipole interactions between water molecules, where the
positive region of one molecule attracts the negative region of another
molecule.
For a hydrogen bond to form, a covalent bond must already exist between a
hydrogen atom and a highly electronegative atom such as oxygen, nitrogen,
or fluorine.
The combination of a strongly polar bond and the lack of shielding effect in a
hydrogen atom is responsible for the relative strength of hydrogen bonds,
which have about 5% of the strength of an average covalent bond.
The attractive forces between water molecules due to hydrogen bonding are
responsible for the surface tension of water, the high boiling point and
specific heat capacity of water, and the ability of water to dissolve polar
substances.
Physical properties of a compound depend on the type of bonding it displays.
Covalent compounds have range of physical properties due to widely varying
intermolecular attractions.
Melting and boiling points of most compounds composed of molecules are
low compared to those of ionic compounds.
Solid molecules have weak intermolecular attractions that need to be broken
to melt.
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However, network solids are extremely stable and do not melt until the
temperature reaches 1000°C or higher.
Diamond is an example of a network solid, where each carbon atom is
covalently bonded to four other carbons, so interconnecting carbon atoms
throughout the diamond.
Cutting a diamond requires breaking a multitude of covalent bonds.
Silicon carbide (SiC) is also a network solid, similar in molecular structure to
diamond.
Silicon carbide has a high melting point of about 2700°C and is used as an
abrasive in grindstones.