To this point all molecules have had an overall charge of zero, however, this is not the case with
polyatomic ions.
o poly = many
o atomic = atoms
o ion = charged particle
o So…polyatomic ion means a molecule of multiple atoms that have a charge.
Polyatomic ions are stable. The ions are found in solutions or solids where the overall charge of
the solid is zero or the solution is zero. What this means is that you can not have a test tube full
of Na+ and no negative charges like a chloride (Cl-1) to cancel out the overall positive charge. IT
CAN NOT BE DONE, a law of physics, and you can’t break the laws of physics.
o the list of polyatomic ions you must commit to memory.
Formula
Name
Formula
Name
NO3NO2CrO42Cr2O72CNSCNMnO4OHO22NH2SO42SO32PO33PO43HPO42H2PO4C2H3O2CH3COO-
nitrate
nitrite
chromate
dichromate
cyanide
thiocyanate
permanganate
hydroxide
peroxide
amide
sulfate
sulfite
phosphite
phosphate
hydrogen phosphate
dihydrogen phosphate
acetate
acetate
ClO4ClO3ClO2ClOIO4IO3IO2IOBrO4BrO3BrO2BrOCO32HCO3HSO4HSO3HSNH4+
perchlorate
chlorate
chlorite
hypochlorite
periodate
iodate
iodite
hypoiodite
perbromate
bromate
bromite
hypobromite
carbonate
hydrogen carbonate
hydrogen sulfate
hydrogen sulfite
hydrogen sulfide
ammonium
Monoatomic Ions to commit to memory (from the Periodic Table)
Names and Formulas for Ionic Compounds:
Compounds from Monoatomic ions:
1.) the name of the cation is exactly the same as the metal! Na is the same name as
Na+1
2.) the name of the anion keeps the root portion (first portion) of the name and then
adds – ide. Cl = chlorine drop the ine and add –ide so that Cl-1 = chloride
3.) the charges must balance – meaning they must cancel out
if the cation has a +2 charge and it is paired with an anion that has a -1
charge you need (2) -1 anions to balance out (1) positive cation.
Ca+2 paired with Cl-1: if we wrote CaCl then we would have (1) net
positive charge left over, so we need (2) Cl-1 species. We write the
formulas with the number of atoms needed as a subscript. Thus Ca+2
paired with Cl-1 becomes CaCl2.
This has often been termed “cross the charges”
Ca+2
Cl-1
Ca1Cl2
write the 1
→ except by convention we do not
This method of crossing the charges can also be used to determine
what the charge of the ion would be if you separated the compound
into its ions.
AlI3
Answer: if we “uncrossed” the charges shows us that the Al would
have a +3 charge and the I would have a -1 charge – and see – we did
not even need the periodic table (but that would confirm our
uncrossing method since Al is a metal and in Group IIIA and I is a
nonmetal and in Group VIIA!! – check for yourself!)
4.) Some metals can form more than one positive ion, meaning they can give up
different numbers of electrons and form different charges. Typically, the
elements that do this are known as the inner transition metals – found on the
periodic table labeled with a “B” instead of the “A” that we have been examining
thus far. For reasons we’ll talk about later . . . these elements can form different
ions. For now, examine the periodic table above and memorize them 
5.) There is an antiquated form of naming these ions, with an ic or ous, therefore I
am not requiring you to learn this method . . . just be aware that it IS out there
and might be helpful in the future so just be familiar with it! Instead, we will use
the roman numeral approach to naming these ions. In this case, the roman
numeral actually indicates the charge for you! Roman numeral II indicates a
charge while Roman numeral III indicates a +3 charge.
+2
For the ous and ic naming: the higher charged ion for a particular
element gets the ic ending while the lower charged ion for a
particular element gets the ous ending
Fe+2 vs. Fe+3
+2
Fe has the lower charge and would be named with the ous ending
while Fe+3 ion has the higher charge and would be named with the
ic ending. The other component to the name is the root. And
unfortunately it is not ironous and ironic. (Of course not!!) These
special ions use the Latin base names given to them.
Using the roman numeral method of naming Fe+2 would be iron(II)
and Fe+3 would be named iron(III).
Compounds from Polyatomic ions:
1.) the polyatomic ion is a group that STAYS TOGETHER!!! It is a unit, an entity unto itself
– do NOT split it up into its individual atoms!! It is no longer a polyatomic atom if you
do that!!
2.) Name the cation first (same as above) and then name the polyatomic atom (from the list
given to you!)
3.) Again you MUST make sure that the overall charge on the compound is neutral once you
combine the cation (anion in the case of NH4+1) with the polyatomic ion. Since these ions
are UNITS/GROUPS/PARTNERS you must indicate that you want more of the total ion
grouping, so we use parentheses with the number needed as a subscript.
Example: Pairing Al+3 with NO3-1
Al has a +3 charge and the charge on the total NO3 species is a -1. Therefore
we need (3) units of NO3-1 in order to have a neutral compound. Thus the
molecular formula for aluminum nitrate will be Al(NO3)3. Again, we are
using the “cross the charges” method! Only now we need 3 sets of NO 3 If
you just wrote NO33 it obviously looks like a nitrogen atom bonded to 33
oxygens!! BAD!!
4.) On the polyatomic ion list there are several ions that have the same root name and only
differed in the number of oxygens present (e.g. BrO4-1, BrO3-1, BrO2-1, BrO-1). The charge
is the same for EACH ion, the only difference is the number of oxygens present. Even
their names are veeeerrrry similar. These are the oxoanions. And there are families of
oxoanions that are apparent on the list above. It is often easies to learn ONE of the
oxoanions and remember that the names change in a systematic way as the number of
oxygens increases or decreases.
When there are only 2 oxoanions in the family (SO4-2 and SO3-2), the ion
with the MOST oxygen atoms gets the –ate ending (sulfate) while the ion
with the LEAST oxygen atoms gets the –ite ending (sulfite).
When there are 4 oxoanions in the family (BrO4-1, BrO3-1, BrO2-1, BrO-1), the
ion with the MOST oxygen atoms gets a prefix – per – and a suffix – ate :
BrO4-1 would be perbromate. The ion with the next most oxygen atoms
drops the per and becomes bromate. The ion with the next most oxygen
atoms becomes bromite (parallels when there are only 2 oxoanions), and
the ion with the least oxygen atoms gets the prefix – hypo – and the suffix ite
and is named hypobromite.
5.) Ionic compounds can do one more thing, adding to their name. They can pick up some
water – termed a hydrate. For ionic hydrates we follow allll of the rules given up to this
point and then indicate the number of water molecule using prefixes given below
followed by hydrate:
Number
1
2
3
4
5
6
7
8
9
10
Prefix
mono
di
tri
tetra
penta
hexa
hepta
octa
nona
deca
Thus: Ba(OH)2∙8H2O would be: barium hydroxide octahydrate
Ba is the metal (cation) which is named barium it has a +2
charge (notice its in group 2A!!) while OH has a -1 charge, thus
we need 2 OH-1 to cancel out the +2 charge of Ba
OH from the polyatomic ion list is hydroxide
Octa from the prefix list stands for 8
Hydrate indicates the presence of water
And: copper(II) nitrate trihydrate would be: Cu(NO3)2∙3H2O
Nitrate is a -1 charge and copper(II) is a +2 charge therefore
when the charges are “crossed” we have 1 Cu and 2 (NO3)
groups
Tri indicates that there are 3 of something
Hydrate indicates water
Naming Acids: Specifically looking at H containing acids (defined as Arrhenius acids).
Typically they are in water, thus in solution. When naming them, we consider the hydrogen to
be a hydro group as the cation bonded to an anion. Specifically you should be familiar with the
binary (2 atoms) acids involving H and the elements in Group VIIA.
Binary Acids: HF, HCl, HBr, HI: use hydro in the name
Named: prefix + nonmetal (drop –ide adding) + ic + acid
HF = hydro
fluoride
ic acid = hydrofluoric acid
HCl = hydro chloride
ic acid = hydrochloric acid
HBr = hydrobromic acid
HI = hydroiodic acid
Oxoacid: from the polyatomic ion sheet, the anions that contain oxygen can be paired
with H thus making an acid (e.g. H2SO4, H2SO3). These names do NOT include hydro!
There is a “fun” saying to remember how to name oxoacids:
-ates become ic’s and -ites become ous’es
So H2SO4 from SO4-2 (sulfate) becomes sulfuric acid (notice no hydro!!!)
H2SO3 from SO3-2 (sulfite) becomes sulfurous acid (notice no hydro!!!)
In situations where per-ate and hypo-ite are used we keep the prefixes but change the
suffixes as above
HBrO4 from BrO4-1 (perbromate) becomes perbromic acid
Binary covalent compounds: formed when 2 elements – specifically nonmetals combine
together to share their electrons. This does not mean 1:1 ratio of the elements, the molecular
formula could be X10Y8 which indicates 18 total atoms – but the bonding is occurring between
two elements – X and Y.
Rules for naming binary covalent compounds:
1. the element with the lower group number on the periodic table is the first
word in the name. The element with the higher group number is the second
word in the name. [Exception!!!!: when a halogen (group VIIA) is covalently
bonded to oxygen, the halogen is named first and the oxygen is named second.
2. If both elements are in the same group, the one with the higher period number
is named first (remember period is the row)
3. The first element is named as the element. If there is more than one of that
element we must indicate how many using the same prefix system that we use
for the naming of hydrates. If there is only one of the first element, by
convention we do not use the prefix mono.
Number
1
2
3
4
5
6
7
8
9
10
Prefix
mono
di
tri
tetra
penta
hexa
hepta
octa
nona
deca
4. The second element is named as its root with the suffix –ide.
5. The suffix will have a numerical prefix.
Examples: PCl5, CO, CO2 H2O
PCl5 = phosphorus pentachloride
CO = carbon monoxide (notice it is not monocarbon monoxide!!)
CO2 = carbon dioxide
H2O = dihydrogen monoxide (which is the “real” name for water!)
Calculating molecular weights of compounds and formula weights of ionic compounds:
Using the number of atoms present in the compound and the molecular weight of the
individual atoms or ions, we can calculate an overall molecular weight for the entire compound.
Molecular weight of the compound = sum of the atomic mass of the elements present in the
compound.
Example: Molecular weight of C2H6
2C atoms = 2 * 12.01g/mole = 24.02g/mole
6H atoms = 6* 1.008g/mole = 6.048g/mole
Then the individual atomic masses are summed 24.02+6.048 = 30.07g/mole
Example: Molecular weight of Al(NO3)3
1 Al = 1* 26.98g/mole = 26.98g/mole
3N = 3* 14.01g/mole = 42.03g/mole
9O (remember 3 sets of NO3 means 9 oxygens total!!) =
9*15.99g/mole = 143.91g/mole
Then take the sum: 26.98+42.03+143.91 = 212.92g/mole