Atomic Structure
History of Atomic Theory from
Democritus to Rutherford
Democritus (approx. 400BC)
 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
 Other philosophers of that time did not agree with his
theories.

Democritus model
Not the model of today
John Dalton (early 1800’s)
Dalton’s 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
aka: 6 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: a) size
b) mass
c) chemical properties.
3. All atoms of different elements are different.
4. Atoms are indivisible and indestructible
5. Different atoms combine in simple/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)
6. In a chemical reaction, atoms are separated, combined, or rearranged
A visual of some of Dalton’s postulates
Dalton’s Atomic Model
•Based on Dalton’s Atomic Theory (6 postulates), most scientists
in the 1800s believed that the atom was like
• a tiny solid ball that could not be broken up
•
into parts.
aka billiard ball model
Not the model of today
Merits of Dalton's atomic theory
•The atomic theory explains the laws of chemical combination (the Law of
Constant Composition and the Law of Multiple Proportions).
•Dalton was the first person to recognize a workable distinction between the
fundamental particle of an element (atom) and that of a compound (molecule).
JJ Thomson (1856-1940)
• Used cathode rays to prove that
Dalton’s Solid-ball model could be
broken into smaller particles
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.
A diagram of another cathode ray set up
Cathode Ray Tubes
Thomson put the tube in a magnetic field.
He predicted that the stream would travel in a straight path.
negative plate
-----------------------------
Predicted pathway
Positive plate
++++++++++++++
Actual pathway
• 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 THAT WERE NEGATIVELY
CHARGED these particles are called electrons.
Cathode Ray Tube 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!
Here are some websites regarding the cathode ray
Experiment
some go beyond what you are accountable for
You do not need to visit these websites
http://chemmovies.unl.edu/ChemAnime/CATHRAYD/CAT
HRAYD.html
https://www.youtube.com/watch?v=GzMh4q-2HjM
Thomson’s ATOMIC MODEL known as the Plum Pudding Model
•Thomson’s Plum Pudding model is a + charge sphere that has (- )charged electrons
scattered inside, like “raisins” in “plum pudding”. Helps if you know what plum pudding is
It is also known as the Chocolate Chip Cookie or Blueberry Muffin Model.
Ah now you can relate
It was previously known that atoms are neutral,
therefore, in Thomson’s model
•Since, the atom is neutral
Thomson’s negative
charges aka electrons
Offset the positive charge
of the sphere
•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.
But Not the model of today
Rutherford (1871-1937)
Took Thomson’s Plum Pudding Model and added to it
IT’s all about the GOLD FOIL ( experiment that is)
Rutherford tested Thomson's hypothesis by devising his "gold foil" experiment.
Rutherford reasoned that if Thomson's model was correct then the mass of the atom
was spread out throughout the atom. Then, if he shot high velocity alpha particles
(helium nuclei) at an atom then there would be very little to deflect the alpha particles.
He decided to test this with a thin film of gold atoms.
Rutherford’s experiment set up is below
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.
Goil foil experiment results again
The Gold Foil Experiment Set Up visuals
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.
•
To explain his observations, Rutherford developed a new model
• called the nuclear model by some and sometimes it is called the planetary model
•The electrons orbit the nucleus like the planets revolve around the sun.
•At this point in time ALL he knew was that there was a POSITIVE CENTER CORE
By 1920 Rutherford had refined the concept of the nucleus and concluded that
the nucleus contained positively charged particles called protons so they can
be added to the model
1935 ish Chadwick shows the nucleus has another
subatomic particle THAT HAS NO ELECTRICAL CHARGE
Aka NEUTRAL and has a mass nearly equal to that of a proton
THIS PARTICLE IS CALLED THE NEUTRON
BELOW IS A Rutherford model containing ALL three subatomic
particles discovered at this time
What is wrong with the Rutherford model
SINCE POSITIVE( the nucleus) ATTRACTS NEGATIVE( the electrons)
and moving objects tend to lose energy
–
Why didn’t this happen