2.  Early ideas about atoms:
 2500 years ago (~400 BC)
 Democritus
 Matter could NOT be divided forever
 Eventually the smallest piece would be obtained
 ATOMOS = “uncuttable”

3.  To Democritus, atoms were small,
hard particles that were all made
of the same material but were
different shapes and sizes.
 Atoms were infinite in number,
always moving and capable of
joining together.
 Theory ignored for 2000 years!

4.  Dalton’s Model of the Atom (~1800)
 Elements are composed of atoms.
 Atoms of the same element are exactly alike.
 Atoms of different elements are different.
 Compounds are formed by the joining of atoms of
two or more elements.

5.  Thomson (~1900)
 Foundation of Modern Chemistry
 Some particles smaller in atom
 “Plum Pudding” model
▪ Positive substance = pudding
▪ Negative electrons = raisins

6.  Rutherford (1911)
 Positive Center = Nucleus
 Negative electrons around it

7.  Bohr + Wave Model (1913)
 Each electron in a specific energy level
 Electrons move in orbits around nucleus.
 Energy level = certain distance from nucleus.
 Impossible to determine exact location of electron
 Probable location based on energy electron has.

8.  Bohr + Wave Model (1913)
 Orbital = Electron Cloud
 Electrons with the lowest energy are found in the
energy level closest to the nucleus
 Electrons with the highest energy are found in the
outermost energy levels, farther from the nucleus.

9. Indivisible Electron Nucleus Orbit
Greek X
Dalton X
Thomson X
Rutherford X X
Bohr X X X
Electron
Cloud
Wave X X X