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Atoms and Their
Structure
Objective
1) Explain Thompsons Cathode ray
experiments and atomic model
2) Explain Milikan’s Oil
3) Name important atomic scientists
History of the Atom
Democritus
All matter is composed of very small,
indivisible particles called a-tomos. 400BC

Who’s Next?

John Dalton (17661844), an English
school teacher and
chemist, studied
the results of
experiments by
other scientists.
Dalton’s Atomic Theory
 John Dalton atomic theory of matter 1803.
 With only a few changes over the years,
Dalton’s theory 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 or destroyed.
Parts of the Atom

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,
discovered that
this solid-ball
model was not
accurate.
Parts of the Atom
 Thomson’s experiments used a
cathode ray tube.
 It is a vacuum tube - all the air has been
pumped out.
Where did Cathode Ray Tubes
(CRT) come from?
Started with Geissler tubes
Crooke’s Tubes
Extremely Low
Pressure
Crookes Tubes
Enter J.J. Thomson
 In 1897 he showed that cathode
rays consist of particles electrons - that conduct
electricity. Thomson also
concluded that electrons are part
of atoms.
 He discovered electrons and
managed to estimate it’s
magnitude (amount) by
performing experiments with
charged particles in gases.
So What’s Going On Here???
 Problem 1: We know the
CRT ray is negative but can
the charge be separated
from the ray?
 Problem 2: Is the CRT ray a
wave or particle?
Voltage source
-
+
Vacuum tube
Metal Disks
Voltage source
-
+
 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.
Voltage source
-
+
 When the electrodes are charged, rays
travel in the tube from the negative
electrode, which is the cathode, to the
positive electrode, the anode.
Voltage source
-
+
 Because these rays originate at the
cathode, they are called cathode rays.
Voltage source
-
+
Voltage source
-
+
Voltage source
+

By adding an electric field,
Voltage source
+

The rays bent toward a positively
charged plate and away from a
negatively charged plate.
Voltage source
+

Objects with like charges repel each
other, and objects with unlike charges
attract each other.
Voltage source
+

By adding an electric field it was found
that the moving rays were negative.
Thomson’s Model
Dalton’s model
From Thomson’s
experiments, atoms
were composed of
electrically charged
particles.
Thomson’s Model
 Sketch
Thomson’s
model of the
atom in your
notes.
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.
Millikan’s Oil Drop Experiment

R.A. Millikan found the charge of an
electron to be -1.60 x 10-19 Coulombs in
his famous oil drop experiment.
The Proton
 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 called
protons.
The Proton
 At this point, it seemed that atoms were
made up of equal numbers of electrons
and protons.
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 and
discovered the nucleus.
Rutherford’s Experiment

The experimenters set up a lead-shielded box
containing radioactive polonium, which emitted
a beam of positively charged subatomic
particles through a small hole.
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,
the alpha particles
should go straight
through.
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
 Alpha particles are deflected by the
nucleus if they get close enough it.
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.
 Most of the atom’s mass is located in
the nucleus.

The Nuclear Model of the Atom

Sketch Rutherford’s model of the atom
in your notes.
Isotopes

In 1910, J.J. Thomson discovered that
neon consisted of atoms of two different
masses.
Isotopes
Carbon-12 and
carbon-14 are
isotopes of one
another.
 They are the
same element
with different
masses.

12
14
6
6
12
C
6
Isotopes
Atoms of an element that are chemically
alike but differ in mass are called
isotopes of the element.
 Because of the discovery of isotopes,
scientists hypothesized that atoms
contained still a third type of particle that
explained these differences in mass.

The Neutron

Isotopes of an element differ in the
number of the subatomic particle called
neutrons.
The Neutron
Calculations showed that the neutron
should have a mass equal to that of a
proton but no electrical charge.
 The existence of this neutral particle,
called a neutron, was confirmed in the
early 1930s.
 James Chadwick is given credit for
discovering the neutron.

Subatomic Particles
Name
Symbol Charge
Relative
mass
Electron
e-
-1
1/2000
Proton
p+
+1
1
Neutron
n0
0
1
ALMOST EVERYTHING
we do in this class
will only deal with
these 3 particles
PROTON
ELECTRON
NEUTRON
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.
 The mass number (A) is the atomic
mass rounded to a whole number.

Modern View of the Atom
The atom has two regions and is
3-dimensional.
 The nucleus is at the center and
contains the protons and neutrons.

AMU
Atomic mass unit
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.
Model of Atoms
Draw this diagram in your notes.
Atomic Number
The atomic number (Z) 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 atoms have no overall
electrical charge, an 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.

SYMBOLS
Draw the following in your notes.
108
Ag
47

The smaller number is the atomic
number. The atomic number equals the
number of protons and the number of
electrons in neutral atoms.
108
Ag
47

The larger number is the mass number.
The mass number equals the number of
protons PLUS the number of neutrons.
mass # - atomic # = # neutrons
108
Ag
47

47 protons,
Silver-108 has ____
47 electrons, and _____
61 neutrons.
____
Model of a Hydrogen Atom

Hydrogen has _____
one proton,
one electron, and _____
zero neutrons.
_____
Naming Isotopes

Put the mass number after the symbol
of the element.
 Carbon – 12.000000 amu
C-12
 Carbon
– 14.003242 amu
C-14
 Lead
– 209.98418 amu
Pb-210
 Lead
– 211.99188 amu
Pb-212
Model of a Helium Atom

Helium has _____
two protons,
two electrons, and _____
two neutrons.
_____
Model of a Boron Atom
5 p+
6 no

Boron has _____
five protons, _____
five
six neutrons.
electrons and _____
Model of a Carbon Atom

Carbon has _____
six protons, _____
six
six neutrons.
electrons and _____
Isotopes

Remember, isotopes of an element have
different mass numbers because they
have different numbers of neutrons, but
they all have the same atomic number.
Isotopes
Subtract the atomic number from the
mass number to determine the number
of neutrons.
 How many neutrons are in each lithium
isotope below?
4 neutrons

3
neutrons
5
neutrons
Information in the Periodic Table

The average atomic mass is the
weighted average mass of all the
naturally occurring isotopes of that
element.
Average Atomic Mass

The average atomic mass of the
element is the sum of the mass
contributions of each isotope.
SYMBOLS
Write the complete symbol for sodium.
23
Na
11

To write the complete symbol for an
element, the atomic number will go in the
lower left corner of the element symbol.
Write the complete symbol for magnesium-26.
26
Mg
12

To write the complete symbol for an
element, the atomic number will go in the
lower left corner of the element symbol.
Symbols Example
19
9

F
Determine the complete symbol for a
fluorine atom with a mass number of 19.
Symbols Example
1) 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 Example
80
35

Br
Determine the complete symbol for a
bromine atom with a mass number of
80.
Symbols Problem
2) 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
3) If an 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
34
Se
Symbols Problem
4) If an 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
91
Pa
Symbols Problem
5) If an 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
(78)
195
78
Pt
EXIT TICKET
PLEASE GET OUT A SHEET OF PAPER.
PUT YOUR NAME AT THE TOP.
ON THE FIRST LINE WRITE
EXIT TICKET – SUBATOMIC PARTICLES
AND TODAY’S DATE
1) Fill in the chart below.
Element
Potassium
Argon
# of
# of
# of
Protons Neutrons Electrons
Mass
#
Atomic
#
19
20
19
39
19
18
22
18
40
18
2) Fill in the chart below. (When numbers are
provided, the isotope represented by each
space may NOT be the most common
isotope or the one closest in atomic mass
to the value on the periodic table.)
Element
# of
# of
# of
Protons Neutrons Electrons
Mass
#
Atomic
#
Chlorine
Oxygen
17
20
17
37
17
8
10
8
18
8
End of Day 1