Significance of a Chemical Formula
•Chemical formulas form the basis
of the language of chemistry and
reveal much information about the
substances they represent.
•To describe the atomic makeup of compounds
chemists use systematic methods for naming
compounds and for writing chemical formulas.
•A chemical formula indicates the
relative number of atoms of each
kind in a chemical compound.
•For a molecular compound, the chemical formula
gives the number of atoms of each element
contained in a single molecule of the compound.
•Example: octane
C8H18
The subscript after the C
indicates that there are 8
carbon atoms in the molecule.
The subscript after the H
indicates that there are 18
hydrogen atoms in the
molecule.
•The chemical formula for an ionic compound
represents one formula unit – the simplest ratio of
the compound’s (+) ions and (-) ions.
•Example: aluminum sulfate
Al2(SO4)3
•Parentheses surround the polyatomic ion (SO4) to
identify it as a unit. The subscript 3 refers to the
Al2(SO4)3
unit.
2 aluminum atoms
3 sulfur atoms and
12 oxygen atoms
•Remember ionic compounds exist as
crystalline solids.
•Aluminum sulfate crystal
Monatomic Ions
•Monatomic ions are ions formed from a single
atom.
•Monatomic cations are identified by the element’s
name (Na+ is sodium).
•When naming monatomic anions, you drop the
ending of the element’s name and add –ide.
•F-, Fluorine → Fluoride
•Many of the transition metals lose different
numbers of electrons and follow the Stock System of
naming ions and elements.
•Example: copper(I) → Cu+
or copper(II) → Cu2+
copper(I) ion – lost one electron
copper(II) ion – lost two electrons
Binary Ionic Compounds
•Binary ionic compounds are compounds composed
of two different elements.
•The total number of positive and negative charges
must be equal.
•The cross over method is a method of balancing the
charges between the ions in an ionic compound.
•Use your Valence Graphic organizer and ion
handout to help you with the charges on ions.
Polyatomic Ions
O
O
N
O
•In a polyatomic ion, two
or more atoms are
covalently bonded
together.
1–
O
O
N
O
•Together, they carry a charge.
•When balancing charges in an ionic compound, the
polyatomic ions acts as a single unit of charge.
1-
2+
Mg
Mg(NO3)2
1-
1-
2+
Mg
Mg(NO3)2
2+
1-
1-
2+
1-
Mg
the charges are balanced
Mg(NO3)2
2+
2-
Remember a parentheses is placed around the polyatomic ion to
identify it as a unit.
Cross Over Method (or Criss-Cross
Method)
•The cross over method is a method of balancing the
charges between the ions in an ionic compound.
Example: aluminum oxide
1) Write the symbols for the ions.
Al3+
O2-
2) Cross over the charges by using the absolute
value of each ion’s charge as the subscript for the
other ion.
Al23+ O32-
3) Check the combined positive and negative
charges to see if they are equal.
2 Al atoms x 3+ = +6
3 O atoms x 2- = -6
0
The correct formula is Al2O3.
Naming Binary Ionic Compounds
•The nomenclature, or naming system, or binary
ionic compounds involves combining the names of
the compound’s positive and negative ions.
•The name of the cation is given first, followed by
the name of the anion (-ide).
•Example: Al2O3
aluminum oxide
•Some elements such as iron, form two or more
cations with different charges.
•The Stock System of nomenclature uses a Roman
numeral to indicate an ion’s charge.
•The numeral is enclosed in parentheses and placed
immediately after the metal’s name.
•Example: Fe2+ - iron(II), Fe3+ - iron(III).
Example: write the name for the following
compound.
SnI4
1) First determine the charge on the Sn ion.
The iodine ion has a charge of 1- and the
formula contains 4 I- ions therefore, the overall
negative charge is 4-.
2) In order for the charges to equal the Sn ion must
have a charge of 4+.
3) The name would be tin(IV) iodide.
Compounds containing Polyatomic Ions
•Most polyatomic ions are negatively charged and
are oxyanions – contain oxygen.
•Compounds containing polyatomic ions are named
in the same way as binary ionic compounds.
•Name the cation first, then the anion.
•Example: Ca(OH)2
Name cation first,
then the anion.
calcium hydroxide
•If more than one oxyanion is formed by the same
two elements (Ex. N and O) the most common ion is
given the ending – ate.
•The one with one less oxygen ends in –ite.
•Example: NO2- - nitrite, NO3-, nitrate.
•Some elements can form more than two types of
oxyanions.
•Example: chlorine can form
•ClO- (hypochlorite)
•ClO2- (chlorite)
•ClO3- (chlorate)
•ClO4- (perchlorate)
Naming Binary Molecular Compounds
•The naming of molecular compounds is based on
the use of prefixes (CO is carbon monoxide, CO2 is
carbon dioxide, P4O10 is tetraphosphorus decoxide).
•The less electronegative element is given first. It is
given a prefix only if it contributes more than one
atom to a molecule.
•The second element is named by (a) prefix giving
number of atoms contributed (b) the root of the
name of the second element, and (c) the ending
–ide.
Numerical Prefixes
Number
1
2
3
4
5
6
7
8
9
10
Prefix
monoditritetrapentahexaheptaoctanonadeca-
The prefix system is illustrated below.
Prefix needed if first
element contributes
more than one atom
+
Name of
first element
Prefix indicating number of
atoms contributed by second
element.
+
Root name of second element
+ ide.