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Dalton’s Atomic Theory
Objectives: To learn about Dalton’s theory of atoms.
To understand and illustrate the law of constant composition.
A
s scientists of the eighteenth century studied the nature of materials,
several things became clear:
1. Most natural materials are mixtures of pure substances.
2. Pure substances are either elements or combinations of elements called
compounds.
3. A given compound always contains the same proportions (by mass) of
the elements. For example, water always contains 8 g of oxygen for
every 1 g of hydrogen, and carbon dioxide always contains 2.7 g of oxygen
for every 1 g of carbon. This principle became known as the law of
constant composition. It means that a given compound always has
the same composition, regardless of where it comes from.
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John Dalton (Figure 3.1), an English scientist and teacher, was aware of
these observations. In about 1808 he offered an explanation for them that
became known as Dalton’s atomic theory. The main ideas of this theory
(model) can be stated as follow
Dalton’s Atomic Theory
1. Elements are made of tiny particles called atoms.
2. All atoms of a given element are identical.
3. The atoms of a given element are different from those of any other
element.
Figure 3.1
John Dalton (1766–1844) was
an English scientist who made
his living as a teacher in Manchester. Although Dalton is best
known for his atomic theory, he
made contributions in many
other areas, including meteorology (he recorded daily weather
conditions for 46 years, producing a total of 200,000 data
entries). A rather shy man, Dalton was colorblind to red (a
special handicap for a chemist)
and suffered from lead poisoning contracted from drinking
stout (strong beer or ale) that
had been drawn through lead
pipes.
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Chapter 3
4. Atoms of one element can combine with atoms of other elements to
form compounds. A given compound always has the same relative
numbers and types of atoms.
5. Atoms are indivisible in chemical processes. That is, atoms are not
created or destroyed in chemical reactions. A chemical reaction simply changes the way the atoms are grouped together.
Dalton’s model successfully explained important observations such as the
law of constant composition. This law makes sense because if a compound
always contains the same relative numbers of atoms, it will always contain
the same proportions by mass of the various elements.
Like most new ideas, Dalton’s model was not accepted immediately.
However, Dalton was convinced he was right and used his model to predict
how a given pair of elements might combine to form more than one compound. For example, nitrogen and oxygen might form a compound containing one atom of nitrogen and one atom of oxygen (written NO), a compound
containing two atoms of nitrogen and one atom of oxygen (written N2O), a
compound containing one atom of nitrogen and two atoms of oxygen
Chemical Foundations: Elements, Atoms, and Ions
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S c i e n c e, Te c h n o l o g y, a n d S o c i e t y
No Laughing Matter
S
ometimes solving one problem leads to another.
One such example involves the catalytic converters now required on all automobiles sold in
much of the world. The purpose of these converters is to remove harmful pollutants such as CO and
NO2 from automobile exhausts. The good news is
that these devices are quite effective and have led
to much cleaner air in congested areas. The bad
news is that these devices produce significant
amounts of nitrous oxide, N2O, commonly known
as laughing gas because when inhaled it produces
relaxation and mild inebriation. It was long used by
dentists to make their patients more tolerant of
some painful dental procedures.
The problem with N2O is not that it is an air pollutant but that it is a “greenhouse gas.” Certain molecules, such as CO2, CH4, N2O, and others, strongly
absorb infrared light (“heat radiation”), which
causes the earth’s atmosphere to retain more of its
Figure 3.2
Dalton pictured compounds as collections of atoms.
Here NO, NO2, and N2O are represented. Note that
the number of atoms of each type in a molecule is
given by a subscript, except that the number 1 is
always assumed and never written.
heat energy. Human activities have significantly increased the concentrations of these gases in the atmosphere. Mounting evidence suggests that the
earth is warming as a result, leading to possible dramatic changes in climate.
A recent study by the Environmental Protection
Agency (EPA) indicates that N2O now accounts for
over 7% of the greenhouse gases in the atmosphere
and that automobiles equipped with catalytic converters produce nearly half of this N2O. Ironically,
N2O is not regulated, because the Clean Air Act of
1970 was written to control smog—not greenhouse
gases. The United States and other industrialized nations are now negotiating to find ways to control
global warming but no agreement is now in place.
The N2O situation illustrates just how complex
environmental issues are. Clean may not necessarily be “green.”
N
O
O
NO
N
O
NO2
N
N
O
N 2O
(written NO2), and so on (Figure 3.2). When the existence of these substances was verified, it was a triumph for Dalton’s model. Because Dalton
was able to predict correctly the formation of multiple compounds between
two elements, his atomic theory became widely accepted.
Focus Questions
Sections 3.1–3.3
1. Give four different chemical meanings for the word element and an
example of each.
2. How many of the Top Ten elements in the human body are also Top
Ten elements in Earth’s crust, ocean, and atmosphere? What are these
elements?
3. List the elements that you think were named after a place.
4. What observations led Dalton to propose his model for the atom?
5. Did Dalton’s model of the atom include protons, neutrons, and
electrons inside the atoms? Why or why not?
3.3
Dalton’s Atomic Theory
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