1. A quick memory trick...
How do you remember whether it is the metal that forms a negative ion or the non-metal???
“M” for metal = “M” for mother
A mother is always giving... Always positive
“N” is for non-metal = “N” is for negative
Negative people always take more than they should

2. Atomic Accounting
A chemical formula is a combination of symbols and letters
It will tell you how many of each atom there is in a molecule
Example:
CO2
Subscript (below writing)
- tells you how many atoms of the element before it
- this subscript 2 = 2 oxygen atoms in this molecule
Why is there no subscript after the C (carbon) atom?
- no number means You don’t have to write the
number if there is only 1

3. Atomic Accounting 3 6 What if the formula was 3CO2?
So... 3CO2 = CO2 + CO2 + CO2
Altogether there are: ___ Carbon atoms
___ Oxygen atoms
We multiplied the co-efficient by the subscript!
Carbon = 3 co-efficient x 1 subscript
Oxygen = 3 co-efficient x 2 subscript
This is called the Co-efficient
- the big number in front of all the symbols
- it tells you how many there are of the entire molecule 3 6

4. You try... How many of each atom are there? Ionic Compounds
Covalent Molecules
Formula
Number of Atoms
NaCl
Na = ____ atoms
Cl = ____ atoms
3 NaCl
CaCl2
Ca = ____ atoms
2 CaCl2
3 Na2S
S = ____ atoms
Formula
Number of Atoms
H2O
H = ____ atoms
O = ____ atoms
3 H2O
CH4
C = ____ atoms
SO2
S = ____ atoms
NO2
N = ____ atoms 1 2 1 1 3 6 3 3 1 1 2 4 1 2 4 2 6 18 3 2

5. Writing Formulas for Ionic Compounds
What if you’re given the name (in words) of an ionic compound and you are asked to come up with the formula (in symbols)?
For example magnesium phosphide = ?
In an ionic compound, the positive charges balance out the negative charges.
The ratio of positive:negative charges gives the proper formula.
The ratio is always written in the most reduced form.
For example, what is the formula for magnesium phosphide?
Must make the positive and negative charges equal so they will balance out
magnesium is Mg2+ phosphorous is P3–
Lowest common multiple of 2 and 3 is 6
3 Mg2+ ions and 2 P3– ions
(will make 6 positive charges and 6 negative charges = BALANCED!)
Mg3P2
See page 188

6. You try… What is the formula for calcium oxide?
calcium is Ca2+ oxygen is O2–
Make the positive and negative charges equal so they will balance out:
1 Ca2+ ion and 1 O2– ions
(2 positive charges and 2 negative charges = BALANCED!)
Ca1O1 – simply written as CaO
Calcium oxide, also known as “quicklime” was once produced by cooking limestone in ancient kilns.

7. Practice Problems

8. Formula of an Ionic Compound with a Multivalent Metal
Some transitional metals are multivalent, meaning they have more than one ion form.
On the periodic table, the most common form of the ion is listed on top.
In the name of the compound, Roman numerals are used following the positive ion to indicate which ion was used.
For example, what is the formula manganese (III) sulphide?
This manganese is Mn3+ sulfur is S2–
Lowest common multiple of 3 and 2 is 6
2 Mn3+ ions and 3 S2– ions
(6 positive, 6 negative = neutral charge)
Mn2S3
See pages
(c) McGraw Hill Ryerson 2007

9. You try… Try the name for TiF4 titanium (IV) fluoride
titanium is Ti4+ or Ti3+ fluorine is F–
There is 1 Ti ion and 4 F– ions
Which titanium ion did we use?
If we had used the Ti3+ then the charge would be 3-, which would not balance the 4- charge of the fluorine, so we must have used the Ti4+
titanium (IV) fluoride

10. Practice Problems