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Atoms and Their
Structure
History of the Atom
Original idea (400 B.C.) came from
Democritus, a Greek philosopher
 Democritus expressed the belief that all
matter is composed of very small, indivisible
particles, which he named atomos.

Who’s Next?

John Dalton (17661844), an English
school teacher and
chemist, studied
the results of
experiments by
other scientists.
Dalton’s Atomic Theory
 Dalton proposed his atomic theory of
matter in 1803.
 Although his theory has been modified
slightly to accommodate new
discoveries, Dalton’s theory was so
insightful that it has remained
essentially intact up to the present time.
Dalton’s Atomic Theory
1. All matter is made of tiny indivisible
particles called atoms.
2. Atoms of the same element are
identical; those of different atoms are
different.
Dalton’s Atomic Theory, cont.
3. Atoms of different elements combine in
whole number ratios to form
compounds
4. Chemical reactions involve the
rearrangement of atoms. No new atoms
are created nor destroyed.
Parts of the Atom
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Because of Dalton’s atomic theory, most
scientists in the 1800s believed that the
atom was like a tiny solid ball that could
not be broken up into parts.
In 1897, a British physicist, J.J. Thomson,
made discoveries that required Dalton’s 1st
postulate to be modified
He discovered that atoms are made of
smaller (subatomic) particles.
Parts of the Atom
 Thomson’s experiments used a cathode
ray tube.
 It is a vacuum tube - all the air has been
pumped out.
Thomson’s Experiment
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Vacuum tube
Metal Disks
Thomson’s Experiment
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 At each end of the tube is a metal
piece called an electrode, which is
connected through the glass to a
metal terminal outside the tube.
Thomson’s Experiment
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 When the electrodes are charged, rays
travel in the tube from the negative
electrode, which is the cathode, to the
positive electrode, the anode.
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 Because these rays originate at the
cathode, they are called cathode rays.
Thomson’s Experiment
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Thomson’s Experiment
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Thomson’s Experiment
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Thomson’s Experiment
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By adding an electric field,
Thomson’s Experiment
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Thomson’s Experiment
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Thomson found that the rays bent
toward a positively charged plate and
away from a negatively charged plate.
Thomson’s Experiment
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He knew that objects with like charges
repel each other, and objects with unlike
charges attract each other.
Thomson’s Experiment
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By adding an electric field he found that
the moving rays were negative.
Thomson’s Experiment
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Thomson concluded that cathode rays are
made up of invisible, negatively charged
particles.
Cathode Ray Tube
Thomson’s Model
 From Thomson’s
experiments,
scientists had to
conclude that
although atoms are
neutral, some of the
subatomic particles
have a charge
(positive or negative).
Thomson’s Model
Matter is not negatively charged, so
atoms can’t be negatively charged
either.
 If atoms contained extremely light,
negatively charged particles, then they
must also contain positively charged
particles — probably with a much
greater mass than electrons.

Thomson’s Model

J.J. Thomson said
the atom was like
plum pudding, a
popular English
dessert.
Thomspon’s Model
 In 1886, scientists discovered that a
cathode-ray tube emitted rays not only
from the cathode but also from the
positively charged anode.
 Years later, scientists determined that
the rays were composed of positively
charged subatomic particles.
Isotopes
In 1910, J.J. Thomson discovered
evidence that atoms of the same type
(elements) can have different masses.
 Neon consisted of atoms of two different
masses.

Isotopes

Atoms of an element that are chemically
alike but differ in mass are called
isotopes of the element.
Ernest Rutherford

In 1909, a team of
scientists led by Ernest
Rutherford in England
carried out the first of
several important
experiments that
revealed an arrangement
far different from the
plum pudding model of
the atom.
Rutherford’s Experiment

The experimenters set up a leadshielded box containing radioactive
polonium, which emitted a beam of
positively charged subatomic particles
through a small hole.
Rutherford’s Experiment

The sheet of gold foil was surrounded
by a screen coated with zinc sulfide,
which glows when struck by the
positively charged particles of the beam.
Lead
block
Polonium
Florescent
Screen
Gold Foil
What Rutherford Expected

The alpha particles to pass through
without changing direction very much.
Because he thought the mass was
evenly distributed in the atom.
What Rutherford Observed
How Rutherford Explained It
To explain the results of the experiment,
Rutherford’s team proposed a new
model of the atom:
 Because most of the particles passed
through the foil, they concluded that the
atom is nearly all empty space.

How Rutherford Explained It

Because so few
particles were
deflected, they
proposed that the
atom has a small,
dense, positively
charged central core,
called a “nucleus.”
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 Alpha particles are deflected by it if they
get close enough to the nucleus.
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The Nuclear Model of the Atom

The new model of the atom as pictured
by Rutherford’s group in 1911 is shown
below.
Explanation of 4 Observations
 Three subatomic particles were
proposed to explain the four
observations made by Thomson and
Rutherford: protons, electrons, and
neutrons.
The Electron
 The first to be discovered, electrons
have a negative charge and almost no
mass (compared to protons and
neutrons).
 Electrons account for the volume of an
atom.
The Proton
 Protons are positively charged,
neutralizing the charge of electrons.
 They have mass, and are located in the
nucleus.
The Neutron
Because of the discovery of isotopes,
scientists predicted that a third particle
would be discovered that exists in the
nucleus, has a mass equal to that of a
proton, but has no charge.
 The existence of this neutral particle,
called a neutron, was confirmed in the
early 1930s.

Subatomic Particles
Name
Symbol Charge
Relative
mass
Electron
e-
-1
1/2000
Proton
p+
+1
1
Neutron
n0
0
1
Modern View of the Atom
The atom has two
regions and is 3dimensional.
 The nucleus is at
the center and
contains the
protons and
neutrons.
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Modern View of the Atom

The electron
cloud is the
region where you
might find an
electron and
most of the
volume of an
atom.
Atomic Number
The atomic number of an element is
the number of protons in the nucleus of
an atom of that element.
 The number of protons determines
identity of an element, as well as many
of its chemical and physical properties.

Atomic Number
Because neutral atoms have no overall
electrical charge, a neutral atom must
have as many electrons as there are
protons in its nucleus.
 Therefore, the atomic number of an
element also tells the number of
electrons in a neutral atom of that
element.
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Masses
The mass of a neutron is almost the
same as the mass of a proton.
 The sum of the protons and neutrons in
the nucleus is the mass number of that
particular atom.
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Isotopes of an element have different
mass numbers because they have
different numbers of neutrons, but they
all have the same atomic number.
Masses
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Isotopes of an element have different
mass numbers because they have
different numbers of neutrons, but they all
have the same atomic number.
Isotopes are always identified by their
mass number, and can be represented in
two ways:
Two equivalent representations of the
carbon – 12 isotope:
C-12
12C
Symbols

Elements can be represented by using
the symbol of the element, the mass
number and the atomic number.
Mass
number
Atomic
number
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X
The mass number is the number of
protons + the number of neutrons.
Symbols

Mass number is represented by the
letter A.
Mass
A
number
Atomic
Z
number
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X
Atomic number is represented by the
letter Z.
Symbols Example

Determine the following for the
fluorine atom depicted below.
a) number of protons (9)
19
b) number of neutrons (10)
c) number of electrons (9)
d) atomic number (9)
e) mass number (19)
F
9
Symbols Problem

Determine the following for the
bromine atom depicted below.
a) number of protons (35)
80
b) number of neutrons (45)
c) number of electrons (35)
d) atomic number (35)
e) mass number (80)
Br
35
Symbols Problem

If a neutral element has an atomic
number of 34 and a mass number of
78 what is the
a) number of protons (34)
b) number of neutrons (44)
c) number of electrons (34)
d) complete symbol
78
(2 ways)
34
Se or Se-78
Symbols Problem

If a neutral element has 91 protons
and 140 neutrons what is the
a) atomic number (91)
b) mass number (231)
c) number of electrons (91)
d) complete symbol
231
(2 ways)
91
Pa or Pa-231
Symbols Problem

If a neutral element has 78 electrons
and 117 neutrons what is the
a) atomic number (78)
b) mass number (195)
c) number of protons
d) complete symbol
195
(2 ways)
78
(78)
Pt or Pt-195
Information in the Periodic Table
The average atomic mass is the
weighted average mass of all the
naturally occurring isotopes of that
element.
 The unit is the Atomic Mass Unit (amu).

Calculating Atomic Mass
Calculating Atomic Mass
Copper exists as a mixture of two
isotopes.
 The lighter isotope (Cu-63), with 29
protons and 34 neutrons, makes up
69.17% of copper atoms.
 The heavier isotope (Cu-65), with 29
protons and 36 neutrons, constitutes the
remaining 30.83% of copper atoms.

Calculating Atomic Mass
To determine the average atomic mass,
first calculate the contribution of each
isotope to the average atomic mass,
being sure to convert each percent to a
fractional abundance.
 Will the average atomic mass of copper
be closer to 63 amu or 65 amu?

Calculating Atomic Mass
Mass contribution = mass of isotope x
abundance of isotope
For Cu-63:
Mass contribution = 62.930 amu x 0.6917 =
43.529 amu
For Cu-65:
Mass contribution = 64.928 amu x 0.3083 =
20.017 amu
Calculating Atomic Mass

The average atomic mass of the
element is the sum of the mass
contributions of each isotope.
Atomic mass Cu = mass contribution Cu-63 +
mass contribution Cu-65
Atomic mass Cu = 43.529 + 20.017 = 63.546 amu
Ions

While changing the number of neutrons
in an atom changes the mass and
creates an isotope, changing the
number of electrons changes the
charge of an atom and creates an ion.
Ions
The charge is the sum of the charges
contributed by the protons and
electrons. Charge = (p+) – (e-)
 The charge of an atom is written as a
superscript on the right side of an
element’s symbol:
Ex. Fluorine has 9 protons with 10
electrons: F1- or simply FSodium has 11 protons with 10
electrons: Na1+or simply Na+

Ions
Write the symbol for the following ions:
1.
oxygen (O) with 10 electrons
2.
beryllium (Be) with 2 electrons
3.
zirconium (Zr) with 37 electrons
3+
Zr
4.
bromine (Br) with 36 electrons
Br-
O22+
Be
Summary of Subatomic Particles
subatomic particle
mass
charge
location
changing the number
creates…
p+
1
+1
nucleus
a different
element
neutron, n0
1
0
nucleus
an isotope
-1
electron
cloud
an ion
proton,
electron,
e-
~0
End of Day 1