Atomic Models

Atomic Models

1. Model has changed throughout history

   • ancient Greeks- everything made from invisible particles
     • experiments bought forward accepted ideas

2. JJ Thompson- 1897

   • found atoms were not solid and indivisible
   • measurements showed small - charge particles
     • electrons
   • "plum pudding model"
     • electrons floating in a positive sphere

3. Rutherford- 1909

   • conducted "gold foil" experiment
     • alpha particles fired at thin sheet of gold
     • plum pudding would deflect most particles slightly by + charge
     • most passed through, some deflected back.
   • came up with nuclear model
     • tiny + charged nucleus
       • surrounded by - electron cloud
     • most of atom empty space
   • modified many times

4. Bohr

   • 4 basic principles
     • 1- electrons only in fixed orbits
     • 2- each shell has fixed energy
     • electron movement between shells
       • 3/4- emits/absorbs fixed freq of EM radiation
   • explained why noble gases are inert
   • shells can only hold fixed numbers of electrons
     • full shell- stable- will not react

5. more than 1 model in use

   • most accurate model uses quantum mechanics
   • Bohr's model easiest to understand

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