Atomic Models

Atomic Models

1. The Accepted Model of the Atom Has Changed Throughout History

   • Ancient Greeks thought that all matter was made from indivisible particles
     • The Greek word atomos means 'uncuttable'
     • indivisible means cannot be divided
   • at the start of the 19thC John Dalton described atoms as solid spheres.
     • different spheres made up different elements

2. Experimental Evidence Showed that Atoms Weren't solid Spheres

   • 1897 J J Thompson
     • He found out that in the atom there are negatively charged particles
       • He called them 'corpuscles'
     • This new model was called the 'plum pudding model'
       • positively charged sphere with negative electrons embedded in it

3. Rutherford showed that the plum pudding model was wrong

   • 1909 famous gold foil experiment
     • they fired alpha parithcles at an extremely thin sheet of gold
       • they expected most of the alpha particles to be deflected very slightly by the positive pudding that made up most of an atom
       • most of the alpha particles passed straight through the gold atoms and a very small number were deflected backwards
     • the plum pudding model could not be right
   • he came up with the nuclear model
     • there is a tiny positively charged nucleus at the centre of the atom
       • the nucleus is surrounded by a cloud of negative electrons
     • most of the atom is empty space

4. Rutherford's Model was modified several times

   • Henry Moseley discovered that the charge of the nucleus increased from one element to another in units of one
   • Rutherford discovered that there was positively charged particles that he called protons
     • the charges of nuclei of different atoms could then be explained
       • The atoms of different elements have a different number of protons in their nucleus
   • the atoms were heavier then they would be if they just contained protons
     • Rutherford predicted that there were other particles in the nucleus, that had mass but no charge
       • James Chadwick eventually discovered the neutron

5. The Bohr Model was a further improvement

   • Scientists realised that electrons in a cloud around the nucleus of an atom would spiral down into the nucleus causing the atom to collapes
     • electrons can only exist in fixed orbits, or shells, and not anywhere in between.
     • each shell has a fixed energy
     • when an electron moves between shells electromagnetic radiation is emitted or absorbed
     • Because the energy os shells is fixed, the radiation will have a fixed frequency
   • the frequencies of radiation emitted and absorbed by atoms were already known from experiments. the bohr model fitted there observations - it looked good.
   • The Bohr model also explained why some elements (the noble gases) are inert. He said that the shells of an atom can only hold fixed numbers of electrons, and that an element's reactivity is due to its electrons. when an atom has full shells of electrons it is stable and does not react

6. There's more then one model of atomic structure in use today

   • We now know that the Bohr model is not perfact - but it's still widely wsed to describe atoms because it's simple and explains many observations from experiments, like bonding and ionisation energy trends.
   • The most accurate model we have today involves complicated quantum mechanics. Basicaally, you can say how likely it is to be at any particular point in the atom. Oh, and electrons can act as waves as well as particles (but you don't need to worry about the details).

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