History of the Atomic Model

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Democritus

460 BC

•Asked the question 'how many times can you break something before it doesn't break anymore?'
•Thought it ended eventually at the smallest possible bit of matter
•Names the basic matter particles atoms.
•He thought the atom was a solid ball
•But nothing progressed because no one cared or believed

Dalton

1800 AC

•Performed experiments using gases and chemicals and proved that atoms do exist
•John Dalton however had no idea of their structure, thought it was the same as democritus’s.
•He worked out atoms due to the evaporation of water and wondered how both water and air could both occupy the same space at the same time
•to establish what effect properties of various individual gases had on the properties of the mixture as a whole he performed various experiments.
•Dalton developed the hypothesis that the sizes of the particles making up various gases must be different.

Thomson

1897

•Discovered the electron and that their charge was negative
•Thomson thought that matter must have a positive charge.
•He experimented using cathode ray tubes.
•He came up with the plum pudding model of the atom.
•Thought the atom was mostly positive and that negative electrons just drifted around the atom
•Thomson’s model then influenced other scientists including his students to make better models.

Rutherford

1909 AC

•Rutherford changed the model of the atom by creating a nucleus.
•Instead of the whole model being occupied by positive model just the middle was.
•He said, “The atom was mostly empty and that the electrons surrounded the positive nucleus.”
•The experiment which was used to create the theory that there is a nucleus in an atom was by measuring the deflection of particles after they were shot through a piece of gold foil. Occasionally through this experiment the particles were scattered at large angles whereas the rest passes straight through with just minimal disturbance. Some particles even bounced directly back.
•This made him believe that most of the foil was made of empty space however there were some tiny, dense clumps of matter throughout.

•This model then influenced his student Bohr to perfect the atomic structure.

Bohr

1913 AC

•Bohr was a Danish scientist who was a student of Rutherford.
•He made a new model which was based
•Bohr changed the orbit of the electron.
•He also created energy levels in the atom (electron shells)
•Created quantum mechanics (new concept regarding energy).
•Bohr accidentally stumbled across Balmer's numerology for the hydrogen spectrum. In a flash Bohr came up with a workable model of the atom.

Chadwick

1932 AC

•James Chadwick came up with the current model.
•Using alpha particles he discovered the neutral atomic particle with a mass close to a proton.
•Therefore he discovered the neutron.
•James Chadwick also worked with Rutherford.