Metals tend to have one, two or three electrons in
their outer orbits and they tend to lose these
electrons when they combine with other elements to
form compounds.
Non-metals tend to have five, six or seven electrons
in their outer orbits so they gain electrons when they
combine with metals to form compounds.
IONIC COMPOUNDS – When atoms lose or gain
electrons to become stable, they form cations or
anions. Cations and anions are oppositely charged
and are attracted to each other – this explains why
some compounds are formed.
These compounds are relatively harmless
compounds because the elements in the compound
are stable. These compounds are called ionic
compounds which are compounds made up of
positive and negative ions that have resulted from
the transfer of electrons from a metal to a nonmetal.
Sodium chloride (table salt) is an ionic compound
made up of sodium ions, Na+, and chloride ions, Cl-.
For ionic compounds that contain only two
elements, one element is always a metal and the
other is a non-metal.
The attraction that holds oppositely charged ions
together in a compound is called an ionic bond.
Aluminum and Chlorine
gas combine to form an
ionic compound called
aluminum chloride. Each
aluminum atom has three
electrons to lose, while
each chlorine atom can
accommodate only one
extra electron. This
means that aluminum
reacts with three chlorine
atoms.
Draw a diagram to show the transfer of electrons
from calcium to fluorine.
Draw a diagram to show the transfer of electrons
from calcium to fluorine.
F 1-
F 1Ca2+
Calcium and Fluorine combine to form calcium
fluoride, an ionic compound. Each calcium atom
has 2 electrons in its valence shell so it wants to
lose both to become stable. Fluorine has 7
electrons in its valence shell so it wants to gain one
electron to become stable. Therefore, one Calcium
atom needs two fluorine atoms to form compound
Calcium Fluoride.
Ionic compounds are compounds made up of one
metal and one non-metal. The elements by
themselves have ionic charges but they come
together to make a neutral compound
(positive charge = negative charge).
Naming Ionic Compounds
Names of ionic compounds have two parts. The
first part refers to the metal ion in the compound
and the second part to the non-metal ion.
Remember that the name of the metal ion remains
the same as the metal atom but the ending of the
name of the second ion (non-metal) changes to
“ide”.
Group 1 = 1 valence electron - forms 1+ ions
Group 2 = 2 valence electrons - forms 2+ ions
Group 13 = 3 valence electrons - forms 3+ ions
Group 14 = 4 valence electrons - forms 4+ ions
But Group 14 is special since it is just as easy to add 4 electrons
and achieve a full valence shell (depending on certain conditions).
So,
Group 14 =
Group 15 =
Group 16 =
Group 17 =
Group 18 =
4 valence electrons- forms 4- ions
5 valence electrons- forms 3- ions
6 valence electrons- forms 2- ions
7 valence electrons- forms 1- ions
8 valence electrons - form NO IONS!!
We can use these valences values in writing
chemical formulas.
Writing Chemical Formulas of Ionic
Compounds
Valence is important in writing the chemical formulas
of ionic compounds. The simplest way to tell what
the valence value of an element will be is to look at
the periodic table.
Writing Chemical Formulas of Ionic
Compounds
“The Zero-Sum Rule” – The sum of all charges in the chemical
formula of the compound must equal zero.
Example 1: Write the chemical formula of magnesium chloride.
Step 1:
Write the symbols of the elements, with the metal on
the left side and the non-metal on the right side.
Mg
Step 2:
Cl
Add the ionic charge of each ion above the symbol.
+2
Mg
-1
Cl
Step 3:
Determine how many ions of each type are required
to bring the total charge to zero. The sum of all
charges in the compound must equal zero.
Total charge:
Step 4:
1(+2) + 2(-1) = 0
Mg
Cl
Write the chemical formula using the (bold)
coefficients in front of each bracket as subscripts.
Mg1Cl2
Step 5:
Do not write the subscript “1” in chemical formulas
because the symbol itself represents on ion.
The chemical formula for magnesium chloride is MgCl2.
Example 2: Write the chemical formula of aluminum oxide.
Step 1:
Write the symbols of the metal and non-metal
elements.
Al
Step 2:
Add the ionic charge of each ion above the symbol.
+3
Al
Step 3:
-2
O
Determine the number of ions required to bring the
total charge to zero.
2(+3)
Al
Step 4:
O
3(-2) = 0
O
The chemical formula of aluminum oxide is
Al2O3.
The Crisscross Method for Determining
Formulas
Step 1:
Write out the chemical symbols for each elements
involved in the ionic compound. Write the metal
(cation) first, followed by the non-metal (anion).
Magnesium
Mg
Step 2:
Chlorine
Cl
Write the ionic charge (valence) above each symbol
based on which group they fall under in the periodic
table.
Mg
2+
1
Cl
12
Notice that there
are no longer
Step 3: Crossover the numbers, drop the signs, and write them as subscripts.
We do not show the 1 because the symbol itself Mg represents one
element. We no longer show a + or – because when the charged ions
bond together their respective valence shell is full and the compound
becomes neutral.
MgCl2
Step 4: Look for the common factor between both subscripts. It one exists,
decrease each subscript by this factor. If not, leave the formula.
MgCl2 (No common factor exists here)
Step 5: Name the compound by changing the ending of the non- metal element
from –ine / -gen to –ide.
MgCl2 = magnesium chloride
Elements with Multiple Ionic Charges
We have a special set of elements that are multivalent (multi =
many, valent = valences). They are located in the Transition
Metals (centre of the periodic table).
Examples are:
Copper
Iron
Lead
Nickel
Manganese
Tin
Gold
Example 1: Write the chemical name of CuBr2.
Step 1: Since we do not know whether the copper ion is +1 or +2,
we reverse our criss-cross to see what the charge will be.
2
Cu
1
1
Br
Reverse the criss-cross
2
Therefore, the ionic charge of copper in this compound is +2.
Step 2: Write the name of the compound. When the metal has
more than one ionic charge you must always indicate the ionic
charge with Roman numerals.
The name of CuBr2 is copper (II) bromide.
The following are some other examples:
Copper can have multiple ionic charges. What are the
possibilities? You can find these charges in your periodic table.
Copper can have either a charge of 1+ or 2+. So for example,
when copper forms a compound with chlorine the chemical name
and chemical formula will be the following:
Copper (I) chloride (CuCl)
Copper (II) chloride (CuCl2)
Iron can have multiple ionic charges. What are the possibilities?
Iron can have a charge of 2+ or 3+. So for example, when iron
forms a compound with oxygen the chemical name and chemical
formula will be the following:
Iron (II) oxide (FeO)
Iron (III) oxide (Fe2O3)