A Pictorial History of Atomic Theory From Ancient Greece to Modern Quantum Physics

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.

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.

John Dalton 1766-1844 • Dalton was a British experimental scientist.

• Dalton’s model of an atom was not that much advanced from the ideas of Democritus.

• He contributed important ideas about atoms.

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.

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.

J.J. Thomson 1856-1940 • Thomson experimented with with particles and found that they did not behave as predicted by Dalton’s idea of an atom. • He discovered the existence of the electron.

Plum Pudding Model 1900 • An atom that was composed of a positively charged sphere and negatively charged particles fit his observations.

Hantaro Nagaoka 1865-1950 • Hantaro Nagaoka was a Japanese physicist. He was educated in Japan and Europe. He taught at the University of Tokyo.

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.

Ernest Rutherford 1871-1937 • Rutherford thought that the atom was mostly empty space – not a huge nucleus.

Planetary Model 1911 • Rutherford still pictured orbiting electrons but randomly in all dimensions around a smaller nucleus.

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.

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.

Planetary with Energy Shells 1913 • He also proposed that the chemical properties of an element are determined by the electrons in the outermost orbit.

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

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.

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.

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.

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

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.