1. What are the structural differences
between ionic and molecular compounds?
2. How do those differences affect
their chemical formulas?
The chemical formula for a covalent compound
is called a molecular formula;
The chemical formula for an ionic compound
is called a formula unit.
The chemical formula for a covalent compound
is called a molecular formula;
Example: H2O is the molecular formula of water
The chemical formula for an ionic compound
is called a formula unit.
Example: NaCl is the formula unit of
common table salt
What’s the dif?
A nice
VISUAL look
at the difference
will help out:
We’ll look at sodium chloride, NaCl,
as an example ionic compound.
NaCl does not exist as
a single unit like a molecule.
Like any ionic compound,
sodium chloride exists as a crystal:
The formula unit for sodium chloride is NaCl.
However, if we look inside the crystal,
the Na+ and Cl- ions are each bonded
to six oppositely charged neighbors:
Let’s shrink
the ions so
we can take
a peek inside
Let’s shrink
the ions so
we can take
a peek inside
We’ll take a
look at this
Cl- ion.
How many
Na+ ions is
the Cl- close
enough to
bond with?
We’ll start
again
from the
head-on view
We’ll start
again
from the
head-on view
And,
how many
Cl- ions is
each Na+ ion
bonded to?
Because each Na+ is bonded to more than one Cl- ion,
and each Cl- is bonded to more than one Na+ ion, then
all the ions in the whole crystal are
part of one single bonded unit,
and the crystal below would be Na32Cl32.
This is impractical, given the enormous numbers
of ions in a crystal large enough to see with our eyes.
So, only the ratio of ions is given (its empirical formula),
which is called a formula
unit.
NaCl is the formula unit for
the sodium chloride crystal.
NaCl is the smallest whole number
ratio of sodium and chloride ions
in the crystal.
This is impractical, given the enormous numbers
of ions in a crystal large enough to see with our eyes.
So, only the ratio of ions is given (its empirical formula),
which is called a formula
unit.
NaCl is the formula unit for
the sodium chloride crystal.
NaCl is the smallest whole number
ratio of sodium and chloride ions
in the crystal.
What about molecules?
water is a molecule.
‘space filling’ model
‘ball and stick’ model
water is covalently bonded hydrogen and oxygen.
‘space filling’ model
‘ball and stick’ model
The H2O molecule remains as a single unit of three
covalently bonded atoms.
In large amounts of H2O, any attraction between
H2O molecules is due to weak forces, and so the
molecules behave as individual units, not one large
bonded unit as in an ionic crystal.
This difference in structure between molecules and
ionic compounds is reflected in their symbols:
This difference in structure between molecules and
ionic compounds is reflected in their symbols:
The molecule is represented by a molecular formula,
showing all the atoms present in the unit.
The ionic compound is represented
by the ratio of its ions, which is called a formula unit.
versus
H2O
NaCl
IN SUMMARY, in a molecule, the atoms given by
the formula are all covalently bonded. In a collection
of those molecules, the molecules are attracted to
each other by weak forces, but no covalent bonds
occur between molecules--the molecules exist as
individual units of covalently bonded atoms.
BUT, in an ionic compound, where the bond is the
electrostatic attraction between oppositely charged
ions, each ion is bonded to each of its oppositely
charged neighbors!
So, for any ionic compound, all the ions in the whole
crystal can be thought of as part of one single bonded unit.
So the formula unit shows the lowest whole number ratio
of the ions, rather than representing ALL the ions in the crystal.