history and developments of the atom

• JOHN DALTON


  In the early 19th century, JOHN DALTON proposed that atoms were solid SPHERES, and different spheres represented different ELEMENTS.
• JJ THOMSON


  By 1897, JJ. THOMSON had discovered ELECTRONS through experiments, leading to the "PLUM PUDDING MODEL", which showed the atom as a SPHERE of POSITIVE charge with NEGATIVELY charged ELECTRONS EMBEDDED within it.
• Ernest Rutherford - Alpha particle scattering experiment

  1. Fired alpha particles at gold foil

  2. Observed that most passed through, but some were deflected more than expected

  3. Suggesting a small, dense, positively charged nucleus at the centre of the atom which could repel the positively charged alpha particles.

  4. Nucleus also surrounded by cloud of electrons.
• The Plum Pudding model


  • Does not explain the results of the alpha particle scattering experiment as PPM Showed the whole atom as a ball of positive charge with no empty space
• Rutherford's model

  • Small, dense, positively charged nucleus at the centre of the atom, surrounded by a cloud of electrons
• NIELS BOHR

  • Idea that ELECTRONS ORBIT the nucleus in fixed SHELLS.
  • Bohr's model suggested that electrons could only occupy certain orbits or ENERGY LEVELS, at FIXED DISTANCES from the nucleus.
  • Bohr carried out further experiments which revealed the nucleus could be SUBDIVIDED into a whole number of smaller particles, each particle having the same amount of POSITIVE charge. These were called PROTONS.
• JAMES CHADWICK

  1932, discovered NEUTRONS, neutral particles in the nucleus, solidifying the nuclear model close to what is accepted today.