Chemistry
Ionic Compounds
Ionic Compounds
Recall Ions
• “+” on the left (metals) because
they lose electrons (become
less negative)
• “-” on the right (non-metals)
because they gain electrons
(become more negative)
• What is the total charge if you
have X+1 and Y-1
- (+1) + (-1) = 0, therefore the
total charge is neutral or 0
Ionic Compounds
Ionic Compounds
• Metal and a non-metal combine to form "Ionic
Compounds".
• The metal atoms loses electrons to form positive ions.
• The non-metal gains the electrons to form negative ions.
• The two ions join together
• The result is an electrically neutral ionic compound.
• Ion + Ion = Ionic Compound
Ionic Compounds
Ionic Compounds
• Ionic compounds are formed when a
metal gives one or more of its
electrons to a non metal.
• When all of the electrons are gone,
the metal’s inner orbit of electrons is
full. So, the metal is stable.
• The Lewis diagram is just the
symbol and the charge
• The atom has lost electrons so it
takes on a positive charge
Ca
Ca+2
Ionic Compounds
Ionic Compounds
• The non-metal receives one or
more electrons to fill its outer
shell and become stable.
• If we draw the Lewis Structure for
a non metal we include the full
valence orbit, brackets and a
charge
• Having gained two electrons, the
Oxygen is now negative.
• The resulting positive metal
and negative nonmetal stick
together by electrostatic
attraction.
O
-2
O
Ionic
Compounds
Ionic Compounds
• Putting the compound
together, we can see that the
overall molecule has a neutral
charge (no charge).
-2
Ca+2
• This particular example is
simple because the Calcium
and Oxygen want to
exchange the same number
of electrons.
• What would happen if they
wanted to exchange different
numbers of electrons?
O
+2 – 2 = 0
Try it! Li and O
Using Lewis Structures:
Ionic bonding between Lithium and Oxygen
1. First draw the Lewis
structures of the atoms on
their own.
2. Now use an arrow to show
the electron transferring
from the Lithium atom to
the Oxygen atom
3. You can see that the
Oxygen atom still does not
have a complete outer
shell. Clearly another
Lithium atom is required to
complete the compound.
Li
O
Li
Using
Lithium Oxide continued
Lewis Structures:
Ionic bonding between Lithium and Oxygen
• The Lithium atoms, having
lost one electron each have
a charge of +1.
Li+
• The Oxygen atom, having
gained two electrons, has a
charge of -2.
-2
Li+
O
• The final Lewis structure is
illustrated here.
• The compound is Li2O.
Gizmo Demo
Illustrate the bonding between Aluminum and Oxygen.
(Note the ratio here is a little harder to find.)
• Draw the Lewis Structures of the
individual atoms.
• Use arrows to show the transfer of
electrons.
• Aluminum has one electron left,
we need another oxygen atom.
• The oxygen atom now needs
more electrons so we need
another aluminum atom.
• The extra electrons on the
Aluminum mean we need another
Oxygen.
• Since, we have been able to make
both atoms stable, we have hit the
correct ratio.
Al
O
Al
O
O
Gizmo Demo
Aluminum Oxide Continued
Aluminum and Oxygen
• Having lost 3 electrons, the
aluminum has a charge of +3.
• Having gained 2 electrons,
the oxygen has a charge of 2.
-2
-2
O
O
Al+3
• Now draw the final Lewis
Structure.
• This structure is designed to
illustrate the transfer of
electrons while bonding.
Al+3
-2
O
Naming Ionic Compounds
Naming Ionic Compounds
1. Write the name of the metal
2. Write the name of the non-metal changing
the end of the non metal name to – ide.
Examples – Sodium Chloride, Calcium Oxide
Writing the formulas for
Ionic Compounds
Writing the Chemical Formula
•
•
The trick to finding the ratio of atoms in the
molecule.
The easiest way is to find the number of
BONDS the ion will make is to again refer to
your period table – as we did last day
Writing the Chemical Formula
Writing the formulas for
Ionic Compounds
1. Number the groups 1-3 going from left to right (skip the
Transition Metals)
2. Number the groups 1-3 going from right to left (skip the
Nobel Gases)
1
2
3
3 2
1
Writing the Chemical Formula
•
•
Writing the formulas for
Ionic Compounds
Those numbers represent the charge the ion
will have
It also tells us the number of bonds it will make
+1
+2
+3
-3 -2 -1
The
Criss-Cross
Rule
Then …
Write out the atoms with their stable charge
Al+3O-2
Cross over the numbers to the opposite atom and
remove the sign – This is the Criss-Cross Rule
Al2O3
If possible reduce to lowest terms.
The Criss-Cross Rule
Example: Aluminum Chloride
Step 1: Aluminum
Chloride
write out name with space
Al+3
Step 2:
Cl-1
write symbols & valence number
Step 3:
Al1
Cl 3
criss-cross valence number as subscripts
Step 4:
combine as formula unit
(“1” is never shown)
- Reduce if possible
AlCl3
The Criss-Cross Rule
Example: Magnesium Oxide
Step 1: Magnesium
Oxide
Step 2:
Mg+2
O-2
Step 3:
Mg 2
O2
Step 4:
Mg2O2
Step 5:
MgO
Multivalent Compounds
• These will include the
transition metals
• When atoms get larger, their
atomic structure can become
more complex.
• This means that some atoms
can have different numbers of
valence electrons in different
situations.
Cu+1
• For example if we look at the
Lewis structure of Copper. It
can have one valence
electron or two valence
electrons.
Cu
Cu
• Clearly, this atom can form
more than one type of
molecule.
-2
O
Cu+1
-2
Cu+2
O
Naming Multivalent Ions
• We need to distinguish between the two or three
different ion forms
• Use roman numerals from I – VII which
corresponds to 1+  7+ ion charges
• For example:
Metal Ion charge
1+
2+
3+
4+
5+
6+
7+
– Nickel can have two ion forms, Ni2+ and Ni3+
– These are named nickel (II) and Nickel (III)
Roman Numeral
I
II
III
IV
V
VI
VII
Multivalent Compounds
Naming Multivalent Compounds
• In terms of naming these
compounds, we need a method of
distinguishing,
CuO
and
Cu2O
• You need to put a roman numeral
to indicate the charge in brackets
in between the metal and non
metal.
• Remember it is the original
charge
• The roman numeral is only included
if the metal is one of the atoms that
can exhibit more than one charge.
• This information will be provided!
e.g. Try to write the name for
the following compounds
without looking at the
next slide
CuF
Copper (I) Fluoride
PbI2
Lead (II) Iodide
CaF2
Calcium Fluoride
How do we know which roman numeral is
being used?
Naming Multivalent Compounds
1) CuF
-
Do the reverse of the criss cross method
Take the subscript and use it as the charge on the opposite symbol
Put the charge for the metal in brackets
Cu 1 F1
Cu +1
F-1
 Copper (I) Flouride
2) PbI2
Pb 1 I2
 Lead (II) Iodide
Pb +2
I-1
• Silver (Ag) – always has a charge of +1
• Zinc (Zn) – always has a charge of +2
• Be careful when you are asked to determine the
roman numeral given the formula (e.g. CuS)
•This would lead you to answer: Copper (I) Sulfide
•However we ALWAYS NEED TO CHECK THAT
THE CHARGE ON THE ANION IS CORRECT
•In this case it would be -1 however, if we look at
the periodic table we know that S is -2
•Therefore we need to multiply both charges by 2
• Copper (II) Sulfide
•MAKE SURE YOU CHECK THAT THE CHARGE
ON THE NON-METAL IS CORRECT!!
Properties of Ionic Compounds
Ionic Compounds
• Many ionic compounds are soluble in water.
• When they dissolve, they separate into ions.
• Water molecules surround each ion preventing it them from
rejoining
Properties of Ionic Compounds
• They are hard, brittle solids
with high melting points
• Many are also electrolytes which
means they conduct electricity
when dissolved in water
• Pure water is a poor conductor of
electricity, dissolved ions improve
this property considerably