1
POLYATOMIC IONS
We have seen that atoms can lose or gain electrons to become ions.
Groups of atoms can also become ions.
These groups of atoms are called polyatomic ions.
Examples:
OH- - hydroxide ion
NO3- - nitrate ion
CO32- - carbonate ion
NH4+ - ammonium ion
Polyatomic ions can form ionic compounds just as monatomic ions.
Example: What is the chemical formula for magnesium hydroxide?
magnesium  Mg2+
hydroxide  OH Mg(OH)2
Note parenthesis indicates we need two of the entire OH- group.
MgOH2 is wrong.
NOMENCLATURE: Refer to handout
Nomenclature of ions
Cations (positive)
- name of ion is same as metal
- with main group metals, Roman numerals are used to indicate the charge of
the ion only if the metal can have more than one charge.
Ex: All alkali metals have only +1 charge
K+ - potassium ion Rb+ - rubidium ion
Ex: All alkaline earth metals have only +2 charge
Mg2+ - magnesium ion Ca2+ - calcium ion
Ex:
Al3+
Sn4+
Pb2+
 aluminum
 tin(IV)
 lead(II)
- with transition metals, the charge of cation is indicated with Roman numerals
Ex:
Fe2+  iron(II)
- many transition metals have only one common charge; thus, using the roman
numeral is optional.
Ni2+, Zn2+, Ag+
- polyatomic cations are given –ium suffix
Ex:
NH4+  ammonium
2
Anions (negative)
- monatomic anions have –ide suffix
Ex:
Cl-  chloride
S2-  sulfide
As3-  arsenide
- two polyatomic ions are exceptions and also have –ide suffix
OHCN-
hydroxide
cyanide
- all other polyatomic ions (oxyanions) have –ite or –ate suffix
- ite is always one less oxygen than -ate
Ex:
SO42sulfate
SO32sulfite
Ex:
ClO4ClO3ClO2ClO-
perchlorate
chlorate
chlorite
hypochlorite
SO42sulfate
2SO3
sulfite
- note sulfite anion has 3 oxygen atoms and sulfate anion has 4 oxygen atoms
Ex:
Ex:
ClO3ClO2-
chlorate
chlorite
- note chlorite anion has 2 oxygen atoms and chlorate anion has 3 oxygen atoms
3
CONSTRUCTING CORRECT IONIC CHEMICAL FORMULAS
When the charges of ions are known, correct chemical formulas are found
by combining the correct number of ions to make the total charge of the
compound zero.
Example: What is the chemical formula for K+ and Br-?
One + charge and one - charge make zero charge.
 1 (+1) + 1 (-1) = 0
Answer is KBr.
Example: What is the chemical formula for Ca2+ and Cl-?
Calcium ion has +2 charge, therefore, two –1 chlorine ions are needed for the
compound to equal zero charge.
Example: What is chemical formula for Na+ and O2-?
Example: What is chemical formula for Fe3+ and S2-?
4
NAMING IONIC COMPOUNDS
1. Write name of cation first (include Roman numeral, if necessary).
2. Write name of anion.
Binary Compounds
Example: NaI
Na+ is metal ion and I- is nonmetal ion
NaI  sodium iodide
Example: SrBr2
Sr2+ is metal ion and Br- is nonmetal ion
SrBr2  strontium bromide
Example: FeCl3
Fe3+ is metal ion and Cl- is nonmetal ion
FeCl3 
Example: potassium oxide
potassium ion is K+ and oxide ion is O2potassium oxide 
Example: zinc nitride
Cation-Polyatomic Anion Compounds
1. Write name of metal ion (or ammonium ion) first
2. Write name of polyatomic anion
Examples:
NaNO3  Na+ and NO3- 
Fe(C2H3O2)2
aluminum cyanide
barium sulfite
ammonium oxalate
5
NAMING MOLECULAR COMPOUNDS
1. Write the name of the element that is farthest from upper right-hand corner first.
2. Indicate number of atoms with numerical prefix.
1 – mono**
6 – hexa
2 – di
7 – hepta
3 – tri
8 – octa
4 – tetra
9 – nono
5 – penta
10 – deca
**
use of the mono prefix is not preferred, except for carbon monoxide.
3.
4.
5.
6.
Add name of second element with –ide suffix.
Indicate number of atoms with numerical prefix.
Note: No numerical prefixes with hydrogen.
Important exceptions to rules
a) H2O – water
b) NH3 – ammonia
c) CH4 – methane
- Hydrocarbons and their derivatives have their own nomenclature system.
Examples
N2O 
P2S3 
boron trifluoride 
carbon tetrachloride 
6
HYDROCARBONS
- compounds with carbon and hydrogen
- simplest hydrocarbons are called alkanes.
- general formula of an alkane is CnH2n+2
Nomenclature of Alkanes
1. Need to count longest continuous chain of carbons
2. Number of carbons gives compound’s prefix
1
meth
6
hex
2
eth
7
hept
3
prop
8
oct
4
but
9
non
5
pent
10
dec
3. All alkanes have the suffix –ane.
Examples: butane C4H10
H
H
H
H
H
C
C
C
C
H
H
H
H
H
Examples: heptane C7H16
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H