Section 5.1: Ionic Compounds
Objectives: Apply ionic charge to
writing formulas for ionic
compounds, Apply formulas to name
ionic compounds, Interpret the
information in a chemical formula
Ionic Compound Formulas
BINARY COMPOUNDS: Contain only two
elements
Can contain more than one ion of each
element (ex: CaF2), but are not
composed of three or more different
elements.
Ionic Compound Formulas
Naming Binary Ionic Compounds:
1) Write the name of the positively charged ion
(metal)
2) Add the name of the negatively charged ion
(nonmetal), modified to end in –ide
Ex: Potassium, Chlorine →
Potassium Chloride
Magnesium, Oxygen → Magnesium Oxide
Ionic Compound Formulas
Try these:
1) Li2O
2) CaCl2
3) Sulfur and Sodium
4) Potassium and Sulfur
5) Fluorine and Sodium
Formulas for Ionic Compounds
Compounds are electrically neutral – the
sums of the charges in an ionic
compound equal 0
 Add subscripts to the element symbols
(ions) until the sum of the ions’ charge is
0.
 The smallest subscript is 1. However,
no subscript needs to be written - the 1
is understood.
Example: NaCl - 1 Sodium, 1 Chlorine

Formulas for Ionic Compounds
If more than one ion of a given element is
in a compound, the subscript indicates
how many ions are present
Example: CaF2 - 1 Calcium, 2 Fluorine
ions
Formulas for Ionic Compounds

In an ionic compound, a formula represents
the smallest ratio of atoms or ions in the
compound
Ex. CaF2 not Ca2F4

In a covalent compound, the smallest unit
of the compound is a molecule
Ex. H2O

A formula is the simplest possible ratio of
ions present.
Formulas for Ionic Compounds
FORMULA UNIT: Simplest ratio of ions in
a compound.
Charges on Ions

Metals tend to lose electrons and
become positive ions

Nonmetals tend to gain electrons and
become negative ions

OXIDATION NUMBER: Charge on an
ion
Charges on Ions
Oxidation numbers for elements are can be
predicted by group number.
Ex: F is in group 17, it gains one outer
electron
Al is in group 13, it loses three outer electrons
 However, oxidation numbers for elements
in groups 3-12 (transition elements) cannot
be predicted by group
 Get out periodic table and make notes!

Writing a simple formula
Write the formula for an ionic compound
containing sodium and sulfur.
1)What are the elements involved?
2) What are the oxidation numbers of each?
3) Sum of the ions must be zero. (add some if
necessary
4) Write the symbols and numbers
(subscripts) for each element, positive ion
(metal) first.
Writing a simple formula
Practice Problems
1) lithium oxide
→ Li2O
2) calcium bromide
→ CaBr2
3) barium and oxygen
→ BaO
4) strontium and iodine
→ SrI2
Writing a simple formula
More Practice
5) Barium and oxygen
6)Strontium and iodine
7) Lithium and chlorine
8) Radium and chlorine
POLYATOMIC ION
Ion that has 2 or more different elements

In a polyatomic ion, a group of atoms is
covalently bonded together when the
atoms share electrons.

Although the individual atoms have no
charge the group as a whole has a charge.
Examples: nitrite NO22-, sulfate SO42-
POLYATOMIC ION
Formulas for polyatomic ions:
 Treat the polyatomic ion as if it were a singleelement ion by keeping it together as a unit.
 The sum must equal zero.
 Multiples of a polyatomic ion in a formula can
be indicated by placing the entire polyatomic
ion in parentheses.
Example: Mg(NO3)2
Note: Never change the subscripts within the
polyatomic ion
POLYATOMIC ION
Sample problems
1) Write a formula for Li+ and CO32 Because CO3 has a –2 charge, two Li
are needed to balance
 Li2CO3
 Check charges: 2(1+) +1(2-) = 0 (sum
must equal zero)
POLYATOMIC ION
2) Write a formula for Al3+ and SO42 Find the least common multiple of 2 and
3→ 6
 Two Al3+ will be needed: 2 (3+) = 6
 Three SO42- will be needed: 3 (2-) = -6
 Al2(SO4)3
 Check charges: 2 (3+) + 3 (2-) = 0
POLYATOMIC ION
3) Ammonium (NH4+) and sulfite (SO32-)
 Because SO3 has a –2 charge, two NH4
are needed to balance
 (NH4)2SO3
 Check Charges: 2 (1+) + 1 (2-) = 0
POLYATOMIC ION
4) Sodium (Na+) and Phosphate (PO43-)
 Because PO4 has a –3 charge, 3 Na are
needed to balance
 Na3PO4
 Check Charges: 3 (1+) + 1 (3-) = 0
POLYATOMIC ION
Formulas:
1) Aluminum and cyanide
2) Lithium and peroxide
3) Ammonium and phosphate
4) Calcium and carbonate
5) Rubidium and nitrate
Names:
1) BaSO4 2) MgCO3 3) Li2SO3
4) K3PO4 5) Al(CN)3
Transition Elements
Can have more than one oxidation
number, except Zn2+ and Ag+ only have
one type of ion
Examples: Cu+ Cu2+

Show Video
Fe2+ Fe3+
Transition Elements
Naming compounds formed from transition
elements:
 Use a Roman numeral to indicate the
oxidation number of the transition element
 Roman numerals are placed in
parentheses after the name of the element
Examples: Cr2+ Chromium (II)
Cr3+ Chromium (III)
Cr6+ Chromium (VI)
Show Video
Transition Elements
Examples
1) Write the formula for Copper (I) and
Sulfite
 Cu+ and SO32 Because SO3 has a –2 charge, 2 Cu will
be needed to balance
 Cu2SO3
Transition Elements
2) Write the name for Pb(NO3)2
 Because NO3 has a –1 charge and there
are 2 of them, the oxidation state of Pb
must be +2
 lead (II) nitrate
Transition Elements
Practice Problems:
 Gold (III) sulfate
 Nickel (II) fluoride
 MnS
 Cu2SO3
 SnF4
 Au(CN)3
HYDRATE
Compound in which there is a specific ratio
of water to ionic compound
 In hydrates, the water molecules are
chemically bonded to ionic compounds
(• H2O)
Ex
CuSO4 • H2O
HYDRATE
HYGROSCOPIC substances: Ionic
compounds can easily become hydrates by
absorbing water molecules from water
vapor in the air
Example: Na2CO3
DELIQUESCENT substances: So
hygroscopic that they take up enough
water from the air to dissolve completely
and form a liquid solution
Example: NaOH
HYDRATE

Compounds that form hydrates are often
are used as drying agents (desiccants)

Heating hydrates can drive off water.

ANHYDROUS compound: One in which
all of the water has been removed.
HYDRATE
To write the formula for a hydrate,
1) Write the formula for the compound
2) Place a dot followed by the number of
water molecules per formula unit of
compound
Ex. CuSO4 • 5H2O
HYDRATE
To name hydrates,
1) Write the regular name for the
compound
2) Use prefix to indicate the number of
water molecules present in front of the
word hydrate
Ex Copper (II) sulfate pentahydrate
HYDRATE
Prefixes to Use in Naming Hydrates:
Molecules of water
Prefix
1
mono2
di3
tri4
tetra5
penta6
hexa7
hepta8
octa9
nona10
deca-
HYDRATE
Examples:
 Calcium chloride pentahydrate
 Aluminum oxide octahydrate
 Li3N∙ 3H2O
 BaS∙ 4H2O
Interpreting Formulas
To represent more than one formula unit of a
compound- place a coefficient before the
formula
Example: two formula units of NaCl –
2NaCl
Formulas summarize how many atoms of
each element are present:
Examples: (NH4)2SO4
2 N, 8H, 1S, 4O
Interpreting Formulas
Try this:
 3Al2(CrO4)3
6 Al, 9 Cr, 36 O

5Ba3(PO4)2
15 Ba, 10 P, 40 O