Ionic Compounds:
Writing Formulas and Names
Empirical Formulas
• Give the smallest whole-number ratio
of elements in the compound.
• Ionic compounds have empirical
formulas only.
Identify the empirical formulas
•
•
•
•
•
•
•
•
H2O
Empirical and possibly Molecular
H2O2 Molecular
Empirical and Ionic
CaF2
Empirical and Ionic
NaCl
Empirical and possibly Molecular
CH4
Molecular
C2H6
Empirical and Ionic
NiO
Empirical and Ionic
FeCl3
Formula Unit
• The simplest ratio of the ions in an ionic
compound.
• KBr means potassium ions and bromide
ions are in a 1-to-1 ratio and is the formula
unit.
• For ionic compounds, the formula mass is
based on the formula unit.
Binary Compounds
• Composed of two elements
–Positive monatomic metal ion
–Negative monatomic nonmetal ion
–Monoatomic ion is a one-atom ion.
Note: A binary compound may contain more
than 2 ions but only 2 kinds of ions like Al2O3.
Oxidation Number or State
• Charge of a monatomic ion.
• Given by right superscript.
Na+1
Ca+2
O-2
• Some elements form only 1 ion.
• Some elements can form more than 1 ion.
Fe+2 and Fe+3
• Try to find patterns.
Oxidation State of Selected
Elements
•
•
•
•
Group 1 ions always +1
Group 2 ions always +2
MOST group 13 ions are +3
Groups 14, 15, 16, and 17 have multiple
oxidation states, both positive and
negative.
• Transition metals (Group B) can have
more than 1 oxidation state but always
positive.
Writing Formulas for Binary Cmpds
• The first rule in writing formulas for ionic
compounds is
POSITIVE ION FIRST
But how do you get the subscripts?
We will start by writing formulas from the ions.
Writing Formulas for Binary Cmpds
• Compounds are electrically neutral –
no charge.
• Take the number of each kind of ion X
the charge on the ion. Sum these
terms. Should get 0.
• Positive and negative charges must
balance out.
Equal but Opposite Charges
• Na+1 and Cl-1: NaCl +1 + (-1) = 0
• Mg+2 and O-2: MgO +2 + (-2) = 0
• Al+3 and P-3: AlP
+3 + (-3) = 0
• Rule: Just write the symbols,
POSITIVE FIRST!
Try a few formulas:
•
•
•
•
•
Li+1 and I-1
Ca+2 and O-2
Al+3 and N-3
K+1 and F-1
Ba+2 and S-2
LiI
CaO
AlN
KF
BaS
Criss-Cross Method
• When the charges don’t cancel out
immediately:
• Mg+2 and Cl-1, CROSS and DROP!
• (Numbers only, forget signs!)
• Mg1Cl2 but if the subscript is 1, forget it!
• MgCl2 means 1 Mg+2 and 2 Cl-1
Check the Math
• MgCl2 means 1 Mg+2 and 2 Cl-1
• (1 X +2) + (2 X -1) = 2 – 2 = 0
• The charges add up to zero!
Try a few formulas:
•
•
•
•
•
•
•
Ca+2 + Cl-1
Na+1 + O-2
Cs+1 + S-2
Al+3 + Cl-1
Al+3 + Se-2
Mg+2 + F-1
K+1 + N-3
CaCl2
Na2O
Cs2S
AlCl3
Al2Se3
MgF2
K3N
Of course, it gets more difficult
•
•
•
•
•
•
•
•
Potassium (K) and Fluorine (F)
KF
Zinc (Zn) and Iodine (I)
ZnI2
Sodium (Na) and Oxygen (O)
Na2O
Magnesium (Mg) and Oxygen (O)
MgO
Aluminum (Al) and Oxygen (O)
Al2O3
Calcium (Ca) and Bromine (Br) CaBr2
Cesium (Cs) and Iodine (I)
CsI
Silver (Ag) and Sulfur (S)
Ag2S
Naming Binary Ionic Compounds
1. Always name the metal (the cation or
positive ion) first.
2. Write the stem of the nonmetal (the anion
or negative ion).
3. Add the ending “ide” to the nonmetal.
Stems of nonmetals
Nitr
Ox
Fluor
Phosph
Sulf
Chlor
Arsen
Selen
Brom
Tellur
Iod
Hydr is the
stem for H
The First Step in Naming
• Look up the positive metal in the
periodic table in your reference
tables.
• If the metal has only one oxidation
state (possible charge), it’s easy.
• If the metal has more than one
oxidation state, there’s an extra step.
Metals with one oxidation state
•
•
•
•
•
•
•
•
CaO
BaS
AlN
LiCl
Al2Se3
Na2O
K3N
MgF2
Calcium Oxide
Barium Sulfide
Aluminum Nitride
Lithium Chloride
Aluminum Selenide
Sodium Oxide
Potassium Nitride
Magnesium Fluoride
Metals with > 1 oxidation state.
• Use the formula to figure out which
oxidation state the metal ion has.
• Ex: Fe can be Fe+2 or Fe+3
• Name FeO and Fe2O3. These are
two different compounds. You cannot
name them iron oxide. Every formula
goes with 1 name and vice-versa.
FeO and Fe2O3
• Compounds are electrically neutral.
• Oxygen is -2.
FeO
1 O which is -2.
The Fe must be +2.
Name:
Iron (II) oxide
The roman numeral II is
the charge on the Fe.
FeO and Fe2O3
• Compounds are electrically neutral.
• Oxygen is -2.
Each Fe is +3.
Iron (III) oxide
Each O is -2.
Fe2O3
Total positive
charge must
be +6.
There are 3 O’s.
3 X (-2) = -6.
Total negative charge.
Name the following
Hint: Positive Always First
•
•
•
•
•
•
•
•
TiCl3
Mn2O4
Co2O3
PdBr2
AuCl3
MoN
MnO
TiO
Titanium (III) chloride
Manganese (IV) oxide
Cobalt (III) oxide
Palladium (II) bromide
Gold (III) chloride
Molybdenum (III) nitride
Manganese (II) oxide
Titanium (II) oxide
POLYATOMIC IONS
• A group of covalently bonded atoms
that carries a charge.
• See Table E. May be +’ve or –’ve.
• Polyatomic ions have “names.”
• Because they are charged,
polyatomic ions can form ionic bonds
with oppositely-charged ions.
SO42-
CO32-
PO43-
OH-
Ternary (or more) Compounds
• Contain 3 or more elements
(uppercase letters)
• Usually have a polyatomic ion
– If the polyatomic is +’ ve, it’s bonded to a
nonmetal
– If the polyatomic is –’ve, it’s bonded to a
metal
– Once in a while, 2 polyatomics are
bonded together.
Formulas with polyatomics
• What’s the formula for the compound
formed from NH4+1 and Cl-1?
• The charges add up to zero, so just
write the symbols, positive first!
NH4Cl
Try a few more:
•
•
•
•
•
•
Na+ and OHK + and HCO3-1
Mg+2 and CO3-2
Li + and NO3NH4 + and CNCa+2 and SO4-2
NaOH
KHCO3
MgCO3
LiNO3
NH4CN
CaSO4
These are more challenging:
•
•
•
•
•
•
•
Mg+2 and (PO4)-3
(NH4)+1 and S-2
Al+3 and (NO3)-1
Fe+2 and OH-1
Hg2+2 and SCN-1
Mg+2 and HCO3Al+3 and C2O42-
Mg3(PO4)2
(NH4)2S
Al(NO3)3
Fe(OH)2
Hg2(SCN)2
Mg(HCO3)2
Al2(C2O4)3
Some of the most challenging are
•
•
•
•
•
•
•
•
Zn(NO3)2
Zinc + Nitrate ion
Mg(OH)2
Magnesium + Hydroxide ion
Lithium + Carbonate ion
Li2CO3
Ammonium ion + Bromine
NH4Br
Potassium + Sulfate ion
K2SO4
Calcium + Phosphate ion
Ca3(PO4)2
Beryllium + Chlorate ion
Be(ClO3)2
Ammonium ion + Sulfate ion (NH4)2SO4
Naming compounds with
polyatomics
• Polyatomic ions have names, given in
Table E.
• Naming is parallel to binary naming.
• Positive always first.
• If +’ve ion is a metal, check to see how
many oxidation states it has. If > 1, name
has a roman numeral.
• If –’ve is polyatomic, 2nd part of name is
name of polyatomic. Don’t modify ending.
Name the following
•
•
•
•
•
•
•
•
NaOH
KHCO3
LiNO3
CaSO4
Al(NO3)3
Fe(OH)2
CuSO4
CuSCN
Sodium hydroxide
Potassium hydrogen carbonate
Lithium nitrate
Calcium sulfate
Aluminum nitrate
Iron (II) hydroxide
Copper (II) sulfate
Copper (I) thiocyanate
Summary for Ionic Compounds
• Name = metal + stem of nonmetal + ide.
• If the metal has more than 1 oxidation
state, it has a roman numeral in the name.
• Formula: positive first, always.
• Compounds are electrically neutral.
• Use oxidation states you know, like O, to
figure out the oxidation states that have
more than 1 possibility.