2.1.1 The changing atom

2.1.1 The changing atom

1. Fifth century greek atom

   • Democritus developed the first idea of the atom
   • He suggested you can divide a sample of matter only a certain n. of times
   • Eventually, you end up with a particle which cannot be split further
     • The particle was called "atomos" = indivisible

2. Early 1800s- Daltons atomic theory

   • Dalton developed his atomic theory and stated:
     • Atoms are tiny particles that make up elements
     • Atoms are indivisible
     • All atoms of a given element are the same
     • Atoms of one element are different to those of every other

3. 1897- 1906 - JJ Thompson discovers e-

   • Scientists discovered cathode rays which were emitted from cathode ray tubes
   • Thompson discovered cathode rays were a stream of particles with the properties:
     • Negative charge
     • Deflected by both magnet and electric field
     • V. V. small mass
   • Thompson concluded the rays must have come from within atoms of the electrodes
     • Suggested atoms were made up of negative e- moving in a 'sea' of + charge
       • Model= plum pudding atom
       • Overall - charge = overall + charge = atom is neutral

4. 1909-1911 - Rutherford's gold leaf experiment

   • An experiment was carried out where they directed a-particles towards a sheet of very thin gold foil
   • They measured any deflection of particles and based on the plum pudding model came to a theory that the foil would hardly deflect a-particles
   • The results surprised them:
     • Most particles weren't deflected at all
     • Some particles deflected at large angles
     • V. few deflected back to the source
   • In 1911, a new model was presented based on the results:
     • The + charge of an atom and most of its mass are at the centre in a nucleus
       • The e- orbit the nucleus
         • Most of an atoms volume is the space between e- and nucleus
           • Overall charges must balance

5. 1913- Bohr's planetary model and Moseley's work on atomic numbers

   • Bohr altered Rutherford's model to allow e- to only follow certain paths or they would spiral into the nucleus
     • This was the planetary model where e- orbit the central nucleus 'sun' in shells
       • The model explains some periodic properties:
         • Specrtal lines
           • Energy of e-
   • Moseley discovered a link between x-ray frequencies and an elements atomic number

6. 1918 - Rutherford discovers the proton

   • Rutherford's discovery explained Moseley's finding that an atom's atomic number linked to x-ray freq.

7. 1923-1926- Wave and particle behaviour

   • Broglie suggested particles can have the nature of both wave and particle
   • Schordinger suggested e- have wave like properties and introduced the idea of atomic orbitals

8. 1932 - Chadwick discovers the neutron

   • Chadwick observed a new type of radiation emitted from some elements made of uncharged particles with the same mass of a proton
     • These uncharged particles are known as neutrons due to having no charge

9. Modern day

   • It is thought that protons and neutrons are made up of even smaller particles called quarks

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