Writing Chemical Formulas and Naming Compounds
In order to successfully write a chemical formula from a name or name a compound from its formula, you must decide
which set of rules to follow. All compounds fall into one of three categories. They are: Ionic, Covalent or Acid and
different rules apply to each category. Let’s run through each. Keep in mind that every formula and every name
has two parts!
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Ionic: (metal + nonmetal)
As you have already learned, all Ionic substances start with a metal or the polyatomic ion ammonium (NH4). That is
how you can tell if the substance is ionic. The second part can be either a nonmetal or another polyatomic ion (all
listed on the back of your periodic table). Here are your rules.
Writing Formulas: (when given the Name)
1. Write the metal’s symbol 1st.
2. Get the oxidation number for the metal from the
PT or from the roman numeral in the name
(transition metals only)
3. Write the second part of the formula based on the
ending
ide – write nonmetal symbol (or
hydroxide formula)
ate/ite – write the polyatomic ion
formula (look on your reference sheet)
4. Get the oxidation number for the second part
from either the PT or from the polyatomic ion’s
charge.
5. Criss-cross oxidation # to get subscripts. (ignore
the + and -) Write polyatomic ions in
parenthesis, if it gets an additional subscript.
6. Things to watch for:
a. Zinc is always +2 and Silver is always +1
in compounds
b. Reduce subscripts to simplest form
Naming Compounds: (when given the formula)
1. Write metal name (or ammonium) as the first
part of the name
2. Look at the second part of the formula and decide
if it is a nonmetal or a polyatomic ion.
a. If it is a nonmetal, write the nonmetal
name with ide ending.
b. If it is a polyatomic ion just write the
polyatomic ion name. (look on your
sheet)
3. Decide if the metal needs a roman numeral
(transition metals)
4. Undo criss-crossing to get Roman numeral.
5. Things to watch for:
a. Zinc and Silver do not need roman
numerals.
b. Sometimes the subscripts were reduced –
keep this in mind for transition metals so
that you give the right Roman numeral.
Ex.
Ex.
K2CrO4
Calcium chloride
AlCl3
Lithium sulfate
MgSO4
Magnesium nitrate
Cu(NO3)2
Iron (III) carbonate
Covalent (Molecular): (nonmetal + nonmetal)
As you have already learned, covalent substances contain only nonmetals. Below are the rules for writing the formula
from the chemical name or naming the chemical from the formula.
Writing Formula:
1. DO NOT look at oxidation #’s!!!!!
2. For the first part of the formula write the first
nonmetal’s symbol and use the prefix to
determine its subscript
3. For the second part, write the second nonmetal’s
symbol, again using the prefix to get subscript
4. Prefix Meanings:
mono or no prefix = 1
di = 2
tri= 3
tetra = 4
penta = 5
hexa = 6
hepta = 7
octa = 8
nona = 9
deca = 10
5. Things to look for:
a. Do not reduce subscripts.
b. DO NOT CRISS-CROSS!!
c. The first word does not need a prefix if
there is only one of that element (no
mono-)
Naming compounds:
1. Write the first nonmetal’s name using the subscript to
assign a prefix. If there is no subscript then there is no
prefix.
2. Write the second nonmetal’s name always with a
prefix and always with ide ending
Ex.
Ex.
dinitrogen hexachloride
N2O4
Silicon dioxide
SCl6
triphosphorus octabromide
SiO
Acid:
All acid formulas start with hydrogen. All acid names have the word “acid” in the name
Memorize the following acids and their formulas:
1. HCl – hydrochloric acid
2. H2SO4 – sulfuric acid
3. HC2H3O2 (CH3COOH) – acetic acid
4. HNO3 – nitric acid
Acid:
All acid formulas start with hydrogen. All acid names have the word “acid” in the name ex. hydrochloric acid. Again
here are the guidelines for writing formulas and naming acids.
Writing formulas:
1. Write H as first part of the formula
2. Identify the second part by whether it has hydro
in the name or not. Do step 3 or 4!!
3. If hydro is in the name, then write a nonmetal as
the second part.
4. If NO hydro is in the name, then use the ending
to determine which polyatomic ion to write.
a. IC – write polyatomic ion with ate
ending
b. OUS – write polyatomic ion with ite
ending
5. Swap and drop oxidation numbers to identify the
subscript on H
Ex. Nitric acid
Naming Compounds:
1. If the acid is binary (just 2 elements) write:
Hydro – nonmetal stem – ic acid
2. If the acid contains a polyatomic ion, change the
ending of the ion name.
a. ATE – changes to ic
b. ITE – changes to ous.
c. Then write acid
d. NO Hydro in the name!!
Ex.
H2CO3
Chromic acid
HI
Hydrochloric acid
HC2H3O2