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Atomic Structure
An atom is the smallest particle of an
element that retains its identity in a
chemical reaction.
 Although early philosophers and
scientists could not observe individual
atoms, they were still able to propose
ideas about the structure of atoms.

Atomic Structure
For thousands of years, people had
many ideas about matter
 Ancient Greeks believed that
everything was made up of the four
elements
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Water, Fire, Earth, Air
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
Around 300 BC, a Greek philosopher,
Democritus stated that everything is
made up of tiny, invisible particles
 He
said the particles were indivisible
 He called them “atomos” – which
means unable to divide
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
Then in the early 1800’s, an English
school teacher, John Dalton
developed the first atomic theory


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His theory was successful because it was
supported with scientific evidence
By using experimental methods, Dalton
transformed Democritus’ ideas on atoms
into a scientific theory.
His theory has 4 main points.
Atomic Structure
1. All elements are composed
of tiny indivisible particles
called atoms.
Atoms of element A
2. Atoms of the same element
are identical. The atoms of
any one element are
different from those of any
other element.
Atoms of
element A
Atoms of
element B
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3. Atoms of different
elements can physically
mix together or can
chemically combine in
simple whole-number
ratios to form
compounds.
Mixture of atoms of
elements A and B
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4. Chemical reactions occur
when atoms are
separated from each
other, joined, or
rearranged in different
combinations.
Atoms of one element are
never changed into atoms of
another element as a result
of a chemical reaction.
Compound made by
chemically combining
atoms of elements A
and B
Atomic Structure
Much of Dalton’s atomic theory is
accepted today.
 One important change, however, is
that atoms are now known to be
divisible.
 Also, atoms of the same element are
not identical.

Atomic Structure
By the late 1800’s, an English physicist,
JJ Thomson contributed to the
development of the model
 He performed an experiment with a
cathode ray tube

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His experiment led to the discovery of the
electron
An electron has one unit of negative
charge, and its mass is 1/1840 the mass of
a hydrogen atom.
Atomic Structure

In 1886, Eugen Goldstein discovered
protons

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If matter has negative particles, it must
also have positive particles.
In 1932, James Chadwick confirmed
the existence of yet another
subatomic particle: the neutron

Neutrons are subatomic particles with no
charge but with a mass nearly equal to
that of a proton.
Atomic Structure

When subatomic particles were
discovered, scientists wondered how
the particles were put together in an
atom.
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Most scientists—including J. J. Thomson—
thought it likely that the electrons were
evenly distributed throughout an atom
filled uniformly with positively charged
material.
Known as “plum pudding” model.
Atomic Structure

Since the mass of an atom is very small
chemists created a new unit to
describe the mass of atoms

Amu (atomic mass unit) is 1/12 the mass of
a Carbon – 12 atom

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Protons are 1.007276 amu
Neutrons are 1.008665 amu
However we round each one to 1 amu
Atomic Structure
The table below summarizes the
properties of subatomic particles.
Properties of Subatomic Particles
Particle
Symbol
Relative
charge
Relative mass (mass
of proton = 1)
Actual mass (g)
Electron
e–
1–
1/1840
9.11  10–28
Proton
p+
1+
1
1.67  10–24
Neutron
n0
0
1
1.67  10–24
Although protons and neutrons are extremely small, theoretical
physicists believe that they are composed of yet smaller subnuclear
particles called quarks.
Atomic Structure

Elements are different because they contain
different numbers of protons.

The atomic number identifies an element.
Atomic number = protons

Mass number = protons + neutrons
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The total number of protons and neutrons in
an atom is called the mass number.
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Remember :Electrons are so much less massive than
protons and neutrons, so their mass is not included in
atomic mass
The mass listed in the periodic table is the
average atomic mass

It is a weighted average of the atomic masses of
naturally occurring isotopes
Atomic Structure

Remember that atoms are electrically
neutral.
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In an atom, protons = electrons
Protons, neutrons, and electrons can be
calculated from atomic number and
mass number.
How many protons, electrons, and
neutrons are in each atom?
boron, fluorine, sodium
Atomic Structure

Isotopes are atoms of the same
element with different masses
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Neutrons were actually discovered due to
isotopes
In an isotope the number of protons and
electrons are the same – only the neutrons
differ
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For example, hydrogen has 3 isotopes:
Isotope
Proton
Neutron
Electron
Hydrogen - 1
Hydrogen – 2
Hydrogen - 3
Note that the correct way to write an
isotope is to write the name, followed by
the mass number.
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
Another way to specify an isotope is to
use the nuclear symbol.

The nuclear symbol includes the chemical
symbol, atomic number, and atomic mass.
Write the nuclear symbol for boron,
fluorine, and sodium
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
The value shown in the periodic table is
the average atomic mass
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It is a weighted average
For example: Chlorine has two isotopes
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Chlorine-35 and Chlorine-37
The abundance is
Cl-35 has an amu of 34.9689 with an
abundance of 75.771%
 Cl-37 has an amu of 36.9659 with an
abundance of 24.229%

Atomic Structure
 Although
Thomson and other
scientists answered some
questions about the atom, many
still remained
 In the early 1900’s, Ernest
Rutherford performed the “Gold
Foil Experiment”

He observed that the alpha particles
went through the gold foil
Atomic Structure

The Rutherford atomic model is known
as the nuclear atom.
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His model inferred that the atom was
almost entirely empty space
Also concluded that the nucleus was
extremely tiny and contained the protons
and the neutrons
The electrons are distributed around the
nucleus and occupy almost all the volume
of the atom.
Atomic Structure

According to this model, the nucleus is
tiny and densely packed compared with
the atom as a whole.

If an atom were the size of a football
stadium, the nucleus would be about the
size of a marble.