Atomic Theory in a Nutshell
440BC Democritus – “atomos” – everything made up of tiny building blocks
350BC Aristotle – everything made up of 4 elements – earth, air, fire, water
Alchemists – mixed different materials together to make new materials.
Discovered medicines, acids and tried to turn materials into gold.
Discovered
“pure” substances
and said there are a lot more than just 4 elements. Were first to id the chem. and phy properties of elements
1803 John Dalton –
“Father of Modern Chemistry” – atoms are like solid billiard balls – atoms of the same element are the same; atoms of different elements are different
1900 J. J. Thomson – up with the
Worked with the CRT (cathode ray tube) and came
“Plum Pudding Model” – negative and a positive
found the atom had 2 parts – a
1909 Ernie Rutherford – Made a radio active gun and shot radiation at a sheet of gold foil. Came up with the Solar System Model – the atom is mostly empty space with a positive center and negative particles orbiting around it like planets around the sun.
1913 Neils Bohr – Electron Cloud Model said the particles are so tiny and moving so quickly that we can’t predict where they are – we can only guess where they are likely to be. Electron Cloud Model sees a atom with a solid center and a fuzzy area with negative particles around it.
This is the model we use today.
1920-30 Schroedinger/Heisenberg Electron Cloud Model…Said the particles are so tiney and moving so quickly that we can’t predict where they are- we can only guess where they are likely to be. Electron Cloud Model sees and atom with a solid center and a fuzzy area with negative particles around it. This is the model we use today.

1932 James Chadwick – Discovered the neutron.
By 1932 we knew the
Atom had 2 parts ( protons, located in the nucleus or center of the atom and electrons, located outside the nucleus ). The electrons were kept in place by the protons as opposite charges attract . But what kept the protons together inside the nucleus? They should have repelled each other. By studying the weight of atoms, he found that protons were held in place by another particle he called “neutron” as they were “neutral” – they had no charge. For every proton there was a neutron that separated it from another neutron keeping the neutrons in place inside the nucleus