Writing Formulas for Compounds 2016 A.notebook
January 11, 2017
Unit 7: Formula's and Equations
NaCl
Writing Formulas
Aim: Writing a chemical formula using the compound
neutrality rule.
Jan 22­12:35 PM
Ionic Compound: a compound formed from two or more ions attracting together after transferring electrons.
Compound Neutrality Rule: All ionic compounds are electrically neutral due to an even transfer of electrons from the cations to the anions and the charges of the opposite ions CANCEL OUT.
Ex: Magnesium fluoride
*show transfer and charges*
Jan 22­12:35 PM
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Writing Formulas for Compounds 2016 A.notebook
January 11, 2017
Types of Compounds
Polyatomic Ion: two or more
nonmetal atoms combined to
form a molecule that is an ion
with an overall charge.
SO4
Binary com
Ternary co
-2
SO4‐2
Formula= SO 4
Charge= ‐2
Jan 23­12:44 PM
What's in a name?
Aluminum oxide; Al2O3
+3
-2
Al O
+3
-2
Al O
The first ion is a positive ion
and always written first. It can
be a metal or polyatomic ion.
-2
O
Aluminum sulfate; Al2(SO4)3 The second ion is a negative ion
and always written second. It
+3
-2
Al SO4
can be a nonmetal ion or
+3
-2
Al SO4
polyatomic ion.
-2
SO4
Jan 22­12:35 PM
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Writing Formulas for Compounds 2016 A.notebook
January 11, 2017
To write a formula:
1. Write the symbols and oxidation states.
a. the first symbol is always a metal or polyatomic ion (Table E)
b. the second symbol is in the P.T. or Table E (polyatomic ion).
•ends in "ide", look on the periodic table and find what element it sounds like, oxide= oxygen. Exceptions= hydroxide, cyanide and peroxide are on Table E.
LCF •ends in "‐ate", "‐ite" or "‐ium" look on Table E.
c. A roman numeral in the name tells you the oxidation state of the 2x3= 6
metal ion with more than one possible charge.
+2
A) Gallium sulfide
+3
LCF
2x3= 6
B) Iron (II) Phosphate
-2
Ga S
+3
-2
Ga S
+2
Al2S3
-3
Fe SO4
+2
-3
Fe SO4
+2
Fe
LCF
2x3= 6
-2
S
+6 + -6 = 0
+3
Fe
+6 + -6 =
Fe3(SO4)2
• You must use parenthesis around more than 1 0 polyatomic ion.
• Make sure the number of cations and anions balance out so the total charge is zero. (Use the LCF= least common factor)
• You want the lowest number of each ion possible.
Jan 22­12:35 PM
Examples
3) Barium phosphate
+2
-3
Ba3(PO4)2
Ba PO4
+2
-3
Ba PO4
+2
Ba
+6 + -6 = 0
4) Sodium sulfide
+
-2
Na2S
Na S
+
Na
+2 + -2 = 0
+2
+3
+6
5) Chromium (VI) chloride Cr
+6
Cr Cl Cl Cl- Cl- Cl- Cl-
CrCl6
+6 + -6 = 0
Jan 22­12:35 PM
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