1. The History and Anatomy of the Atom
Unit Four
The History and Anatomy of the Atom

2. History of the Atom Democritus 460-370 B.C.
He was a pre-Socratic philosopher known as “the laughing philosopher.”
Democritus was extremely wealthy and went around Greece purchasing writings of other philosophers.
He argued that matter consisted of tiny particles called atoms-derived from atomos meaning “indivisible.”

3. History of the Atom Aristotle 384-322 B.C.
He was a Greek philosopher under the tutelage of Plato who eventually taught Alexander the Great.
Contrary to Democritus, he believed that matter consisted of five elements: Earth, Air, Fire, Water, and Aether.

4. History of the Atom

5. History of the Atom John Dalton 1766-1844
He was born into a Quaker family. His father worked as a weaver.
Dalton became a teacher at the ripe age of 15 with his older brother Jonathan.
In 1793, he began teaching at the New College in Manchester.

6. History of the Atom
Beginning in 1787, John Dalton began keeping a meteorological journal of observations. He continued this practice for 57 years; recording over 200,000 observations.
In 1794, Dalton presented his first paper on color-blindness.
Dalton, in 1800, began developing his Atomic Theory based on the law of conservation of mass, the law of definite proportions, and the law of multiple proportions.

7. Dalton’s Atomic Theory 1808
All matter is made of atoms.
Atoms of different elements have different appearance and properties.
Atoms can be rearranged but they cannot be subdivided, created, or destroyed.
Atoms can combine in whole-number ratios to form new compounds.

8. Revision to Dalton’s Theory
Dalton’s model of the atom consisted of electrons embedded within a positive fluid.
Due to later discoveries, Dalton’s view that atoms could not be divided had to be revised.

9. The Structure of the Atom: The Electron
Sir Joseph John Thomson
In the late 1800’s, Thomson was conducting experiments with vacuum tubes called Cathode Ray Tube (CRT’s).
He modified his tube to contain a cathode (-) and an anode (+). Later, he added a paddle wheel and a phosphorus plate.

10. The Structure of the Atom: The Electron
Thomson noticed two things which led to a hypothesis of the electron:
The production of shadows on a plate.
The spinning the wheel within the CRT.

11. The Structure of the Atom: The Mass of the Electron
Robert Millikan
He was a Nobel Laureate in physics because of his discovery of the mass of an electron.
Millikan was the president of Caltech for 24 years.

12. The Structure of the Atom: The Mass of the Electron
In 1909, Millikan conducted an experiment to determine the mass of the electron.
He placed oil drops between an electrical field and determined the precise mass of an electron at 1/1837 the mass of a hydrogen atom or simply zero.

13. The Structure of the Atom: The Charge/Mass of a Proton
Ernest Rutherford
He was a New Zealand born physicist who became known as the father of nuclear physics.
Proposed the orbital view of the atom.

14. The Structure of the Atom: The Charge/Mass of the Proton
Gold Foil Experiment: Alpha particles were aimed at gold foil. Flashes of light were viewed on a detecting screen at random locations.

15. The Structure of the Atom: The Charge/Mass of the Proton
Since the charge of the alpha particles were positive, Rutherford correctly speculated that the nucleus must contain a positive particle.
He referred to this positive particle as a proton and later found the mass to be amu or simply 1.

16. The Structure of the Atom: The Charge/Mass of the Neutron
James Chadwick
An English physicist who won the Nobel prize for discovering the neutron using a similar technique as Rutherford. Later used to split nuclei or fission.
The neutron is neutral and has a mass of amu or simply 1.

17. Nuclear Forces or Strong Forces
The force which holds atoms together is called nuclear or strong forces.
These forces are extremely powerful!

18. Summary of Subatomic Particles
J.J. Thomson-Electron and negative charge.
Millikan-Mass of electron; zero.
Rutherford-Proton in the nucleus with a charge of +1 and a mass of 1.
Chadwick-Neutron in the nucleus with a neutral charge and a mass of 1.

19. Models of the Atom John Dalton J.J. Thomson Rutherford Bohr
Electron Cloud
Billiard Ball Model
Plum-Pudding Model
+ Nucleus with – electrons zipping around
Electrons orbit around nucleus
Electrons exist in probable locations around the nucleus

20. Atomic Number Super easy to remember: ATOMIC # = # OF PROTONS
* Found on the periodic table just above the elemental symbol.

21. Mass Number MASS # = # OF PROTONS + # OF NEUTRONS
Can be found on the periodic table under the elemental symbol. You must round to the nearest whole number. Ex. Carbon is rounded to 12.

22. Isotopes
Isotopes are atoms of the same element that have different numbers of neutrons and thus different mass numbers.
Example: Hydrogen has 3 isotopes; protium, deuterium, and tritium.
***Remember: The number of protons never changes! If you change the number of protons, you change the element.

23. Average Atomic Mass
To calculate the average atomic mass, each exact atomic mass is multiplied by its percent abundance. Then, add the results together and round. See example: Nitrogen
Mass #
Exact Mass
% Abund. 14
99.63 15
0.37

24. Calculating Avg. At. Mass
( ) (0.9963) +
( ) (0.0037) =
Notice that this number is equal to the number on the periodic table!!!