Chapter 20 Section 3
Writing Formulas and Naming Compounds
Compound Names and Formulas
• Compounds have specific names and
formulas so you can tell how many and what
atoms are in the compound/molecule.
• For example, the formula for Carbon Dioxide
is CO2. This tells us that Carbon Dioxide has
one atom of carbon and two atoms of
oxygen.
Important Vocabulary
• Binary compound: a compound composed of 2 elements. They can be
covalent or ionic.
• Oxidation number: tells you how many electrons an atom has gained,
lost, or shared to become stable.
• Polyatomic ion: a positively or negatively charged, covalently bonded
group of atoms.
• Cation: a positively charged ion.
• Anion: a negatively charged ion.
Naming Ionic Compounds
• Cations: Use the element’s name.
• Anion: Add “ide” to the end of the element’s name.
Cation is
ALWAYS
FIRST
Sodium Chloride
Anion is
ALWAYS
LAST
Ionic compounds MUST have a total charge of zero, meaning that all
negative and positive charges cancel each other out!!
Na+1 + Cl-1 = NaCl = sodium chloride = all charges are cancelled out!
Ca+2 + F-1 + F-1 = CaF2 = calcium fluoride = all charges are cancelled out!
Would a transition metal forming a compound
with a nonmetal still be considered an ionic
compound?
• YES! You are still dealing with a metal and a nonmetal forming a
compound. The rules for naming these and writing the formulas are a
little different though….
Oxidation numbers are important! You can
kind of think of think of them like charges.
Aluminum will always
have an oxidation
number of +3.
Al+3
Zinc will always have
an oxidation number
of +2.
Zn+2
Silver will always have an oxidation
number of +1.
Ag+1
Transition Metal Naming
• Transition metals show their charges as Roman
Numerals because their charges don’t follow a set
pattern and they often change.
FeO = Fe+2 + O-2
Iron (II) Oxide
Fe2O3 = Fe+3 + O-2
Iron (III) Oxide
Practice Makes Perfect!
• What is the formula for Lithium Oxide?
• Li2O
• What is the name of this compound: CaBr2 ?
• Calcium Bromide
• What is the formula for Iron (III) Sulfide?
• Fe2S3
• What is the name of this compound: CuCl2 ?
• Copper (II) Chloride
Complex compounds
• Compounds with polyatomic ions are named ALMOST EXACTLY the
same way that ionic compounds are.
• Cations: Use the element’s name.
• Anions: Use the polyatomic ion’s name.
***The exception to the rule would be if your cation happens to be
a polyatomic ion. There are only two that we’ll use: ammonium and
dimercury. If the cation bonds to an element and not another
polyatomic ion, drop the ending of the element and add “ide.”
ALL CHARGES MUST CANCEL!
USE PARENTHESES WHEN YOU NEED MORE THAN ONE OF A
PARTICULAR POLYATOMIC ION!
More Practice…
• What is the formula for Potassium Carbonate?
• K2CO3
• What is the name of this compound: NH4CH3COO ?
• Ammonium acetate
• What is the formula for Magnesium Nitrate?
• Mg(NO3)2
Covalent Naming
• To name covalent compounds, you must put a prefix in front of
each element’s name. Add “ide” to the last element’s name.
1.mono-
6. hexa-
2.di-
7. hepta-
3.tri-
8. octa-
4.tetra-
9. nona-
5.penta-
10. deca-
***Often, the prefix “mono” is omitted, but is
sometimes used for emphasis.
Practice, Practice, Practice…
• What is the formula for Dihydrogen Monoxide?
• H2O
• What is the name of this compound: H2S ?
• Dihydrogen Sulfide (or monosulfide)