Atomic Structure

Atomic Structure

1. Developing the Model of the Atom

   • 1804
     • John Dalton agreed with Democritus that all matter was made up of ting spheres (atoms) that couldn't be broken up
       • But he believed that different each element was made up of different type of atom
   • nearly 100 years later
     • J.J Thomson discovered the electrons that could be removed from atoms
       • John Dalton wasn't quite right
         • J.J Thomson suggested atoms were spheres of positive charge with negative electrons stuck in them
           • The Plum Pudding model
   • 1909
     • Scientists in Rutherford's lab conducted the alpha scattering experiment
       • By firing Alpha particles at thin Gold foil
         • According to the plum pudding model, they expected the alpha particles to pass straight through the gold foil, or only be slightly be deflected
           • Most particles went straight through the thin gold foil, but some were deflected more than expected
             • This proved the Plum Pudding model was not right
               • Because some of the particles were deflected back, scientists realised most of the mass of the atom must be concentrated at the centre in a tiny nucleus
                 • The nucleus must have had a positive charge since it repelled the positive alpha particles.
                 • They also realised that most of the atom was empty space because most of the alpha particles passed straight through
                   • Nuclear Model
                     • A positively charged nucleus surrounded by a cloud of negative elctrons
                       • Neils Bohr said that electrons orbiting the nucleus do so at certain distances called energy levels.
                         • Evidence from further experiments changed the nuclear model to have a nucleus made up of a group of particles called (Protons) which all had a positive charge which added up to the overall charge of the nucleus
     • 1932
       • James Chadwick proved the existence of the neutrons, which explained the imbalance between the atomic number and the mass number

2. The current model of the atom

   • The nucleus is tiny but makes up most of the mass of the atom
     • It contains protons and neutrons
       • Neutrons have a neutral charge whilst protons have a positive charge
         • The protons give the nucleus an overall positive charge
   • Rest of the atom is just empty space.
     • Negative electrons orbit the nucleus really fast.
       • they give the atom its overall size
         • The number of protons=the number of electrons, because the protons and electrons have the equal but opposite charge, the charges cancel each other out so the atom has no overall charge.
       • if the electrons gain energy by absorbing EM radiation, they move to a higher energy level, further away from the nucleus
       • If the electrons release EM radiation, they move to a lower energy level, closer to the nucleus

3. Isotopes

   • Isotopes are different forms of the same element
     • Isotopes of an element have the same number of proton but different number of neutrons
       • so they have the same atomic number but different mass number
         • the number of protons is the atomic number of the element
   • All elements have different isotopes, but there are usually one or two which are stable
     • Unstable isotopes tend to decay by radioactive decay
       • Radioactive decay is when unstable isotopes tend to decay into other elements and give out radiation as they try to become more stable

4. Alpha Particle

   • Helium Nuclei
     • Contains 2 protons and 2 neutrons
       • They don't penetrate far into materials
         • They only travel a few cm in the air