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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 compoundsCovalent (molecular)
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
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.
• The charge on the ion is called oxidation number.
Single atom ions Polyatomic ions Na+ Cl- Mg2+
Ca2+ S2- O2-
B3+ N3- Li+
P3- Al3+ I-
NH4+ HCO3
-
CO32- C2H3O2
-
PO43- H3O
+
• 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 gains 2 electron and becomes O2- 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 gains 3 electron and becomes N3- 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 6 7
Roman symbol I II III IV V 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 (neutral compound) x = +2
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 (neutral compound) x = +3
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 (neutral compound) x = +4
So the name of the compound is nickel(IV) nitride
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
Examples on Writing formula unit of binary ionic compounds (with variable oxidation number)
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 NH4 phosphate PO4
nitrite NO2 nitrate NO3
sulfite SO3 sulfate SO4
carbonate CO3 hydrogen carbonate HCO3
+ 3-
- -
2- 2-
2- -
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 AnhydrousCuSO4.5H2O
Copper(II) sulfate pentahydrate
blue crystals
CuSO4
Copper(II) sulfate
white crystals
Hydrate AnhydrousCoCl2.6H2O
Cobalt(II) chloride hexahydrate
Pink crystals
CoCl2
Cobalt(II) chloride
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 2
NH4 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
