Unit Four: Chemical Formulas
Chapter 4
A + X = AX
Unit Objectives
Upon competition of this unit, you will be able
to:
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List names and symbols of common elements
Differentiate between chemical symbols and
formulas
Differentiate between subscripts and coefficients
Given a formula, state the number of atoms of
each element present
Define compound, ion, polyatomic ion, and
oxidation number
List the names, symbols, and oxidation numbers
or charges of common ions
Objectives Continued
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List the common elements that normally occur as
diatomic molecules
Write formulas for chemical compounds using
oxidation numbers
Distinguish atoms, ions, and molecules
Define, recognize, and give examples of binary
compounds
Name compounds from their chemical formula
Recognize and give examples of compounds
containing polyatomic ions
Differentiate between molecular and empirical
formulas
Define, recognize, and give examples of a formula
unit
Names and Symbols
Chemical Symbols are shorthand
representations of the elements
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See Table 4-1
Symbols usually come from the first letter
of the name, or in some cases the Latin
version of the name
 Ac = Actinium
 Al = Aluminum
 I = Iodine
 Fe = Iron
Chemical Formulas
A Chemical Formula is a combination of
symbols that represents the composition of
a compound
A formula shows two things
1.
2.
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Elements Present
Relative number of atoms of each element in the
compound
H2O = Hydrogen and Oxygen present, 2
Hydrogen's and 1 Oxygen
Oxidation Number
A charged atom is called an ion
An ion made of more than one atom is
called a polyatomic ion
When a single atom takes on a charge,
that charge is known as the oxidation
number of the atom
See table 4-3 and 4-4 on page 65
Ionic Compounds
Table salt, NaCl, is an ionic compound
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Table 4-3 shows that Na has a 1+ charge and Cl
has a 1- charge
 They combine in a 1:1 ratio to give a net charge of 0
 1 + (1-) = 0
 Therefore, the formula is NaCl
An ionic compound is composed of charged
particles
Neutral atoms combine to form molecules.
Diatomic Molecules
A diatomic molecule is composed of
two atoms of the same element
Seven common elements occur as
diatomic molecules
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You will need to memorize these
They are: Chlorine (Cl2), Hydrogen (H2),
Nitrogen (N2), Oxygen (O2), Fluorine (F2),
Bromine (Br2), and Iodine (I2)
Any time these element exist on their own,
they will be found as diatomic molecules
 I.e., You will never find just one hydrogen
floating around
Practice
Write the formula for a compound of
Hydrogen and Bromine
Write the formula for a compound of Ca
and Cl
Write the formula for a compound of
Aluminum Hydroxide
Practice
Write the formula for a compound of
Hydrogen and Bromine
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HBr
Write the formula for a compound of Ca
and Cl
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CaCl2
Write the formula for a compound of
Aluminum Hydroxide
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Al(OH)3
Points to remember
If an ion has a roman numeral after its
symbol, that means that it has the
charge indicated by the roman numeral
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Pb(II) = Lead 2+
Cr(III) = Chromium 3+
Use parenthesis when you have more
than one polyatomic ion in a compound
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Mg(CN)2
Assignment
Complete Exercises 1-3 on page 67 of
the text
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Worth a possible 10 points
Due: Tomorrow
Naming Compounds
Many compounds have two names,
common and chemical
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NaHCO3 is sodium hydrogen carbonate, but
is also known as baking soda
We will use systematic naming
Naming Binary Compounds
Compounds containing only two
elements are called binary
compounds
To name a binary compound
1.
2.
Write the name of the element having a
positive charge
Then add the name of the negative
element, modifying the ending to -ide
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Example: HBr = Hydrogen Bromide
Naming Binary Compounds cont.
Some elements have more than one possible
charge
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When we name these, we use roman numerals
after the name of the element to show its
oxidation number
 NO = Nitrogen (II) Oxide
(see table 4-6)
Prefixes
In an older naming system, prefixes were used to
indicate the number of atoms (table 4-7)
Prefix
Mono
Di
# of atoms
1
2
Tri
Tetra
Penta
3
4
5
Hexa
Hepta
Octa
6
7
8
Common Acids
The names of the common acids do not
follow the rules of systematic naming
You will have to memorize the
names and formulas of the
common acids
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See table 4-9 on page 69 of the text
Organic Compounds
Organics are named using a different
set of rules
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Count the number of carbon atoms in a
chain
 The beginning of the name will be in table 4-10
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Then add suffix –ane
Example: C6H14
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Hexane
Assignment
Complete Exercises 4-8 on page70
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Worth a possible 10 points
Due:
Molecular and Empirical Formulas
The formulas for compounds that exist as
molecules are called molecular formulas
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Hydrogen peroxide is H202
Shows actual number of atoms in the molecule
An empirical formula indicates the simplest
whole-number ratio of atoms or ions in a
compound
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For hydrogen peroxide, the empirical formula
would be HO
Coefficients
The formula of a compound represents a
definite amount of that compound, this is
called a formula unit
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One formula unit of water is H2O
If you had two formula units of water, it would be
written as 2H2O
The “2” is called a coefficient
 Remember from algebra, if you have 2 x’s (or x+x), you
would say you had 2x
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Coefficients are used to represent the number of
formula units
Assignment
Complete Exercises 9-10 on page 72
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Worth a possible 10 points
Due: By the end of class
Chapter Review
Complete the following questions on
pages 73-75
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13-19, 22-40, 44, 47, 48, 49, and 50
Worth a possible 25 points
Due before the test