Init 10/28/2010 by Daniel R. Barnes
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IONIC COMPOUND RE-CAP
Ionic compounds typically consist of a metal and a nonmetal.
Nonmetals generally have much higher electronegativities than
metals, so the nonmetal will steal electrons from the metal.
+
IONIC COMPOUND RE-CAP
Ionic compounds typically consist of a metal and a nonmetal.
Nonmetals generally have much higher electronegativities than
metals, so the nonmetal will steal electrons from the metal.
As a result, the nonmetal atom becomes a negatively-charged ion,
and the metal atom becomes a positively-charged ion.
Because they are now oppositely-charged, the negative nonmetal
ion and the positive metal ion are attracted to each other.
+
This electrostaic attraction constitutes an ionic bond between the
metal ion and nonmetal ion.
Because of their attraction for each other, the positive metal ions
and the negative nonmetal ions will form an orderly crystal
structure.
IONIC COMPOUND RE-CAP
Although ionic compounds, like salt, are made of positive ions and
negative ions, the amount of positive charge always equals the
amount of negative charge.
Even though it’s made of plusses and minuses, an ionic
compound, overall, is neutral.
Each ion in an ionic compound will have an electrical charge that
depends upon its location on the periodic table.
Please note the numbers at the tops of the columns.
The ones that matter are the ones with the letter “A” after
them
These numbers tell you . . .
. . . how many valence electrons each element has.
This number is super-important, because it tells you how
many electrons the atom is going to lose or gain.
Remember that atoms like to either completely empty or
completely fill their outer valence shells.
How many electrons an atom loses or gains determines the
atom’s electric charge when it becomes an ion.
BINARY IONIC COMPOUND EXAMPLES
1. What is the chemical formula of sodium chloride?
Sodium = Na = from group 1A  loses 1 e-  Na+
Step one: determine the symbol and charge of each ion.
Sodium has one valence electron, so it has two options.
It can lose the electron to empty its valence shell, or
it can gain seven more electrons to fill its valence shell.
Since losing one electron involves fewer electrons than gaining
seven, sodium loses one electron.
Remember that because electrons are negative, losing electrons
makes an atom positive.
BINARY IONIC COMPOUND EXAMPLES
1. What is the chemical formula of sodium chloride?
Sodium = Na = from group 1A  loses 1 e-  Na+
Chlorine = Cl = from group 7A  gains 1 e-  ClChlorine has seven valence electrons, so it has two options.
It can lose all seven valence electrons to empty its valence shell,
or it can gain one extra electron to fill its valence shell.
Since gaining one electron involves fewer electrons than losing
seven, chlorine gains one electron.
Remember that because electrons are negative, gaining electrons
makes an atom negative.
Now we know the symbols and the charges of the two ions in
sodium chloride.
BINARY IONIC COMPOUND EXAMPLES
1. What is the chemical formula of sodium chloride?
Sodium = Na = from group 1A  loses 1 e-  Na+
Chlorine = Cl = from group 7A  gains 1 e-  ClNa Cl
Now, we need to figure out how many of each ion will make a
single formula unit of the compound.
We need to make sure that the number of plusses equals the
number of minuses.
This one is easy because the sodium ion is plus one and the
chlorine ion is minus one.
The two ions are electrically equal but opposite, so one of each
ion will be fine.
BINARY IONIC COMPOUND EXAMPLES
2. What is the chemical formula of beryllium chloride?
Beryllium = Be = from group 2A  loses 2 e-  Be2+
Chlorine = Cl = from group 7A  gains 1 e-  ClBe2+
ClBeCl2
Cl
S’s:
+2
-2
Now, we need to figure out how many of each ion will make a
single formula unit of the compound.
Beryllium chloride is going to be a little more complicated than
sodium chloride because there are two plusses but only one
minus.
The solution is pretty simple, though. Just add another chloride
ion, and you’ve got a total of two minuses.
BINARY IONIC COMPOUND EXAMPLES
3. What is the chemical formula of potassium oxide?
Potassium = K = from group 1A  loses 1 e-  K+
Oxygen = O = from group 6A  gains 2 e-  O2K+
K+
O2-
K2O
S’s: +2
-2
Potassium oxide has a problem similar to beryllium chloride, but
this time, there’s more minus than plus.
Remember that ionic compounds form when nonmetals steal
electrons from metals.
A neutral oxygen atom is hungry for two electrons, but a
potassium atom is only going to give up one.
It’s going to take two potassium ions to balance out against a
single oxide ion.
BINARY IONIC COMPOUND EXAMPLES
4. What is the formula of aluminum oxide?
Aluminum = Al = from group 3A  loses 3 e-  Al3+
Oxygen = O = from group 6A  gains 2 e-  O2Al3+
Al3+
O2O2-
With aluminum oxide, the plusses don’t equal the minuses either.
This one’s going to be a bit trickier to balance. There are two
ways to go about it. We’ll do the “one ion at a time” method first.
At first, there are 3 plusses but only 2 minuses, so what we need
is more minuses. We get this by adding another oxide ion.
Unfortunately, there is now a total of 4 minuses, which is too
much. We need another aluminum ion.
BINARY IONIC COMPOUND EXAMPLES
4. What is the formula of aluminum oxide?
Aluminum = Al = from group 3A  loses 3 e-  Al3+
Oxygen = O = from group 6A  gains 2 e-  O2Al3+
Al3+
O2O2O2-
Al2O3
S’s: +6
-6
Oh boy. Now we have more plusses than minuses again.
One more oxide ion should help.
Holy cow! I think it’s balanced now!
Yay for correct answers! Yay!
(There is a quicker way than this, though, called the “criss-cross”
method. We’ll show it to you a little later.)
BINARY IONIC COMPOUND EXAMPLES
5. What is the formula of iron (III) bromide?
Iron = Fe  if (III)  loses 3 e-  Fe3+
Bromine = Br = from group 7A  gains 1 e-  Br -
S’s:
Fe3+
Br Br Br -
+3
-3
FeBr3
The “(III)” means that the iron is in the +3 “oxidation state”.
For now, consider Roman numerals to indicate positive charge.
Now, we have the usual problem of getting plusses equal to
minuses. Keep adding ions until + = - .
BINARY IONIC COMPOUND EXAMPLES
6. What is the formula of iron (II) nitride?
Iron = Fe  if (II)  loses 2 e-  Fe2+
Nitrogen = N = from group 5A  gains 3 e-  N3Fe2+
Fe2+
Fe2+
N3N3-
Fe3N2
S’s: +6
-6
Last time, it was iron (III), and now it’s iron (II). As it turns out, iron
has more than one possible oxidation state. That’s true of a lot of
elements.
Once again, though, the Roman numeral tells you . . .
. . . the positive charge of the metal ion.
PATTERN?
DO
SEE
Sodium chloride is made of the ions Na+ & ClThe formula of sodium chloride is
NaCl
Beryllium chloride is made of the ions Be2+ & ClThe formula of beryllium chloride is
BeCl2
Potassium oxide is made of the ions K+ & O2The formula of potassium oxide is
K2O
Aluminum oxide is made of the ions Al3+ & O2The formula of aluminum oxide is
Al2O3
Iron (III) bromide is made of the ions Fe3+ & Br The formula of iron (III) bromide is
FeBr3
Iron (II) nitride is made of the ions Fe2+ & N3The formula of iron (II) nitride is
Fe3N2
YOU
A
Sodium chloride is made of the ions Na+ & ClThe formula of sodium chloride is
NaCl
Beryllium chloride is made of the ions Be2+ & ClThe formula of beryllium chloride is
BeCl2
Potassium oxide is made of the ions K+ & O2The formula of potassium oxide is
K2O
Aluminum oxide is made of the ions Al3+ & O2The formula of aluminum oxide is
Al2O3
Iron (III) bromide is made of the ions Fe3+ & Br The formula of iron (III) bromide is
FeBr3
Iron (II) nitride is made of the ions Fe2+ & N3The formula of iron (II) nitride is
Fe3N2
The “CrissCross” short
cut usually
works just fine.
Usually . . .
BINARY IONIC COMPOUND EXAMPLES
7. What is the formula of tin (IV) oxide?
Tin = Sn  if (IV)  loses 4 e-  Sn4+
Oxygen = O = from group 6A  gains 2 e-  O2Sn4+
O2O2-4
SnO2
S’s: +4
The one-ion-at-a-time method gives us . . .
Sn4+
The criss-cross method gives us . . .
If you do the criss-cross method and
you get subscripts that all can be
divided by the same number, you
should divide them all by the same
number.
O2- Since you can
divide both
Sn2O4
two and four
by two, do so,
and you get
our original
answer.
2
1
=
4
2
In math, if you get 2/4 as your answer, you’re supposed to . . .
. . . reduce the fraction to . . .
You do this by dividing the top and bottom of the fraction by the
same number. (In this case, dividing the top and the bottom by
two)
If, in chemistry, you get . . .
Sn2O4
You should reduce it to . . .
SnO2
Remember this if you decide to use the “criss-cross” method
instead of the “one ion at a time” method.
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A salt crystal may go on and on, but this presentation is over.
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