Chapter 7:
Chemical Formulas and
Chemical Compounds
Coach Kelsoe
Chemistry
Pages 218-249
Chapter 7-1:
Chemical Names and Formulas
Coach Kelsoe
Chemistry
Pages 219-231
Section 7-1 Objectives





Explain the significance of a chemical
formula.
Determine the formula of an ionic compound
formed between two given ions.
Name an ionic compound given its formula.
Using prefixes, name a binary molecular
compound from its formula.
Write the formula of a binary molecular
compound given its name.
Chemical Names and Formulas

The total number of natural and synthetic
chemicals runs in the millions, so often times
it is easier to give chemicals their own names:




Calcium carbonate –
Limestone
Sodium chloride –
Table salt
Dihydrogen monoxide – Water
Sodium bicarbonate –
Baking soda
Chemical Names and Formulas


The only problem with common names is that
you can’t tell about its chemical composition.
There is a systematic method for naming
compounds and for writing chemical formulas.
Ionic Compounds vs.
Molecular Compounds



Ionic compounds are compounds that are
held together by ionic bonds.
Molecular compounds are compounds that
are held together by covalent bonds.
So how do you know the difference?
Electronegativity values, silly!
Significance of a Chemical
Formula


A chemical formula indicates the relative
number of atoms of each kind in a chemical
compound.
For a molecular compound, the chemical
formula reveals the number of atoms of each
element contained in a single molecule of the
compound.
Significance of a Chemical
Formula


The chemical formula for an ionic compound
represents one formula unit–the simplest
ratio of the compound’s cations and anions.
Note the parentheses to identify the anion as
a unit. The subscript 3 refers to the entire
unit. With no subscript, assume one atom.
Monatomic Ions




By gaining or losing electrons, many maingroup elements form ions with noble-gas
configurations.
Group 1 elements tend to form 1+ cations,
group 2 elements tend to form 2+, etc.
Ions formed from a single atom are known as
monatomic ions.
Group 17 elements tend to form 1- anions.
Monatomic Ions


Not all main-group elements readily form
ions. Rather than gaining or losing electrons,
carbon and silicon atoms form covalent
bonds. Tin and lead tend to lose the two
electrons in their p orbitals, but retain the two
in the s orbital. What charge would they
have?
Elements from the d-block form 2+, 3+, or in
a few cases 1+ or 4+ cations.
Naming Monatomic Ions

Monatomic cations are identified simply by
the elements name, followed by “cation.”



Potassium cation
Magnesium cation
Naming monatomic anions requires dropping
the ending of the element’s name and
replacing it with the suffix –ide.


Chlorine  Chloride anion
Nitrogen  Nitride anion
Some Common Monatomic
Ions
Main-group elements
1+
Lithium
2+
3+
Li+
Sodium
Na+ Magnesium
Potassium
K+
Calcium
Mg2+ Aluminum
Ca2+
Rubidium
Rb+ Strontium
Sr2+
Cesium
Cs+ Barium
Ba2+
1-
2-
Fluoride
F-
Chloride
Cl- Sulfide
Bromide
Br-
Iodide
I-
Oxide
Al3+
3O2- Nitride
S2-
N3-
Some Common Monatomic
Ions
d-block elements
1+
2+
3+
4+
Copper(I)
Cu+
Cadmium
Cd2+ Chromium(III)
Cr3+
Lead(IV)
Silver
Ag+
Chromium(II)
Cr2+
Iron(III)
Fe3+
Vanadium(IV)
Cobalt(II)
Co2+
Lead(III)
Pb3+
Tin(IV)
Copper(II)
Cu2+
Vanadium(III)
Iron(II)
Fe2+
Lead(II)
Pb2+
Manganese(II) Mn2+
Mercury(II)
Hg2+
Nickel (II)
Ni2+
Tin(II)
Sn2+
Vanadium(II)
Zinc
V2+
Zn2+
V3+
Pb4+
V4+
Sn4+
Binary Ionic Compounds


Compounds composed of two different
elements are known as binary compounds.
In a binary compound, the total numbers of
positive charges and negative charges must
be equal! Therefore the formula for such a
compound can be written given the identities
of the compound’s ions. For example:
magnesium bromide. We do NOT put the
charges in the final formula!
A Quick Math Review

Subscripts are the small numbers written to
the right of an element’s symbol.


Coefficients are the large numbers written to
the left of an element’s symbol.


O2, H2O, CH4O
2O2, 5H2O, 19CH4O
With polyatomic ions, the subscripts are
multiplied:

(SO4)2, (HSO4)3, (H2PO4)4
“Crossing Over”

As an aid to determining subscripts in
formulas for ionic compounds, the positive
and negative charges can be “crossed over.”



Al3+ O2Al2O3
We can check to make sure this is correct by
multiplying the original charge by the new
subscripts to make sure the number is the
same for both the atoms in the molecule.
Naming Binary Ionic
Compounds


The nomenclature, or naming system, of
binary ionic compounds involves combining
the names of the compound’s positive and
negative ions.
For most simple ionic compounds, the ratio of
the ions is not indicated in the compound’s
name because it is understood based on the
relative charges of the compound’s ions.

Al2O3  Aluminum oxide
Sample Problem 1


Write the formula for the binary ionic compound
formed between calcium and iodine.
2+
 Ca
I CaI2
Write the formula for the binary ionic compound
formed between calcium and sulfur.
2+
 Ca
S2 CaS
Sample Problems 1

Write formulas for the binary ionic
compounds formed between the following
elements:





Potassium and iodine
Magnesium and chlorine
Sodium and sulfur
Aluminum and sulfur
Aluminum and nitrogen
Sample Problems 1

Name the binary ionic compounds indicated
by the following formulas:






AgCl
ZnO
CaBr2
SrF2
BaO
CaCl2
The Stock System of
Nomenclature


Some elements, such as iron, form two or
more cations with different charges. To
distinguish the ions formed by such elements,
the Stock system of nomenclature is used.
This system uses a Roman numeral to
indicate an ion’s charge. The numeral is
enclosed in parentheses and placed
immediately after the metal name.

Fe2+  iron(II)
Fe3+  iron(III)
The Stock System of
Nomenclature

Names of metals that commonly form only
one cation do not include a Roman numeral.



Na+  Sodium
Ba2+  Barium
There is no element that commonly forms
more than one monatomic anion.
Naming a binary ionic compound according to
the Stock system is done as so:


CuCl2  copper(II) chloride
CuF3  copper(III) fluoride
Sample Problem 2

Write the formula and give the name for the
compound formed by the ions Cr3+ and F-.


CrF3
Chromium(III) fluoride
Sample Problems 2

Write the formula and give the name for the
following compounds formed between the
following ions:






Cu2+ and BrFe2+ and O2Pb2+ and ClHg2+ and S2Sn2+ and FFe3+ and O2-






CuBr2; copper(II) bromide
FeO; iron(II) oxide
PbCl2; lead(II) chloride
HgS; mercury(II) sulfide
SnF2; tin(II) fluoride
Fe2O3; iron(III) oxide
Sample Problems 2

Give the names for the following compounds:




CuO
CoF3
SnI4
FeS




Copper(II) oxide
Cobalt(III) fluoride
Tin(IV) iodide
Iron(II) sulfide
Compounds Containing
Polyatomic Ions



The list of polyatomic ions and their charges
can be found on page 226 in your text.
All but the ammonium ion are negatively
charged and most are oxyanions,
polyatomic ions that contain oxygen.
It isn’t uncommon for two oxyanions to be
formed from the same two elements:

Nitrogen and oxygen form nitrite and nitrate.
Compounds Containing
Polyatomic Ions

When naming compounds containing
oxyanions, the most common ion is given the
ending –ate. The ion with one less oxygen
atom is given the ending –ite.


NO2  Nitrite
NO3  Nitrate
If two elements form more than two different
oxyanions, an anion with one less oxygen
than the –ite anion is given the prefix hypo-.

ClO2  Chlorite
ClO  Hypochlorite
Compounds Containing
Polyatomic Ions


An anion with more oxygen than the –ate
anion is given the prefix per-.
Compounds containing polyatomic ions are
named in the same manner as binary ionic
compounds.


AgNO3  Silver nitrate; AgNO2  Silver nitrite
When more than one polyatomic ion is
present in a compound, use parentheses.

Al2(SO4)3  Aluminum sulfate
Sample Problems 3

Write the formula for tin(IV) sulfate.



Sn4+
SO42Sn(SO4)2
Write the formulas for the following ionic
compounds:





Sodium iodide
Calcium chloride
Potassium sulfide
Lithium nitrate
Copper(II) sulfate





NaI
CaCl2
K2S
LiNO3
CuSO4
Sample Problems 3

Give the name for the following compounds:






Ag2O
Ca(OH)2
KClO3
NH4OH
FeCrO4
KClO






Silver oxide
Calcium hydroxide
Potassium chlorate
Ammonium hydroxide
Iron(II) chromate
Potassium hypochlorite
Naming Binary Molecular
Compounds



Unlike ionic compounds, molecular
compounds are composed of individual
covalently bonded units, or molecules.
Chemists use two nomenclature systems to
name binary molecules: the Stock system and
the prefix system.
The prefix system uses numerical prefixes to
identify the number of atoms in the molecule.
Naming Binary Molecular
Compounds


For example, CCl4 is called carbon tetrachloride.
The prefix tetra- means “four.”
The prefix will come before both molecules. The
prefix mono- can sometimes be left off.
1
2
3
4
5
monoditritetrapenta-
6
7
8
9
10
hexaheptaoctanonadeca-
Rules for Naming Binary
Molecular Compounds
1.
2.
The less-electronegative element is given first. It
is given a prefix only if it contributes more than
one atom to a molecule of the compound.
The second element is named by combining (a) a
prefix indicating the number of atoms contributed
by the element, (b) the root of the name of the
second element, and (c) the ending –ide. With
few exceptions, the ending –ide indicates that a
compound contains only two elements.
Rules for Naming Binary
Molecular Compounds
3.
The o or a at the end of a prefix is usually dropped
when the word following the prefix begins with
another vowel, e.g., monoxide or pentoxide.

Tetraphosphorus decoxide

P4O10

Phosphorus comes first because it is less
electronegative than oxygen.

The order of nonmetals in binary compound
names and formulas- C, P, N, H, S, I, Br, Cl, O, F.
Sample Problem 4

Give the name for As2O5.


Diarsenic pentoxide
Name the following binary molecular
compounds:



SO3
ICl3
PBr5



Sulfur trioxide
Iodine trichloride
Phosphorus pentabromide
Sample Problem 4

Write the formula for oxygen difluoride.


OF2
Write formulas for the following compounds:



Carbon tetraiodide
Phosphorus trichloride
Dinitrogen trioxide



CI4
PCl3
N2O3
Acids and Salts



Most acids used in the laboratory can be
classified as either binary acids or oxyacids.
Binary acids are acids that consist of two
elements, usually a hydrogen and one of the
halogens – F, Cl, Br, or I.
Oxyacids are acids that contain hydrogen,
oxygen, and a third element, usually a
nonmetal.
Acids and Salts


Acids were first recognized as a specific class
of compounds based on their properties in
solutions of water. Consequently, in
chemical nomenclature, the term acid usually
refers to a solution in water of one of these
special compounds rather than to the
compound itself.
For example, hydrochloric acid refers to a
water solution of the molecular compound
hydrogen chloride, HCl.
Acids and Salts



Many polyatomic ions are produced by the
loss of hydrogen ions from oxyacids.
An ionic compound composed of a cation and
the anion from an acid is often referred to as
a salt.
Table salt, NaCl, contains the anion from
hydrochloric acid. Calcium sulfate, CaSO4, is
a salt containing an anion from sulfuric acid.
Acids and Salts


Some salts contain anions in which one or
more hydrogen atoms from the acid are
retained. Such anions are named by adding
the word hydrogen or the prefix bi- to the
anion name.
The best known such anion comes from
carbonic acid, H2CO3.