Transcript Slide 1 - Effingham County Schools

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Early Greek Theories
• 440 B.C. - Democritus thought matter could
not be divided indefinitely. He called
nature’s basic particle the “atom”, from the
Greek word meaning “indivisible”.
Democritus
Aristotle
• 350 B.C - Aristotle modified an earlier
theory that matter was made of four
“elements”: earth, fire, water, air.
• Aristotle was wrong. However, his
theory persisted for 2000 years.
Foundations of the Atom
•In the late 1700s chemists believed the modern
definition of an element as a substance that
cannot be further broken down by ordinary
chemical means.
•It was also clear that elements combine to form
compounds that have different physical and
chemical properties than those of the elements
that form them.
Na + Cl → NaCl
Different properties
•Chemists at this time could not agree whether
elements always combined in the same ratio
when forming a particular compound.
•When a substance or substances are
transformed into one or more new substances, it
is caused by a chemical reaction.
Forming
lead
iodide
•In the 1790s, scientists began to analyze
chemical reactions. This lead to the discovery
of several basic laws.
•The law of conservation of mass (matter (mass)
is neither created nor destroyed during ordinary
physical or chemical changes).
•Antoine Lavoisier, a French chemist, verified
this law by experimentation in 1789.
The Law of
Conservation
Of Mass
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•In 1799, Joseph Proust, a French chemist,
showed that a given compound always
contains exactly the same proportion of
elements by mass.
•The principle of the constant composition of
compounds, originally called “Proust’s law”, is
now known as the law of definite proportions.
•The law of definite proportions (a chemical
compound contains the same proportions by
mass regardless of the size of the sample or
source of the compound.
•For example, sodium chloride (table salt)
always consists of 39.34% by mass of the
element sodium, Na, and 60.66% by mass of
the element chlorine, Cl.
•Proust’s discovery stimulated John Dalton, an
English schoolteacher, to think about atoms as
the particles that might compose elements.
•Dalton discovered another principle that
convinced him of the existence of atoms.
•He noted, for example, that carbon and
oxygen form two different compounds that
contain different relative amounts of carbon
and oxygen (CO and CO2).
•This led to the law of multiple proportions in
1803.
•The law of multiple proportions (if two or more
different compounds are composed of the
same two elements, then the ratio of the
masses of the second element combined with
a certain mass of the first element is always a
ratio of small whole numbers.
•For example, the elements carbon and
oxygen form two compounds, carbon dioxide
and carbon monoxide. CO molecules are
always composed of one C atom and one O
atom. CO2 molecules are always one C atom
and two O atoms.
Law of Multiple Proportions
Dalton’s Atomic Theory
•In 1808, John Dalton proposed an explanation
for the three laws (Atomic Theory).
•All matter is composed of extremely small particles
called atoms.
•Atoms of a given element are identical in size,
mass, and other properties; atoms of different
elements differ in size, mass and other properties.
•Atoms cannot be subdivided, created, or
destroyed.
•Atoms of different elements combine in simple
whole-number ratios to form chemical compounds.
•In chemical reactions, atoms are combined,
separated, or rearranged.
Modern Atomic Theory
•Aspects of Dalton’s theory have been proven
incorrect (we know now that atoms are
divisible and a given element can have atoms
with different masses).