1
Chapter 6
Section 6.2
Representing
Chemical
Compounds
2
Chemical Formulas
The composition of a
chemical compound
is represented by a
chemical formula.
3
A chemical formula
shows the kinds and
numbers of atoms in
the smallest
representative unit of
the substance.
4
Molecular Formulas
The chemical
formula of a
molecular compound
is called a
molecular formula.
5
A molecular formula
shows the kind and
number of atoms in a
molecule of a
compound.
6
A water molecule is
made up of two(2)
hydrogen atoms and
one(1) oxygen atom.
The molecular
formula for water is
H2O.
7
A subscript written
after a symbol
indicates the number
of atoms of the
element whose
symbol it follows.
8
In the molecular
formula for ethane,
C2H6, the formula
shows that one
molecule of ethane
contains 2 atoms of
carbon and 6 atoms
of hydrogen.
9
The molecular
formula shows
nothing about the
arrangement of the
atoms within the
molecule.
10
For the molecular
compound ammonia,
the molecular
formula is
13
One way to show the
structure.
14
One way to show the
structure.
15
One way to show the
structure.
16
Formula Unit
Chemical Formulas
can also be written
for ionic compounds
Ionic compounds
don’t exist as
molecules.
17
An ionic compound is
represented by a
formula unit.
A formula unit is the
lowest whole-number
ratio of ions in the
compound.
18
For sodium chloride,
the lowest wholenumber ratio of ions is
+
1:1 (one Na to one Cl ).
Thus the formula unit
for sodium chloride is
NaCl.
19
The charges on the
ions are not shown
when writing the
formula unit of the
compound.
20
The sum of the ionic
charges in a formula
unit must equal
zero(0).
21
Ionic compounds
exist as collections
of positively and
negatively charged
ions arranged in
repeating three
dimensional
patterns.
22
The Laws of Definite
and Multiple
Proportions
23
100g of MgS breaks
down into 43.15g
Mg and 56.87g S.
The ratio of these
masses is
43.15g
 0.75841 : 1
56.87g
24
MgS obeys the Law of
Definite Proportions
which says that in
samples of any chemical
compound, the masses
of elements are always
in the same proportions.
25
Dalton stated the Law of
Multiple Proportions: When
two elements form more
than one compound, the
different masses of one
element that combine with
the same mass of the other
element are in the ratio of
small whole numbers.
26
In hydrogen
peroxide, H2O2 , 32 g
of oxygen combine
with 2 g of hydrogen
In water, H2O , 16 g of
oxygen combine with
2 g of hydrogen.
27
Calculating the ratio
of O in H2O2 to the O
in H2O we get:
32g
 2.0 or 2 : 1
16g
28
29