1. CHAPTER 3 – The Structure of the Atom
PART I – Early Theories of Matter

2. Elements of the Ancient Greeks
Ancient Greeks believed that matter was composed of
four elements:
fire, earth, air, water.
elements were linked to their properties.
these beliefs were upheld until 1800’s.
Hot
AIR
FIRE
Dry
Wet
WATER
EARTH
Cold

3. DEMOCRITUS (440 B.C.)
first to propose that matter consisted of particles which were indivisible.
called these particle “atomos” (atoms)
said that matter was composed of empty space through which atoms moved.

4. ARISTOTLE ( B.C.)
one of the most influential Greek philosophers.
did not believe “empty space” could exist.
rejected Democritus’s ideas.
his beliefs went unchallenged for nearly 2000 years.

5. JOHN DALTON (1803)
developed his atomic theory based upon ideas of Democritus and experiments carried out by other scientists.
his atomic theory is considered the basis of our modern atomic theory, thus he is regarded as the “father of the modern atomic theory”.

6. DALTON’S ATOMIC THEORY
1. All matter is composed of atoms. (Democritus)
2. All atoms of a given element are identical in size,
*mass, and chemical properties.
* later proven false when isotopes were discovered.
3. Atoms cannot be *created, *divided into smaller
particles, or destroyed.
4. In a chemical reaction, atoms are separated, combined, or rearranged.
(Law of Conservation of Mass – by Lavoisier)
* can occur in nuclear reactions/true for chemical reactions.
5. Different atoms combine in simple whole-number
ratios to form compounds.
(Law of Definite Proportions – from work by Joseph Proust)

7. J.J. Thomson - 1897 He began a series of cathode ray tube experiments.
He was able to determine the charge to mass ratio of the charged particle. He found the mass to be extremely small compared to the lightest known atom (H).
He concluded atoms must be divisible. He identified the first subatomic particle, the electron.

8. SUBATOMIC PARTICLE DISCOVERIES
I. The electron was discovered using the cathode ray tube.
Metal electrodes are located on each end. of the gas-filled tube.
The cathode is the (-) end and the anode is the (+) end.
A stream of light rays was emitted from the cathode end,
therefore, they were called cathode rays.
Through experiments with this tube, scientists found that
the cathode rays were negatively charged.

9. Thomson cont.
Thomson’s discovery was important because it meant there were particles smaller than the atom.
Thomson knew that most substances in everyday life were neutral.
Since the mass of an electron was so small, what made up the rest of the mass of an atom?
He concluded that there must be both positive and negative charges in an atom.
His idea became known as the plum pudding model.

10. Robert Millikan (1868 –1953)
Determined the charge of the electron with his oil drop experiment.
The charge has been equated to a single unit of negative charge (-1).
He also determined the actual mass of the electron
Small drops of oil, which had picked up extra electrons, were allowed to fall between two electrically charged plates. Millikan monitored the drops, measuring how the voltage on the plates affected their rate of fall. From his data, he calculated the charges on the drops and the the charge of a single electron.

11. Ernest Rutherford (1871 – 1937)
He became interested in how (+) charged alpha particles interacted with solid matter.
His experiment involved bombarding a piece of
gold foil with alpha particle.s
Based on the Thomson model of the atom,
(uniform distribution of (+) and (-) charges) he predicted that most of the particles would pass through with only minor deflections of the alpha particles.
Expected results:

12. Gold Foil Experiment
He expected only minor deflections of the large, positively charged alpha particles.
He also thought since the positive charge within the gold atom was uniformly distributed, that it would not alter the paths of the particles either.

13. Gold Foil Results
He was amazed to discover that a few of the alpha particles were deflected at large angles. Some even came straight back at the source.
He concluded the plum pudding model to be incorrect. He concluded that atoms must have a tiny dense region, centrally located, called the nucleus which contained all the atom’s positive charge and virtually all it’s mass.