Pirate Chemistry
Polyatomic Ions
We saw in the previous section how monatomic ions (1 atom per ion) are named and put together. There are other types of ions, though, that are more complicated. These are called
polyatomic ions– multiple atoms that share a combined charge. How and why these atoms
group together to make one charge is not important right now. What is important is how they
are named and how they go together.
Most polyatomic ions are anions; ions with a negative charge. Below is a list of many of them:
Negative Ions (anions)
Acetate
C2H3O2
Bicarbonate
HCO3-1
Carbonate
CO3-2
-1
Chlorate
ClO3
Chlorite
ClO2-1
Chromate
CrO4-2
Cyanide
CN-1
Cyanate
CNO-1
Dichromate
-1
-2
Cr2O7
Dihydrogen phosphate H2PO4-1
Hydrogen phosphate
HPO4-2
Hydrogen sulfate
HSO4-1
Hydroxide
OH-1
Hypochlorite
-1
ClO
-1
Nitrate
NO3
Nitrite
NO2-1
Oxalate
C2O4-2
Perchlorate
ClO4-1
Permanganate
MnO4-1
Peroxide
O2-2
Phosphate
PO4-3
Phosphite
PO3
-3
Sulfate
SO4-2
Sulfite
SO3-2
Thiocyanate
SCN-1
Thiosulfate
S2O3-2
Positive Ions (cations)
Ammonium
NH4+1
Ammonium is the only polyatomic cation we will use.
Notice how most of these ions end in either the “-ate” or
“-ite” endings. In the previous section we only had the
“-ide” ending. Remember that if you have a monatomic
ion (straight from the periodic table and not from this
list), it will always end with the “ide” suffix.
Parentheses
When we combine polyatomic ions, we will often have to
use parentheses to correctly list the formula. We use parentheses when we have multiple polyatomic ions. Let’s look
at an example:
Calcium and Nitrate
Ca+2 NO3-1
Nitrate is on the list opposite.
Ca NO3
Criss-cross the charges
Ca NO3 2
There is one calcium
There are two nitrates
Right now it looks like 32!
Use parentheses
Ca(NO3)2
Final formula
One calcium
Two nitrates
Let’s practice this a bit on the next page.
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner
of a picture and want it removed, email csmith@d211.org, and it will be.
Pirate Chemistry
Example 1:
K+1
Potassium Permanganate
MnO4-1
KMnO4
Example 2:
These charges cancel each other out.
Final formula
Magnesium Sulfate
Mg+2 SO4-2
These charges cancel each other out.
MgSO4
Final formula
Example 3:
Sodium phosphate
Na+1
PO4-3
These charges DON’T cancel each other out.
Na
PO4
Criss-cross the charges
Na3PO4
Final formula
Notice that we don’t use parentheses
We only use parentheses when we have multiple polyatomic ions
Example 4:
Aluminum dichromate
Al+3
Cr2O7-2
These charges DON’T cancel each other out.
Al
Cr2O7
Criss-cross the charges
Al2(Cr2O7)3
Final formula
Notice that we don’t use parentheses for the aluminum. It’s not a polyatomic ion.
Notice that we do use parentheses for the dichromate. We have three of them and it is a polyatomic
ion.
Example 5:
Ammonium Thiosulfate
NH4+1 S2O3-2
These charges DON’T cancel each other out.
NH4
Criss-cross the charges
S2O3
(NH4)2S2O3
Final formula
Notice that this time, the ammonium needs parentheses; there are two of them. The thiosulfate
does NOT need parentheses; there is only one S2O3-2 ion.
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner
of a picture and want it removed, email csmith@d211.org, and it will be.
Pirate Chemistry
Transition Metals
Thus far we have only dealt with ions that are in the main groups. These are groups I—VIII A
on the periodic table. These are the main group elements:
Charges
+1
0
+2
+3
+/-4
-3 -2 -1
With few exceptions, these are easy to use. Remember, though, that we have been ignoring
large sections of the periodic table. These are the transition metals. The transition metals are
the elements in the middle
of the Periodic Table. The
reason that we have been
ignoring them is that they
get complicated. Most of
the transition metals have
the ability to go to more
than one charge. For instance iron, Fe, can be either
the Fe+2 charge or the Fe+3
charge. Copper, Cu, can be
either the Cu+1 charge or the
Cu+2 charge. Why does it
matter?
http://www.bbc.co.uk/schools/gcsebitesize/science/images/6_the_transition_metals.gif
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner
of a picture and want it removed, email csmith@d211.org, and it will be.
Pirate Chemistry
If Fe has a +2 charge, it will result in one formula. For instance:
Fe+2
SO4-2
FeSO4
If Fe has a +3 charge, it will result in a different formula:
Fe+3
SO4-2
Fe2(SO4)3
Notice that in the top example you end up with a different formula than the bottom example
even though you are using the same atoms. FeSO4 and Fe2(SO4)3 are different substances as
their pictures show:
http://upload.wikimedia.org/wikipedia/commons/a/ab/Iron(II)-sulfate-heptahydrate-sample.jpg
FeSO4 is a green crystal.
http://img.alibaba.com/photo/244383957/Iron_III_sulfate.summ.jpg
Fe2(SO4)3 is a yellow crystal.
How do we deal with this? They both can’t have the same name, iron sulfate. To solve this,
Chemists use a Roman Numeral to designate the charge on the transition metal ion. For
example:
Fe+2
Fe+3
iron (II) ion
iron (III) ion
Notice that the Roman numeral (II) matches the charge
Notice that the Roman numeral (III) matches the charge
Thus our problem is solved:
Cation
Anion
Formula
Name
Fe+2
SO4-2
FeSO4
Iron (II) sulfate
Fe+3
SO4-2
Fe2(SO4)3
Iron (III) sulfate
Cu+1
O-2
Cu2O
Copper (I) oxide
Cu+2
O-2
CuO
Copper (II) oxide
Please note that the roman numeral does not usually match the number of the atoms present. In
iron (II) sulfate there is only 1 Fe atom present, FeSO4. In copper (I) oxide, there are 2 Cu atoms present, Cu2O while in copper (II) oxide there is only 1 Cu present, CuO. The roman numeral tells us the charge on the ion, not the number of them present.
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner
of a picture and want it removed, email csmith@d211.org, and it will be.
Pirate Chemistry
Questions
1. Write the formula for the following ions:
A. Acetate
B. Bicarbonate
C. Thiocyanate
D. Peroxide
E. Ammonium
2. Write the name of the following ions:
A. CN-1
B. Cl-1
C. ClO3-1
D. SO4-2
E. PO3-3
3. Why are Roman numerals necessary for most transition metals?
4. Write the formula for the following ionic compounds:
A. Calcium iodide
B. Sodium oxide
C. Potassium oxalate
D. Ammonium hypochlorite
E. Aluminum hydroxide
F. Iron (III) phosphate
G. Copper (II) nitrite
H. Silver chloride
I. Zinc cyanate
5. Write the name for each of the following ionic compounds:
A. KCN
B. (NH4)2O
C. AlPO4
D. CaSO3
E. Na2CrO4
F. LiH2PO4
G. Mg(OH)2
H. Cu2CO3
I. Fe2(S2O3)3
J. ZnCl2
6. Each of these formulas has something wrong with it. Fix it:
A. MgCl3
B. Na3SO4
C. CaOH2
D. Fe(III)Br3
E. Na+1I-1
F. OCa
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner
of a picture and want it removed, email csmith@d211.org, and it will be.