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Chemical and Molecular Formulas
Chemical and Molecular Formulas
Q: How can two elements combine to form
more than one chemical compound?
A: Letters of the alphabet can be combined in
many different ways to form words, the atoms
of 2 or more elements can also be combined
in different ways to form more than one type
of compound
• consider elements A&B: AB, A2B2, AB2 …
• what does the subscript 2 represent in the
chemical formula, H2O?
Chemical Formulas
• a chemical formula shows the kinds and
numbers of atoms in the smallest
representative unit of the substance
• chemists have identified more than 10
million chemical compounds
• some are molecular, such as proteins and
hormones in your body, and others are
ionic, such as the salts in body fluids
Chemical Formulas
• For monatomic
elements, they are
simply represented by
their atomic symbols,
He, Ne
• If the molecules of an
element have more
than one atom, a
number is used as a
subscript
• For example: the 7
diatomic molecules
• H2
• F2
• O2
• N2
• Cl2
• Br2
• I2
Molecular Formulas
Examples
• A molecular formula
shows the kinds and • H2O: subscripts indicate
the # of atoms of each
numbers of atoms
element
present in a molecule
• 2 hydrogen
of a compound
• 1 oxygen (the subscript 1 is omitted)
• subscripts always go
after the atom it is
•
One
molecule
of
ethane,
representing
C2H6 contains:
• 2 carbons
• 6 hydrogen
Structure
Although a
molecular
formula shows
the composition
of a molecule, it
tells you
nothing about
the molecule’s
structure. It
doesn’t show
you how the
atoms are
arranged
Ionic Compounds
Formula Units
• Formula units are written for ionic
compounds, they DO NOT represent a
molecule
• there are no separate molecular units, only
a continuous array of ions
• a formula unit is the lowest whole- number
ratio of ions in a compound
Ionic Compounds
• The ionic compound
magnesium chloride
contains magnesium
cations (Mg2+) and
chloride ions (Cl-).
• The ratio of
magnesium cations
to chloride ions is 1:2
• (1
Mg2+
to 2
Cl-)
• this makes the
compound
electrically neutral
• How about Aluminum chloride?
What is the ratio of aluminum
to chloride ions?
• 1:3 = AlCl3
The Law of Definite Proportions
states that in samples of any chemical
compound, the masses of the elements
are always in the same proportions
(consistent with Dalton’s Atomic Theoryatoms combine in simple whole-number
ratios)
The Law of Multiple Proportions
states that whenever 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
Chemical and Molecular Formula
Review Questions
• Differentiate between a molecular formula
and a formula unit.
• a molecular formula shows the number of
each kind of atom in a molecule of the
compound. The formula unit shows the
lowest whole-number ratio of ions in a
compound
Chemical and Molecular Formula
Review Questions
• Which law is illustrated by this statement:
“When carbon and oxygen form the
compounds carbon monoxide and carbon
dioxide, the different masses of carbon
that combine with the same mass of
oxygen are in the ratio of 2:1”?
• law of multiple proportions
Chemical and Molecular Formula
Review Questions
• - Which law is illustrated by this statement:
“In every sample of carbon monoxide, the
mass ratio of carbon to oxygen is 3:4”?
• law of definite proportions
Chemical and Molecular Formula
Review Questions
• If ionic compounds are composed of
charged particles (ions), why isn’t every
ionic compound either positively or
negatively charged?
• The net positive charge on the cations is
exactly balanced by the net negative
charge on the anions.
Chemical and Molecular Formula
Review Questions
• Would you expect the following pairs of
atoms to combine chemically to give an
ionic or a molecular compound?
•
•
•
•
•
•
Li and S
O ans S
Al and O
F and Cl
I and K
H and N
•
•
•
•
•
•
I
M
I
M
I
M
Chemical and Molecular Formula
Review Questions
Identify the number and kinds of atoms present
in a molecule of each compound
•
•
•
•
•
ascorbic acid (vitamin C), C6H8O6
monosodium glutamate (MSG), C5H8O4Na
sucrose (table sugar), C12H22O11
trinitrotoluene (TNT), C7H5N3O6
ammonium nitrate (fertilizer), NH4NO3