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.