Atomic Structure
History of Atomic Theory
Compiled & created by Ms Shivani Kaur Sihra
Democritus (460 - 370 BC)
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Was the first person to come up with the idea
of atom
Believed that all matter was composed of
indivisible particles he called “ATOMS”
Which is derived from the Greek word
“Atomos” – meaning indivisible
He also believed that different atoms:
 Are different sizes
 Have different properties
Other philosophers of that time did not agree
with his theories.
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.
Dalton’s Atomic Laws
1. Law of Conservation of Mass
Matter cannot be created or
destroyed in any physical or
chemical process, just transferred.
2.
Law of Constant Composition
When atoms combine to form molecules,
the ratio of atoms is constant.
Example – H2O will always have 2 times
as many Hydrogen atoms as Oxygen.
Dalton’s Atomic Laws
3. Law of Multiple Proportions – 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
The Law of Conservation of Mass
•When a chemical reaction occurs, mass
is neither created nor destroyed but only
changed.
JJ Thomson (1856-1940)
• Used cathode rays to
prove that Dalton’s Solidball model could be
broken into smaller
particles
•Thomson is credited
with discovering
electrons
Cathode Ray Tubes
• Cathode rays had been used for
some time before Thompson’s
experiments. A cathode ray tube
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 glow in the tube
which travels from cathode, across the tube to the
anode.
A Maltese cross made of thin mica is placed in the
between the cathode and the walls of the tube. When the
experiment is repeated the glow doe not extend to the
other end. The shadow of the cross is produced behind it
making Thomson believe that the glow was a beam of
light travelling like a wave in a straight line .
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• 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 glow had a stream of
negatively charged particles which he named electrons .
The particulate nature is evident from a modification of the
same experiment where a paddle wheel is placed in the path
of the rays and it starts spinning as if it has been hit by fast
moving particulate bodies.
•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.
Rutherford (1871-1937)
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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
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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
The Gold Foil Experiment
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.
Conclusions from Rutherford’s
Gold Foil Experiment (memorize this!)
THE NUCLEAR MODEL ATOM
• 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.
Bohr (1885-1962)
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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.
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•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
Review
Who is the father of atomic theory?
Dalton
What was the first model of the atom?
Dalton’s Tiny Ball Model
What are Dalton’s 3 Laws?
Law of Conservation of Mass, Law of
Constant Composition, Law of Multiple
Porportion
Review
How were Thomson’s and Dalton’s model
different?
Dalton’s model was 1 sphere that cannot
be divided, Thomson had the plum pudding
where electrons are randomly spread
throughout a positively charged sphere.
What did Thomson find out?
Atoms have electrons, they have a - charge
Review
What were Rutherford’s conclusions from the
Gold Foil Experiment?
 Atom has a positively charged nucleus
 electrons are outside,
 atoms are mostly empty
 Nucleus contains most of the mass.