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Introduction to Chemical
Compounds
Why do elements combine?
So Far We’ve Learned:
That atoms are made up of Protons, Neutrons,
and Electrons
The identity of an atom is determined by the
number of protons in the nucleus
BUT, compounds form due to the interactions
between electrons
How are Electrons
Involved?
•I.
Oxidation Numbers
Using the Periodic Table, we can predict an
element’s oxidation number.
“Oxidation Number” means the charge of an ion
(can be + or -), a particle which has gained or
lost electrons.
A (-) charge = gained electrons
A (+) charge = lost electrons
General Rules:
Metals will ALWAYS form + ions
Non-metals will ALWAYS form – ions
Chemical compounds form from the interaction
between oppositely charged ions:
•
Na+1 + Cl-1 -------> NaCl
•
sodium ion
+ chloride ion
---->
sodium chloride
BIG IDEA:
Every element is trying to rearrange its
electrons (by gaining or losing them) in order
to “look like” a noble gas element.
This is called achieving a “Noble Gas
Configuration. (Full outer electron shell)
Atoms do this by getting (usually) 8 electrons
in their outer electron shell (Octet Rule)
Predicting Oxidation Number by Position
When Ions Combine, the
Compound MUST be
Neutral
All (+) charges must be cancelled by (-)
charges
Need to combine ions in smallest proportion
that will cancel charges.
Try These:
Li and Cl
Mg and Cl
Al and Cl
Li and O
ANSWERS:
LiCl
•
Li+1 + Cl-1
MgCl2
•
Mg+2 + 2Cl-1
AlCl3
•
Al+3 + 3Cl-1
Li2O
Anatomy of a Chemical
Formula
1. Symbols for the elements in the compound
2. Numbers called subscripts that indicate how
many atoms of an element are present:
•
AlCl3 = 1 Al and 3 Cl
•
This should makes sense because Al has a +3
charge and Cl has a -1 charge
3. The subscript is ONLY associated with the
element symbol to the immediate left.
TRY THESE:
Roman numerals indicate value of positive charge
Polyatomic Ions
“Poly-” means many
Polyatomic ions are charged particles that
consist of more than one atom
These ions can combine just like ions formed
from single elements
Common Polyatomic Ions
Just One Catch!
If more than one polyatomic ion is needed, the
formula for the compound will need to have
parenthesis around this ion
Example: Mg+2 + PO4-3 ----> Mg3(PO4)2
Polyatomic Ions May be
Used Like Other Ions
Just like Sodium (Na+) can combine with
chloride (Cl-1), to make NaCl
A polyatomic ion called ammonium (NH4+1)
can combine with chloride (Cl-1) to make
NH4Cl
The N and 4 H’s that make up NH4 ,
collectively loose 1 electron to become NH4+1
Ion Cut-Out Lab
Goal: Make Compounds
Using Paper Models of Ions
KBr
Postassium Bromide
K3PO4
Postassium Phosphate
Goal for today: Complete as much of chart
as possible. Do NOT Name compounds.
Naming Ionic
Compounds
1. Determine the ions that make up the compound.
(Use Periodic Table or “Common Ions Chart”)
2. Write the name of the POSITIVE ion first
3. Write the name of the NEGATIVE ion second.
(Sometimes the name of this ion will change slightly)
Negative Ion Names
Name ending
changes to
“-ide”
Non-metal elements in columns 14-17 will form
negative ions with names ending in “-ide”.
Examples:
•Chlorine
becomes chloride
•Fluorine
becomes fluoride
•Oxygen
becomes oxide
•Phosphorous
•Nitrogen
becomes phosphide
becomes nitride
If polyatomic ions are
used...
Naming is easy!
Just put the names of the ions together
(positive first, negative second)
No Name changes are needed
•
Example:
•
Mg3(PO4)2 is named Magnesium phosphate
Positive ion
Negative ion
Transition Metals
If the positive ion is a transition metal,
the name of the compound MUST include
a Roman Numeral!
Here’s an example: There are two “iron chlorides”:
FeCl2 is iron (II) chloride
FeCl3 is iron (III) chloride
The Roman Numeral indicates the charge of the metal ion