A Pictorial History of Atomic Theory

1
A Pictorial History of Atomic Theory

• From Ancient Greece to Modern Quantum Physics

2
Democritus 460-370 BCE

• Democritus was a Greek philosopher and not an
  experimental scientist.
• He thought that there would be a particle that
  would be indivisible.

3
Ancient Greek Model

• He called this particle atomos or uncuttable.
• It is not a detailed or useful model.
• He thought some were smooth and some were sharp.

4
John Dalton 1766-1844

• Dalton was a British experimental scientist.
• Daltons model of an atom was not that much
  advanced from the ideas of Democritus.
• He contributed important ideas about atoms.

5
Solid Sphere Model

• Atoms cannot be broken into smaller pieces.
• In any element, all the atoms are exactly alike.
• Atoms of 2 or more elements can combine to form
  compounds.

6
Solid Sphere Model

• Atoms of each element have a unique mass.
• The masses of the elements in a compound are
  always in a constant ratio.

7
J.J. Thomson 1856-1940

• Thomson experimented with with particles and
  found that they did not behave as predicted by
  Daltons idea of an atom.
• He discovered the existence of the electron.

8
Plum Pudding Model 1900

• An atom that was composed of a positively charged
  sphere and negatively charged particles fit his
  observations.

9
Hantaro Nagaoka 1865-1950

• Hantaro Nagaoka was a Japanese physicist. He was
  educated in Japan and Europe. He taught at the
  University of Tokyo.

10
Saturnian Model 1904

• He proposed an atom with a large positively
  charged nucleus and negatively charged particles
  orbiting it.
• His idea was like Saturn with a huge planet
  holding orbiting rings.

11
Ernest Rutherford 1871-1937

• Rutherford thought that the atom was mostly empty
  space not a huge nucleus.

12
Planetary Model 1911

• Rutherford still pictured orbiting electrons but
  randomly in all dimensions around a smaller
  nucleus.

13
Niels Bohr 1885-1962

• Bohr was a Danish physicist.
• He found that these models did not explain what
  he was observing.
• They did not explain how energy was absorbed or
  released in chemical reactions.

14
Planetary with Energy Shells 1913

• He proposed that electrons moved from one energy
  shell to another rather than randomly and that
  this explained how the energy is absorbed or
  released.

15
Planetary with Energy Shells 1913

• He also proposed that the chemical properties of
  an element are determined by the electrons in the
  outermost orbit.

16
Many Scientists

• Contributions of many scientists have made
  advancements in our concept of the atom.

17
Electron Cloud Model

• We do not think in terms of an orbital path
  anymore but in orbital regions where there is a
  probability of the electron being.

18
Sir James Chadwick 1891-1974

• Chadwick discovered a missing piece of the puzzle
  in the structure of the atom.
• The weight of the protons and electrons did not
  add up to the total weight of the atom.

19
Neutrons in Nucleus

• Chadwick discovered that there was another
  particle in the nucleus (as massive as the proton
  but without any charge).
• There are usually an equal number of protons and
  neutrons.

20
Neutrons in Nucleus

• Sometimes there are different numbers of neutrons
  than protons. This changes the atomic mass of the
  atom. We call these isotopes.

21
Modern Model of Atom

• Our current understanding is of a complicated
  atomic structure.
• There are many sub atomic particles and forces.
• Electrons travel in orbitals.