Science 10 Unit 2
YAY, WE’RE DONE 4.1 !
• GREAT JOB: Now you understand how to draw models of ionic and
covalent compounds!! (Right? )
• WHAT’S NEXT??
• Now you will learn how to properly write the chemical formula of ionic and
covalent compounds
(e.g., H20 is the chemical formula for water)
• You will also learn how to properly name ionic and covalent compounds
(e.g., magnesium phosphide is the name of the ionic compound formed from
magnesium and phosphorus)
• Writing the name and chemical formula for a compound is done differently if
it’s an ionic or covalent compound.
(Can’t chemistry ever be simple?)
• Today, we will focus on the method for ionic compounds;
next class, we’ll cover the method for covalent compounds.
SKILL 1: Writing the chemical formula for
an ionic compound
KEY IDEA: In an ionic compound, the positive charges must
balance the negative charges.
E.g.: What is the chemical formula for calcium oxide?
• “oxide” means oxygen, so there are calcium and oxygen in the compound
• look up the ions that calcium and oxygen typically form in an ionic
compound on your Periodic Table …
Calcium becomes Ca2+
Oxygen becomes O2-
• Notice the charges balance if there is one Ca2+ ion and one O2– ion,
so the chemical formula is written as:
Ca1O1
Wrong!!
(actually, the subscript 1 is
never written…)
CaO
Right!
But what if one ion of each do not balance each other out?
E.g.: What is the formula for magnesium phosphide?
• “phosphide” means phosphorus, so look up the ions that magnesium and
phosphorus form…
Magnesium is Mg2+
Phosphorous is P3-
• Notice one ion of each will NOT balance the charges!
• Can you figure out how many Mg ions and P ions will be needed to
exactly balance each other out??
Lowest common multiple of 2 and 3 is 6
 3 Mg2+ ions needed and 2 P3– ions
needed to balance charges
And the chemical formula is…
Mg3P2
Mg2+
P3-
Mg2+
P3-
Mg2+
6+
Mg3P2
6-
The crystal lattice formed has 2 phosphide ions for every 3 magnesium
ions, so charges are balanced!
YOU TRY: What is the formula for aluminum fluoride?
Aluminum is Al3+
Fluorine is F -
To balance charges, 3 Fluoride ions are needed
for every 1 Aluminum ion…
AlF3
** Notice that the metal is always written first!!
(Useful, but unnecessary if you understand that + and - charges must balance!)
Lead sulphide
1. Write both symbols
(+ion first) with ion charge
2. Criss-cross the charges
and write them as subscripts
Pb
4+
S
Pb 2 S 4
3. Reduce if possible
Pb1S2
4. Remember that the subscript
1 is never written
PbS2
2-
In the shortcut example, you may have been confused because lead has more
than one combining capacity
(i.e., more than one ion charge listed on the Periodic Table)
e.g.
Pb4+ Pb2+
Cu2+ Cu1+
If this is the case, the name of the compound will always
include a roman numeral to tell you which combining capacity
to use…
1+
I
5+
V
2+
II
6+
VI
3+
III
7+
VII
4+
IV
e.g.
Iron II oxide
Iron III oxide
Fe 2+
Fe 3+
FeO
O 2-
O 2-
Fe2O3
So Iron &
Oxygen
can form
two
different
compounds
together!
PRACTICE!
Try Practice Problems:
p.188 #1 a-f & #2 a-f
p.190 #1 a-f
(It says in the text where to find the
answers, so PLEASE check your work as
you go and get help if needed!!)
SKILL 2: Writing the name of an ionic compound
KEY IDEA: Metal + Non-metal (with “-ide” ending)
1. Write the name of the metal first.
2. Write the name of the non-metal second,
and change the ending to “-ide”
3. ONLY IF THE METAL HAS 2 OR MORE ION CHARGES,
include a roman numeral to indicate which one it is.
e.g.
NaCl
Sodium chloride
MgF2
Magnesium fluoride
CuCl2
copper has 2 ions:
which one is it?
Cu 1+
Cu 2+
To find out which copper ion it is, look at the charge on
the anion and balance the charges in reverse.
CuCl2
Cu 2+
Cl 1Cl 12-
from periodic table
Therefore it is copper II chloride
(Can also be written as copper (II) chloride)
YOU TRY: What is the name for Al2S3?
Aluminum is NOT multivalent, so we don’t have
to worry about Roman numerals
Aluminum Sulphide
YOU TRY: What is the name for Au3N?
Gold is multivalent, so we must worry about
Roman numerals!!
Gold has 2 ions:
Nitrogen is: N 3To balance charges, which
Au ion must it be?
Au3+
Au+
Gold (I) Nitride
PRACTICE!
Try Practice Problems:
• p.187 #1 a-f
• p.191 #1 a-j (the multivalent type!)
Again, check your answers as you go!
ACK! Can you handle ONE MORE complication?
A polyatomic ion is a group of atoms that together act as an ion.
(Think about what that means.)
The group of atoms is given a name.
Ion Charge
e.g.
SO4-2
OH-1
PO4-3
Sulphate ion
Hydroxide ion
Phosphate ion
-2
-1
-3
Good news: All the polyatomic ions you need to know, and their charges, are
included in your Data Pages! Have a look! Can you find the 3 examples?
KEY IDEA: When you see a polyatomic ion, treat it as if it is
just one atom and follow the rules as usual.
e.g.
K +1
Formula:
PO4 -3
K3PO4
Name: Potassium phosphate
Important note:
Don’t change the
endings of
polyatomic ions!
YOU TRY: If the ions below form an ionic bond, write the
formula and name.
Al
+3
SO4
-2
formula
name
Al2(SO4)3
Aluminum sulphate
Brackets are necessary around the
SO4 to indicate 3 sulphate ions, not
43 oxygen atoms!
Cu
+2
SO4
-2
formula
name
CuSO4
Copper (II) sulphate
Don’t forget the roman numeral
since copper has more than one
combining capacity
PRACTICE!
Try Practice Problems:
p.193 #1 a-e & #2 a-e
(Check answers!!)
Homework
If you feel shaky on this stuff, give yourself some more time to
pick it up. You can do it! I suggest you read through text
p.184-191. Each time you hit a set of “Practice Problems,”
try doing some more of them and checking your answers. I
suggest this for everyone but especially those feeling a little
overwhelmed.
Once you feel comfortable with the material, then:
1. Read and highlight workbook p.65-66
2. Complete workbook p.68-69


Be ready to have the workbook marked next class along with the
workbook from last day.
Don’t let yourself get behind here. We’re plowing forward next class and
will be writing a test before the break!!