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Atomic Structure
History of Atomic Theory
Democritus- 400 BC

Greek philosopher
credited for
creating the idea
and naming the
atom
(“indestructible”)

Dalton’s Atomic Theory - 1803
• Believed atoms were
solid balls of matter
that are indestructible.
Dalton’s Atomic Theory
• He believed atoms of one element are
different from atoms of another element but
didn’t know how they were different
J.J. Thomson – 1900
•In 1897, a British physicist, J.J. Thomson, discovered
that this solid-ball model was not accurate.
•Through experiments with a
Cathode Ray Tube, he
identified a subatomic particle
with a negative charge
•Stoney, another scientist,
named it “electron”
Cathode Ray Tube

It is a sealed
glass tube
that is a
vacuum
(meaning all
the air has
been taken
out of it)
Cathode Ray Tube Experiment
•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.
•These electrodes become electrically charged
when they are connected to a high-voltage
electrical 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
Cathode Ray Tube
Experiment
• Because these rays
originate at the
cathode, they are
called cathode rays.
• Thomson found that
the rays bent away
from a negatively
charged plate and
toward a positively
charged plate and
•He knew that objects with like
charges repel each other, and
objects with unlike charges
attract each other.
Cathode Ray Tube Experiment Video Clip
Cathode-Ray Tube
•Thomson concluded that cathode rays are
made up of invisible, negatively charged
particles referred to as electrons.
•These electrons had to come from the matter
(atoms) of the negative electrode.
•Since the electrodes used could be made
from a variety of metals, Thomson
concluded that all atoms must have
electrons!
A New Atomic Model
•From Thomson’s experiments, scientists
had to conclude that atoms were not just
neutral spheres, but somehow were
composed of electrically charged particles.
•Matter is not negatively charged, so
atoms can’t be negatively charged either.
Thomson’s Plum Pudding Model
•Thomson proposed the following model of
the atom
•It is now a positively charged sphere with
electrons imbedded inside, like “raisins” in
“plum pudding”.
•The balance of positive and negative
charge supported the reality of a neutral
atom.
Rutherford - 1911

Using a gold foil experiment, he
discovered and named the nucleus of
the atom.
Rutherford’s Gold Foil Experiment - 1911
•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.
• A source produces
a stream of
positively charged
alpha particles
•The stream is directed straight towards
the gold foil.
• The expectation was the particles would
pass straight through the foil.
• But a small fraction of the alpha
particles were deflected or bounced off
the foil.
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.
The Nuclear Model of the Atom
Conclusions from Rutherford’s
Gold Foil Experiment
• 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 Atomic Model
“Planetary Model”
The electrons orbit the nucleus like the
planets revolve around the sun.
Neils Bohr – 1913
 Neils Bohr (1885-1962), a Danish scientist,
who worked with Rutherford, wondered why the
negatively charged electrons did not collapse
into the positively charge nucleus.
•He proposed that electrons must have
enough energy to keep them in constant
motion around the nucleus.
•Electrons have energy of motion that
enables them to overcome the attraction of
the positive nucleus.
Energy Level Model
•This energy keeps
the electrons moving
around the nucleus.
•Bohr’s view of the
atom, which he
proposed in 1913, was
called the energy level
model.
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
Electron Cloud Model
•As a result of continuing research
throughout the 20th century, scientists
today realize that energy levels are not
neat, planet-like orbits around the
nucleus of an atom.
•Instead, they are spherical regions of
space around the nucleus in which
electrons are most likely to be found.
Electron Cloud Model
•Electrons themselves take up little space but
travel rapidly through the space surrounding the
nucleus.
•These spherical regions where electrons travel
may be depicted as clouds around the nucleus.
•The space around the nucleus of an atom where the
atom’s electrons are found is called the electron cloud.
•Each cloud is determined by the probability of an
electron’s location. An electron will be found in its
cloud about 90% of the time.
Electron Cloud Model
Nucleus
Electron
clouds
No electrons
here.