Discovery of the Atom

Discovery of the Atom

1. John Dalton

   • Developed an atomic theory in 1800s
   • Worked out atomic weights + invented symbols for atoms and molecules.
   • Conclusions: All matter is made of atoms. Atoms cannot be broken down into anything simpler. All the atoms of a particlular element are identical to each otherand different from the atoms of other elements. Atom are rearranged in a chemical reaction. Compounds are formed when two or more different atoms join together.
   • The theory was developed and changed as new evidence was discovered.

2. JJ Thomson

   • He discovered the electron in 1897
   • It showed that the atom contained smaller pieces which Dalton had not thought was possible

3. Ernest Ruthereford's nuclear atom

   • In 1911, Rutherford used experimental evidence to show that the atom must contain a central nucleas.
   • Further evidence that an atom contained smaller pieces.

4. Bohr's electron orbits

   • He further developed Rutherford's theory.
   • Used experimental evidence to support the idea that electrons occupy particular orbits or shells around the nucleus of an atom.
   • This is an example of how a theory may change as new evidence is found and how a scientific explanation is provisional but may become more convincing when predictions based on it are confirmed later on.
   • Early model -> atom imagined as a sphere of positive charge with negatively charged electrons dotted around inside . -> experiment carried out to support this -> unexpected results.

5. Geiger and Marsden's experiment

   • Rutherford designed an experiment to test this model.
   • It was carried out by his assistants.
   • Beam of alpha particles -> aimed at thin gold foil and their passage through the foil detected.
   • The scientists expected the alpha particles to pass straight through the foil but something also happened.
   • Some of the particles emerged from the foil at different angles and some even came straight back.
   • The scientists realised that the positively charged alpha particles were being repelled and deflected by a tiny concentration of positive charge in the atom.
   • This meant that what they had previously thought the atom was represented by a nuclear model of the atom.