Atomic Theory
Mark Jayson G. Ang
History of the Atom
Scientists and Their Contribution to
the Model of an Atom
History of the Atom - Timeline
1766 – 1844
Antoine Lavoisier
Thomson
makesJ.J.
a substantial
the
number discovers
of contributions
electron
and
to the
field of
proposes the
Chemistry
Plum Pudding
Model 1871
in 1897
– 1937
Niels Bohr
proposes
the Bohr
Model in
1913
1887 – 1961
James
Chadwick
discovered
the neutron
in in 1932
1700s
1800s
1900s
460 – 370 BC
0
Democritus
proposes
the 1st atomic
theory
1743 – 1794
Erwin
John Dalton
Ernest Rutherford
Schrodinger
proposes performs
his
the Gold Foil
describes
1891 – 1974
atomic theory
Experiment
in
in 1909
the electron
1803
cloud in 1926
1885 – 1962
Click on picture for more information
1856 – 1940
Democritus
(460 BC – 370 BC)
• Proposed an Atomic Theory
(along with his mentor
Leucippus) which states that
all atoms are small, hard,
indivisible and indestructible
particles made of a single
material formed into
different shapes and sizes.
• Aristotle did not support his
atomic theory
Image taken from: https://reichchemistry.wikispaces.com/T.+Glenn+
Time+Line+Project
Atoms
Atomos
• His theory: Matter could
not be divided into
smaller and smaller
pieces forever, eventually
the smallest possible
piece would be obtained.
• This piece would be
indivisible.
• He named the smallest
piece of matter “atomos,”
meaning “not to be cut.”
Atoms
Atomos
 To Democritus, atoms
were small, hard
particles that were all
made of the same
material but were
different shapes and
sizes.
 Atoms were infinite in
number, always
moving and capable
of joining together.
This theory was ignored and
forgotten for more than 2000
years!
Why?
• The eminent
philosophers
of the time,
Aristotle and
Plato, had a
more
respected,
(and
ultimately
wrong)
theory.
Aristotle and Plato favored the earth, fire, air
and water approach to the nature of matter.
Their ideas held sway because of their
eminence as philosophers. The atomos idea
was buried for approximately 2000 years.
Antoine Lavoisier
(1743 – 1794)
Image taken from:
www.ldeo.columbia.edu/.../v1001/geo
time2.html
 Known as the “Father of
Modern Chemistry”
 Was the first person to
generate a list of thirty-three
elements in his textbook
 Devised the metric system
 Discovered/proposed that
combustion occurs when oxygen
combines with other elements
 Discovered/proposed the Law
of Conservation of Mass (or
Matter) which states, in a
chemical reaction, matter is
neither created nor destroyed
John Dalton
(1766 – 1844)
 In 1803, proposed an Atomic Theory
which states:
o All substances are made of atoms;
atoms are small particles that
cannot be created, divided, or
destroyed.
o Atoms of the same element are
exactly alike, and atoms of
different elements are different
o Atoms join with other atoms to
make new substances
 Calculated the of many various atomic
weights elements
 Was a teacher at a very young age
 Was color blind
Image taken from:
chemistry.about.com/.../JohnDalton.htm
John Dalton
(1766 – 1844)
• Dalton is the “Father of
Atomic Theory”
• Dalton’s ideas were so
brilliant that they have
remained essentially
intact up to the present
time and has only been
slightly corrected.
Dalton’s Atomic Theory (1803)
aka: 5 Postulates
1. All matter is composed of extremely small
particles called atoms. (I agree with Democritus!)
2. All atoms of a given element are
identical, having the same:
- size
- mass
- chemical properties.
3. All atoms of different elements are
different.
Dalton’s Atomic Theory (1803)
aka: 5 Postulates
4. Atoms cannot be created, divided into
smaller particles, or destroyed.
**In a chemical reaction, atoms of different elements
are separated, joined or rearranged. They are
never changed into the atoms of another element.
We will learn more later**
5. Atoms combine in definite whole
number ratios to make compounds
(you can’t have a ½ of a Carbon bonding with
Oxygen; it’s a whole atom or no atom)
Dalton’s Atomic Model
•Based on Dalton’s Atomic
Theory (5 postulates), most
scientists in the 1800s believed
that the atom was like a tiny
solid ball that could not be
broken up into parts.
•Dalton was credited for the
three Atomic Laws that were
proven after his time.
John Dalton
(1766 – 1844)
Daltons Model of atom was accepted
for about 100 years because he used it
to support two fundamental Law of
nature
 Law of Conservation of Mass
 Law of Definite Composition

Image taken from:
chemistry.about.com/.../JohnDalton.htm
Law of Conservation of Mass
 It was discovered by Antoine Laurent
Lavoisier
 The total mass of any chemical or physical
change does not change. The amount of
substance may change, the properties may
change, but the total amount of matter
remains constant.
Law of Conservation of Mass
Ex.
5g of solid butter melts in 5g of liquid buter
63.55g Cu + 16.00g O = 79.55g of CuO
79.55g material before the change =
79.55g material after the change
Law of Definite Composition
 It was formulated by Joseph Proust (17541826)
 When elements combine to form compounds,
they do so in definite proportions (fraction)
by mass.
 The mass fraction is obtained by dividing
the mass of the element by the total mass of
the compound.
 The percent by mass is the mass fraction
expressed as a percentage
Law of Definite Composition
Ex.
Consider the substance of water (H2O)
Analysis by mass
Mass Fraction
2.0g H
2/18 = 0.11 H
16g O
16/18 = 0.89 O
18.0 g by mass
1.00 part
% by mass
0.11 x 100% = 11% H
0.89 x 100% = 89% O
100% by mass
Law of Multiple Proportion
If two elements can combine to form more than
one compound, then the ratio of the second
element combined with a certain mass of the first
element is always a ratio of small whole numbers.
Example:
CO vs. CO2
Formula
Ratio of N:O
J.J. Thomson
(1856 – 1940)
Image taken from:
www.wired.com/.../news/2008/04/d
ayintech_0430
 Proved that an atom can be divided
into smaller parts
 While experimenting with cathoderay tubes, discovered corpuscles,
which were later called electrons
 Stated that the atom is neutral
 In 1897, proposed the Plum Pudding
Model which states that atoms
mostly consist of positively charged
material with negatively charged
particles (electrons) located
throughout the positive material
 Won a Nobel Prize
 Discovered the electrons
Cathode Ray Tubes
• Cathode rays had been used for
some time before Thompson’s
experiments.
• A cathode ray is a tube that has
a piece of metal, called an
electrode, at each end. Each
electrode is connected to a power
source (battery).
• When the power is turned on, the electrodes
become charged and produce a stream of charged
particles. They travel from cathode, across the tube
to the anode.
Cathode Ray Tubes
• Thomson put the tube
in a magnetic field.
He predicted that the
stream would travel in
a straight path.
• Instead, he found that
the path curved away
from a negatively
charged plate and
toward a positively
charged plate
• Why?
-----------------------------
++++++++++++++
•Like charges repel each other,
and objects with unlike charges
attract each other, Thomson
concluded that the stream of
charged particles had electrons
in them.
Cathode Ray Tubes Experiment
•Thompson Concluded:
•Cathode rays are made up of
invisible, negatively charged
particles called Electrons.
•These electrons had to come from the matter (atoms)
of the negative electrode.
•Since the electrodes could be made from a variety of
metals, then all atoms must contain electrons!
Thomson’s Plum Pudding Model
•Thomson’s Plum Pudding model is a +
charge sphere that has (- )charged
electrons scattered inside, like “raisins” in
“plum pudding”.
•Overall, the atom is neutral atom
because the atom had the same number
of positive and negative charges.
•From Thomson’s experiments, scientists concluded that
atoms were not just neutral spheres, but somehow were
composed of electrically charged particles.
•The balance of positive and negative charge supports the
neutral atom.
Ernest Rutherford
(1871 – 1937)
 In 1909, performed the Gold
Foil Experiment. Did extensive
work on radioactivity (alpha &
beta particles, gamma
rays/waves) and was referred
to as the “Father of Nuclear
Physics”
 Won a Nobel Prize
 Was a student of J.J. Thomson
 Was on the New Zealand $100
bill
Image taken from:
http://www.scientificweb.com/en/Physics/Biographies/Er
nestRutherford.html
Rutherford (1871-1937)
• Took Thomson’s Plum
Pudding Model and added
to it
• Used the “Gold Foil
Experiment” to discover the
existence of:
– An atomic Nucleus
– Protons (in later experiments)
• You must be able to
explain the Gold Foil
Experiment…it will be on
the CST
Gold Foil Experiment
• Rutherford directed a narrow
beam of alpha particles (+
charges) at a thin piece of
gold foil.
• Based on observations from
other experiments involving
alpha particles, he predicted
that the (+) charges would
go through the foil
Results from Gold Foil Experiment
•Rutherford found that every
once and a while, a + particle
was deflected bounced back.
(about 1% of the time)
•Why?
•Because the + charge hit a
central mass of positive charge
and was repelled.
The Gold Foil Experiment
The Nuclear Model of the Atom
•To explain the results of the experiment,
Rutherford’s team proposed a new model of
the atom.
Conclusions from Rutherford’s
Gold Foil Experiment (memorize this!)
• The atom contains a positively charged
“nucleus”
•This nucleus contains almost all of the mass of
the atom, but occupies a very small volume of
the atom.
•The negatively charged electrons occupied most
of the volume of the atom.
• The atom is mostly empty space.
Rutherford’s Planetary Model
• To explain his
observations,
Rutherford developed a
new model
•The electrons orbit
the nucleus like the
planets revolve around
the sun.
Niels Bohr
(1885 – 1962)
Image taken from:
commons.wikimedia.org/wiki/File:Ni
els_Bohr.jpg
 In 1913, proposed the Bohr
Model, which suggests that
electrons travel around the
nucleus of an atom in orbits or
definite paths. Additionally, the
electron can jump from a path in
one level to a path in another
level (depending on their energy)
 Won a Nobel Prize
 Worked with Ernest Rutherford
Bohr (1885-1962)
• Worked in Rutherford’s lab
• Wondered why – electrons
are not attracted to the +
nucleus and cluster around
it
• Disproved Rutherford’s
Planetary Model
• Experimented with light and
its interaction with matter to
develop a new model.
Bohr’s Energy Level Model
Energy Level Model: Electrons are
arranged in circles around the
nucleus. Each circle has a different
energy.
•Electrons are in constant motion,
traveling around the circle at the
speed of light.

•Electrons can “jump” from one circle
to the next
•But they can’t go to the nucleus they
traveling too fast to be fully attracted.
Bohr’s Energy Level Model
 He proposed the following:
1. Protons and neutrons are in the nucleus
2. Electrons can only be certain distances from
the nucleus.
3. The electrons orbit the nucleus at fixed
energy levels.
4. The electrons must absorb or emit a fixed
amount of energy to travel between these
energy levels
Erwin Schrodinger
(1887-1961)
• In 1926, he further explained
the nature of electrons in an
atom by stating that the exact
location of an electron cannot be
stated; therefore, it is more
accurate to view the electrons in
regions called electron clouds;
electron clouds are places where
the electrons are likely to be
found
• Did extensive work on the Wave
formula  Schrodinger equation
• Won a Nobel Prize
Image taken from:
nobelprize.org/.../1933/schrodinger
-bio.html
James Chadwick
(1891 – 1974)
 Realized that the atomic mass of
most elements was double the
number of protons  discovery
of the neutron in 1932
 Worked on the Manhattan
Project
 Worked with Ernest Rutherford
 Won a Nobel Prize
Image taken from:
www.wired.com/.../news/2009/02/d
ayintech_0227
Progression of the Atomic Model
-
-
-
- --+-
-
+
- -
The structure of an atom, according to:
Electron Cloud
Democritus
James
Ernest
Erwin
Neils
Schrodinger
Chadwick
Rutherford
Bohr&
J.J.
Thomson
John Dalton
Atomic mass, Atomic Number,
and Isotopes
Subatomic Particles
Particle
Symbol
Charge
Relative
Mass
Electron
e-
1-
0
Proton
p+
+
1
Neutron
n
0
1
LecturePLUS Timberlake
42
Location of Subatomic Particles
10-13 cm
electrons
protons
nucleus
neutrons
10-8 cm
LecturePLUS Timberlake
43
Atomic Number
Counts the number
of
protons
in an atom
LecturePLUS Timberlake
44
Atomic Number on the Periodic
Table
Atomic Number
Symbol
11
Na
LecturePLUS Timberlake
45
All atoms of an element have
the same number of protons
11 protons
Sodium
11
Na
LecturePLUS Timberlake
46
Learning Check
State the number of protons for atoms of
each of the following:
A. Nitrogen
1) 5 protons
2) 7 protons 3) 14 protons
B. Sulfur
1) 32 protons
2) 16 protons
3) 6 protons
C. Barium
1) 137 protons 2) 81 protons
LecturePLUS Timberlake
3) 56 protons
47
Solution
State the number of protons for atoms of each
of the following:
A. Nitrogen
2) 7 protons
B. Sulfur
2) 16 protons
C. Barium
3) 56 protons
LecturePLUS Timberlake
48
Mass Number
Counts the number
of
protons and neutrons
in an atom
LecturePLUS Timberlake
49
Atomic Symbols
 Show the mass number and atomic number
 Give the symbol of the element
mass number
23 Na
atomic number
sodium-23
11
LecturePLUS Timberlake
50
Number of Electrons
 An atom is neutral
 The net charge is zero
 Number of protons = Number of electrons
 Atomic number = Number of electrons
LecturePLUS Timberlake
51
Number of Neutron
 Mass Number - Atomic number
LecturePLUS Timberlake
52
Subatomic Particles in Some
Atoms
16
O
31
P
65
8
15
30
8 p+
8n
8 e-
15 p+
16 n
15 eLecturePLUS Timberlake
Zn
30 p+
35 n
30 e53
Isotopes
 Atoms with the same number of protons,
but different numbers of neutrons.
 Atoms of the same element (same atomic
number) with different mass numbers
Isotopes of chlorine
35Cl
37Cl
17
17
chlorine - 35
chlorine - 37
LecturePLUS Timberlake
54
Learning Check
Naturally occurring carbon consists of three
isotopes, 12C, 13C, and 14C. State the number of
protons, neutrons, and electrons in each of
these carbon atoms.
12C
13C
14C
6
6
6
#p _______
_______
_______
#n _______
_______
_______
#e _______
_______
_______
LecturePLUS Timberlake
55
Solution
12C
13C
14C
6
6
6
#p
6
6
6
#n
6
7
8
#e
6
6
6
LecturePLUS Timberlake
56
Learning Check
An atom of zinc has a mass number of 65.
A. Number of protons in the zinc atom
1) 30
2) 35
3) 65
B. Number of neutrons in the zinc atom
1) 30
2) 35
3) 65
C. What is the mass number of a zinc isotope
with 37 neutrons?
1) 37
2) LecturePLUS
65
3)
67
Timberlake
57
Solution
An atom of zinc has a mass number of 65.
A. Number of protons in the zinc atom
1) 30
B. Number of neutrons in the zinc atom
2) 35
C. What is the mass number of a zinc isotope
with 37 neutrons?
3) 67
LecturePLUS Timberlake
58
Learning Check
Write the atomic symbols for atoms with
the following:
A. 8 p+, 8 n, 8 e-
___________
B. 17p+, 20n, 17e-
___________
C. 47p+, 60 n, 47 e-
___________
LecturePLUS Timberlake
59
Solution
16O
A. 8 p+, 8 n, 8 eB. 17p+, 20n, 17e-
8
37Cl
17
107Ag
C. 47p+, 60 n, 47 e47
LecturePLUS Timberlake
60
Atomic Mass on the Periodic
Table
Atomic Number
Symbol
Atomic Mass
11
Na
22.99
LecturePLUS Timberlake
61
Atomic Mass
Atomic mass is the weighted average
mass of all the atomic masses of the
isotopes of that atom.
LecturePLUS Timberlake
62
End of Presentation