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As early as 400 BC, a few people believed that
atoms are the building blocks of all matter.
Yet, until recently, even scientists had never
seen evidence of atoms.
More than 2000 years later, scientists found
evidence that supported this idea.
The Greek philospher Democritus proposed in 440 B.C.
that matter was made of very tiny particles he called
atomos. (Greek: “atomos”= not to be cut)
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Before there were chemists, there were alchemists—
scientists who wanted to find methods to turn common
metals into gold.
The alchemists practiced, often in secret, throughout the
world during the Middle Ages.
Although they were never successful in their quest for gold,
alchemists provided much information and helped establish
chemistry as a science.
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In 1808, John Dalton, an English school
teacher proposed the first Atomic Theory.
Dalton showed that elements are composed
of only one kind of atom and compounds are
made of two or more kinds of atoms.
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1. All matter is composed of extremely small
particles called atoms, which cannot be
subdivided, created, or destroyed. According
to Dalton, atoms cannot be broken into
smaller pieces.
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2. Atoms of a given element are identical in
their physical and chemical properties. In any
element, all atoms are exactly alike. Atoms of
each element have the same mass.
Carbon atom
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3. Atoms of different elements differ in their
physical and chemical properties. Atoms of
different elements are different, particularly
in their mass.
Hydrogen atom
Oxygen atom
Silicon atom
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4. Atoms of different elements combine in
simple, whole-number ratios to form
compounds.
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5. In chemical reactions, atoms are combined,
separated, or rearranged but never created,
destroyed, or changed. According to Dalton,
atoms are indestructible.
The combustion of methane
and oxygen forms water and
carbon dioxide. Note that the
numbers and types of atoms
are the same. All atoms are
accounted for at the end.
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Law of Definite Proportions: A chemical
compound always contains the same
elements in exactly the same proportions by
weight or mass.
For Example: Every sample of table salt has
the same proportions of sodium
and chlorine, NaCl. It is always
a 1:1 ratio of Na:Cl.
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Law of Multiple Proportions: When two
elements combine to form compounds, the
mass of one element that combines with a
given mass of the other is in the ratio of small
whole numbers.
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Law of Conservation of Mass: Mass cannot be
created or destroyed in ordinary chemical
and physical changes.
Today, scientists can divide an atom into even smaller
particles and can destroy and create atoms.
 For example, in 2006, at Russia's Joint Institute for
Nuclear Research, scientists produced 3 atoms with
118 protons. All it took was smashing "bullets" of
calcium at a target of Californium about
10,000,000,000,000,000,000 times.
 Each atom lived only a fraction of a
millisecond before decaying!
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In the mid-1800s scientists discovered that
atoms can be broken into pieces after all. The
smaller parts that make up atoms are called
subatomic particles.
Electrons (- charged)
Protons (+ charged)
Neutrons (neutral)
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An English physicist, J. J. Thomson discovered
electrons by using cathode rays. He was
awarded the Nobel Prize in 1906 for his work.
Using a cathode ray, he was able to
show that atoms have some distinct
particles with a negative charge.
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Electrons are subatomic particles that have a
negative charge. They orbit the nucleus of an
atom and are extremely small, making up
only a tiny fraction of the mass.
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J. J. Thomson proposed a model of an atom
called the Plum-Pudding model. This model
had negatively charged electrons embedded
in a ball of positive charge.
Ernest Rutherford, a student
of Thomson, disproved this
when he discovered that
atoms have a nucleus.
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Rutherford carried out an experiment where
they shot a beam of small, positively charged
particles (called alpha particles) at a thin gold
foil. He measured the angles at which the
particles were deflected.
He found that most of
the particles went straight
through the foil. Only a
few were deflected.
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf
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The nucleus is the dense, central portion of
the atom.
The nucleus has all of the positive charge,
nearly all of the mass but only a very small
fraction of the volume of the atom.
If an atom the size of a sewing pin was
placed in the middle of the 50 yard line
on a football field, the electrons would
be orbiting around the goal posts! Most
of an atom is empty space!
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Protons are subatomic particles that have a
positive charge and are found in the nucleus.
The number of protons in the nucleus is the
atomic number, which identifies the
element.
Neutrons are subatomic
particles that have no
charge and are located
in the nucleus. Neutrons
add mass to the atom.