What is a compound?
A substance in which the atoms of two or
more different elements combine together
Sodium chloride NaCl
Carbon dioxide CO2
Calcium carbonate CaCO3
Aluminum sulfate Al2(SO4)3
Dinitrogen pentoxide N2O5
Copper(II) chloride CuCl2
Why do atoms of elements combine
together?
To reach the state of chemical stability
There are two ways to achieve chemical
stability:
1. Transfer of valence electrons between
atoms.
A new ionic compound is formed (NaCl)
2. Share of valence electrons between
atoms.
A new covalent (molecular) compound is
formed (H2O)
Ionic compounds
 Composed of oppositely charged
ions.
 Crystalline solids at room
temperature.
 Hard and brittle.
 High melting point.
 Dissolve in water.
 In solid state they don’t conduct
electricity, but their liquid state and
solutions in water conduct
electricity
Covalent (molecular)
compounds
 Composed of molecules.
 Many are liquid and gases, some
are solid.
 The solid one have low melting
point.
 Some dissolve in water, many don’t
dissolve in Water.
 Don’t conduct electricity in pure
state.
Ionic
compounds
What is an ion?
• An ion is a single atom (element) or a group of
atoms that have a charge.
• A group of atoms that have a charge is called
polyatomic ion.
Single atom ions
Na+
Ca2+
B3+
P3-
Cl- Mg2+
S2- O2N3- Li+
Al3+ I-
Polyatomic ions
NH4+
HCO3-
CO32 -
C2H3O2-
PO43 -
H3O+
• The charge on the ion is called oxidation number.
• Ions formed from groups 1, 2, 13, 15,16, and 17 of
the periodic table have a fixed oxidation number.
• Elements in groups 1, 2, 13 loose electrons so they
have positive oxidation number equal to number of
electrons they loose and they keep the name of the
element.
• Elements in groups 15, 16, 17 may gain electrons
so they have negative oxidation number equal to
number of electrons they gain and the element
name is modified to end with -ide
An ionic compound is composed of a
positive ion and a negative ion.
If the two ions (positive and negative) are
single atom ions the ionic compound is
called binary ionic compound.
To name a binary ionic compound we start
with the name of the positive ion and then
the name of negative ion
Examples of naming binary ionic
compounds (with fixed oxidation number)
NaCl
Na is sodium and it is in group 1 so it
looses 1 electron and becomes Na+ and
called sodium ion
Cl is chlorine and it is in group 17 so it
gains 1 electron and becomes Cl and
called chloride ion
So the name of the compound is sodium
chloride
Examples of naming binary ionic
compounds (with fixed oxidation number)
Al2O3
Al is aluminum and it is in group 13 so it
looses 3 electrons and becomes Al3+ and
called aluminum ion
O is oxygen and it is in group 16 so it
2gains 2 electron and becomes O and
called oxide ion
So the name of the compound is
aluminum oxide
Examples of naming binary ionic
compounds (with fixed oxidation number)
Ca3N2
Ca is calcium and it is in group 2 so it
looses 2 electron and becomes Ca2+ and
called calcium ion
N is nitrogen and it is in group 15 so it
3
gains 3 electron and becomes N and
called nitride ion
So the name of the compound is calcium
nitride
Writing formula unit of binary ionic compounds
Formula unit is the simplest ratio of ions in a
compound.
To write the formula unit of a binary ionic compound:
1. write the symbols of the elements in the compound in the
same order as in the name.
2. Above the symbols write the oxidation number of each
element without the charge.
3. Simplify the oxidation numbers if you can.
4. Write the simplified oxidation number of each element as
a subscript for the other element.
(if the simplified oxidation number is 1 don’t write it)
Examples on Writing formula unit of
binary ionic compounds
Sodium fluoride
+1
-1
Na
F
The formula unit is NaF
Examples on Writing formula unit of
binary ionic compounds
Calcium oxide
1
1
2
2
Ca
O
The formula unit is CaO
Examples on Writing formula unit of
binary ionic compounds
Aluminum sulfide
3
2
Al
S
The formula unit is Al2S3
Some ions of elements have more than one oxidation
number (like the transition elements). The oxidation
number of such elements is always positive
When such elements form ionic compound the
oxidation number is represented by a roman numeral
following the element’s name.
For example:
• Fe with oxidation number +2 or Fe2+ is written iron(II)
• Fe with oxidation number +3 or Fe3+ is written iron(III)
• Copper(I) is Cu with oxidation number +1 or Cu1+
• Copper(II) is Cu with oxidation number +2 or Cu2+
Remember:
Numbers
1
2
3
4
5
Roman
symbol
I
II
III
IV
V
6
7
VI VII
Examples of naming binary ionic compounds
(when positive ion has more than one
oxidation number)
FeO
Fe is iron and it has more than one oxidation
number. Assume the oxidation number is x.
O is oxygen and its oxidation number is -2.
So
1(x) + 1(-2) = 0
x = +2
(neutral compound)
So the name of the compound is iron(II) oxide
Examples of naming binary ionic compounds
(when positive ion has more than one
oxidation number)
Fe2O3
Fe is iron and it has more than one oxidation
number. Assume the oxidation number is x.
O is oxygen and its oxidation number is -2.
So
2(x) + 3(-2) = 0
x = +3
(neutral compound)
So the name of the compound is iron(III) oxide
Examples of naming binary ionic compounds
(when positive ion has more than one
oxidation number)
Ni3N4
Ni is nickel and it has more than one oxidation
number. Assume the oxidation number is x.
N is nitrogen and its oxidation number is -3.
So
3(x) + 4(-3) = 0
x = +4
(neutral compound)
So the name of the compound is nickel(IV) nitride
Examples on Writing formula unit of
binary ionic compounds (with variable
oxidation number)
Copper(I) sulfide
1
2
Cu
S
The formula unit is Cu2S
Examples on Writing formula unit of
binary ionic compounds (with variable
oxidation number)
Copper(II) sulfide
1
1
2
2
Cu
S
The formula unit is CuS
Examples on Writing formula unit of
binary ionic compounds (with variable
oxidation number)
Nickel(III) Phosphide
1
1
3
3
Ni
P
The formula unit is NiP
Examples on Writing formula unit of
binary ionic compounds (with variable
oxidation number)
Cobalt(III) oxide
3
2
Co
O
The formula unit is Co2O3
Complex ionic compounds
Some ionic compounds are composed of an ion of an
element and a polyatomic ion.
A polyatomic ion is A group of covalently bonded
atoms that acts together as one charged atom
(ion).
Examples of polyatomic ions:
ammonium
nitrite
sulfite
+
NH4
NO2
2SO3
carbonate
2CO3
phosphate
3PO4
nitrate NO3
sulfate SO42-
hydrogen carbonate
HCO3
Examples on naming complex ionic
compounds
Name the compound (NH4)2S?
Solution:
NH4 is ammonium and has oxidation
number Of +1
S is sulfur with oxidation number -2
So the name of the compound is
ammonium sulfide
Examples on naming complex ionic
compounds
Name the compound CaCO3?
Solution:
Ca is calcium and has oxidation number
Of +2
CO3 is carbonate with oxidation number
-2
So the name of the compound is calcium
carbonate
Examples on Writing formula unit of
complex ionic compounds
Write the formula of Aluminum hydroxide?
3
1
Al
OH
The formula unit is Al(OH)3
Examples on Writing formula unit of
complex ionic compounds
Write the formula of Magnesium
phosphate?
2
3
Mg
PO4
The formula unit is Mg3(PO4)2
Examples on Writing formula unit of
complex ionic compounds
Write the formula of Beryllium sulfate?
1
1
2
2
Be
SO4
The formula unit is BeSO4
Hydrates
A hydrate is an ionic compound that has
water chemically attached to its ions.
If the water was removed from the hydrate
it becomes anhydrous
Hydrate
CuSO4.5H2O
Anhydrous
CuSO4
Copper(II) sulfate pentahydrate Copper(II) sulfate
blue crystals
white crystals
Hydrate
CoCl2.6H2O
Anhydrous
CoCl2
Cobalt(II) chloride hexahydrate Cobalt(II) chloride
Pink crystals
blue crystals
Some ionic compounds can absorb water from air to
become hydrates. These are called hygroscopic
substances (example: Na2CO3)
If a hygroscopic absorbs enough water from air to
become a liquid solution then it is called
deliquescent. (Example: NaOH)
Hygroscopic substances are used as drying agents
or desiccants.
Naming and writing formulas of
hydrates
To name or write the
formula of a hydrate
you have to be familiar
with the Greek prefixes
of numbers.
Examples on naming hydrates
Name the compound CaSO4.2H2O?
Solution:
Ca is calcium with oxidation number
+2 and
SO4 is sulfate with oxidation number -2
So the name of the compound is
Calcium sulfate dihydrate
(gypsum)
Examples on naming hydrates
Name the compound NaOH.7H2O?
Solution:
Na is sodium with oxidation number +1
OH is hydroxide with oxidation number
-1
So the name of the compound is
sodium hydroxide heptahydrate
Examples on Writing formula unit of
hydrates
Write the formula of copper(II) sulfate
pentahydrate?
1
1
2
2
Cu
SO4
The formula unit is CuSO4.5H2O
Examples on Writing formula unit of
hydrates
Write the formula of ammonium sulfite
monohydrate?
1
NH4
2
SO3
The formula unit is (NH4)2SO3.H2O
Interpreting formulas
The smallest unit of an ionic compound is
called formula unit.
So the formula unit of sodium chloride is
NaCl
the formula unit of calcium sulfate is
CaSO4
When we write 3CaSO4, it means we have 3
formula units of calcium sulfate.
What does a formula of a compound tell us?
It tells us the elements found in the
compound and the exact number of atoms
of each compound.
Example:
What information does the formula of Al2O3
tell you about the number of atoms of each
element that are present?
It tells us that there are 2 atoms of
aluminum and 3 atoms of oxygen
Example:
What information does the formula of
Mg(OH)2 tell you about the number of
atoms of each element that are present?
It tells us that there are 1 atom of
magnesium, 2 atoms of oxygen and 2
atoms of hydrogen
Example:
What information does the formula of
2Ca(NO3)2.2H2O tell you about the number
of atoms of each element that are present?
It tells us that there are 2 atoms of
calcium, 4 atoms of nitrogen, 16 atoms of
oxygen and 8 atoms of hydrogen
Properties of Ionic Compounds
 Composed of oppositely charged ions.




Crystalline solids at room temperature.
Hard and brittle.
High melting point.
Dissolve in water.
 In solid state they don’t conduct
electricity, but their liquid state and
solutions in water conduct electricity