Chemical Formulas and Chemical Compounds
Study Guide
Naming ionic compounds:
1. Name the monatomic cation using the element name and simply use the name of the
positive polyatomic ion.
2. Name the monatomic anion using the root of the element name with the suffix –ide
and us the name for the negative polyatomic ion.
***You must include Roman numerals in parenthesis only if the formula includes an ion that
may have more than one charge associated with it.
Sodium chloride
Magnesium bromide
Iron(III) sulfate
Ammonium carbonate
Writing ionic formulas:
1. Assign charges to each of the ions present.
2. Arrange the ions so that the cation is on the left and the anion is on the right.
3. Cross the charges and drop the sign.
Al2(SO4)3
Al3+
SO42-
***You must include parenthesis only if the formula includes more than one polyatomic ion.
NaCl
MgBr2
Fe2(SO4)3
(NH4)2CO3
Naming molecular compounds: (Old School---prefix system)
1. A prefix is used with the name of the first element (less electronegative) only if
more than one atom is present.
2. The second element is named by combining (a) a prefix if more than one compound
can be formed by the two elements, (b) the root of the name of the second element,
and (c) the ending –ide.
Dinitrogen
monoxide
Carbon tetrafluoride
Dinitrogen tetroxide
Phosphorus trichloride
Writing molecular formulas:
1. Write the symbol of the element followed by a subscript that represents the
number of atoms of that element present in the molecule.
N2O
CF4
N2O4
PCl3
Naming molecular compounds: (Stock system)
1. Name the element that is less electronegative and write the oxidation state as a
Roman numeral in parenthesis.
2. Use the root of the element name of the more electronegative element and the
suffix –ide.
Nitrogen(I) oxide
Carbon(IV) fluoride
Nitrogen(II) oxide
Phosphorus(III)
chloride
Naming binary acids:
1. Start with the prefix hydro-, then the root of the element name, and end with the
suffix –ic.
2. Add the word acid.
Hydrochloric acid
HCl
Hydrobromic acid
HBr
Hydroiodic acid
HI
Hydrosulfuric acid
H 2S
Naming oxyacids:
1. Name the polyatomic ion and change the suffixes –ate to –ic and –ite to –ous…leave
the prefixes alone.
2. Add the word acid.
Chloric acid
HClO3
Hypoiodous acid
HIO
Nitric acid
HNO3
Sulfuric acid
H2SO4
NOTE ABOUT NAMING: Compounds that are written with a charge (indicated by superscript) are
polyatomic ions and must be named accordingly.
Naming polyatomic ions:
1. Memorize the charge, name, and formula of all of the polyatomic ions that end with the
suffix –ate.
2. Use the following system to figure out the names of the other polyatomic ions:
Change in Formula
Prefix-Suffix
+1 Oxygen
Per-ate
Memorize formula, name, and charge
-ate
-1 Oxygen
------ite
-2 Oxygen
Hypo-ite
Perchlorate ion
ClO4-
Chlorate ion
ClO3-
Chlorite ion
ClO2-
Hypochlorite ion
ClO-
Rules for Assigning Oxidation Numbers:
1. An uncombined element has an oxidation number of zero.
2. A monatomic ion has an oxidation number equal to the charge of the ion.
3. Fluorine has an oxidation number of -1 in all of its compounds, because it is the most
electronegative element.
4. Oxygen usually has an oxidation number of -2, except in peroxides (-1), superoxides
(-½), and in compounds containing halogens (+2).
5. Hydrogen has an oxidation number of +1 in all compounds with elements of greater
electronegativity, and an oxidation number of -1 in compounds with metals.
6. The more electronegative element in a binary compound is assigned the oxidation
number equal to its charge if it were an ion.
7. The algebraic sum of all the oxidation numbers in a neutral compound is zero.
8. The algebraic sum of oxidation numbers of all atoms in a polyatomic ion is equal to the
charge of the ion.
UF6
GeCl2
SO42-
Uranium = +6, Fluorine = -1
Germanium = +2, Chlorine = -1
Oxygen = -2, Sulfur = +6
Formula Mass:
1. Write a correct formula…oxidation numbers/charges must sum to equal zero, unless the
compound is a polyatomic ion.
2. Use the Periodic Table to get the atomic mass for each element in the compound.
3. Multiply the atomic mass of the element by the total number of atoms of that element in
the compound. Repeat this for each element in the compound.
4. Add the products from step 3 together.
Ca(NO3) 2
Formula
Formula
Formula
Formula
mass
mass
mass
mass
= 1Ca + 2N + 6O
= 40.08u + 2(14.0067u) + 6(15.9994u)
= 164.0898u
= 164.09u
Molar Mass:
1. Calculate the formula mass (see above).
2. Change the units to grams per mole.
Ca(NO3) 2
Molar mass = 164.09g/mol
Conversions with molar mass:
1. Use the factor label method.
2. Consult your notes to fill in the triangle.
Percent Composition:
1. Calculate the formula mass of the compound.
2. Determine the number of atoms of each element that are present in the compound.
3. Substitute the values from above into the following equation:
Mass of element
x 100%
Mass of compound
Empirical Formula:
% composition
mass
moles
Molecular Formula:
(empirical formula)x = (molecular formula)
x = Molecular formula mass
Empirical formula mass
ratio