1. Writing Ionic Formulas

2. What is a binary compound?
Binary compound – a compound that is composed of 2 elements
NaCl LiCl NaF Li2O
Oxidation # - tells you how many electrons an atom has gained or lost to become stable.
(It is the charge on an ion!)
K Mg O 2- Cl -

3. Oxidation Numbers Reviewed
3 -

4. Reminder: When writing chemical formulas, the individual ions in a compound carry charges, but the overall compound is neutral!

5. How to Write a Chemical Formula
Determine the oxidation #s (use PT)
Write the cation and its oxidation # first
Write the anion and its oxidation # next
Crisscross the numbers only and write the oxidation # as the subscript
Reduce if necessary
Check by adding charges to equal 0

6. What is the formula for Potassium Sulfide

7. Polyatomic Ions Polyatomic Ions = ions that contain more than 1 atom
Ammonium
NH4+
Acetate
C2H3O2-
Chlorate
ClO3-
Nitrate
NO3-
Hydroxide
OH-
Carbonate
CO32-
Sulfate
SO42-
Phosphate
PO43-

8. Using Polyatomic Ions
Polyatomic ions are treated the same way as binary compounds, but…
If there is a subscript at the end of the polyatomic ion and there is a crossed subscript, the ion is placed in parentheses
Ex: Magnesium and Phosphate
Mg3(PO4)2

9. Example Problems Copper (II) Phosphite Lithium Hydroxide
Sodium Carbonate
Magnesium Fluoride

10. Naming from Formulas
Write the name of cation first (use the same name as the periodic table or polyatomic ion!)
Write the name of the anion second
If the anion is a single element, change the ending to –ide
If the anion is polyatomic, use the polyatomic name (don’t change it!)

11. Examples
1. NaCl =
2. MgS =
3. Rb2SO4 =
4. NH4NO3 =

12. Writing Covalent Formulas
Both elements/ions are anions (negative), so there is NO crossing of charges!
The subscripts in the formula refer to the number of atoms used to make the molecule.
Memorize the following prefixes:
Mono- 4. Tetra- 7. Hepta-
Di Penta- 8. Octa-
Tri- 6. Hexa- 9. Nona-
10. Deca-

13. Practice Write the formula for the following molecules:
diphosphorus pentachloride =
Carbon monoxide =
Trinitrogen tetrachloride =

14. Rules for Naming Molecular Compounds
When writing the names for the compounds (when given a formula), use the subscripts to determine prefixes.
Do NOT cross/switch the subscript/prefix.
Do NOT use the prefix for 1 (mono) with the first ion. Only use it for the second ion.
Remember to change the second ion name’s ending to -ide

15. Practice Write the name of the molecule from its formula: CCl4 = SO3 =
H2O =