The changing atom

The changing atom

1. The greek atom

   • They thought matter was made out of particles.
   • You could only break down matter a certain amount of times, until the atom could not be split any further.

2. John Dalton

   • His atomic theory was...
     • Atoms are tiny particles that make up elements
     • Atoms cannot be divided
     • All atoms of a give element were the same
     • Atoms of one element are different from those of other elements.
   • he developed the first table of atomic masses
   • You can still apply many of his predictions to todays model of the atom.

3. Joseph John (JJ) Thompson

   • Used the cathode ray to discover the electron.
   • He discovered that cathode rays were a stream of particles and every particle had the following properties...
     • They had a negative charge
     • They could be deflected with a magnetic and electric field
     • It had an extremely small mass
   • He concluded that the electrons must have come from within the atoms of the cathode-ray tube's electrodes.
   • He proposed that the atom was made up of negatively charged electrons floating in 'sea' of positiove charge, inspiring the plum pudding model of the atom.

4. Ernest Rutherford

   • Directed α particles at a thin sheet of gold foil., then measured any deflection.
   • He had predicted no deflections at all in the α particles at all. Instead he found that...
     • Most of the particles had not been deflected at all.
     • A very small percentage of particles were deflected through large angles.
     • A very few particles were deflected back towards the source,
   • He then proposed the nuclear atom
     • The positive charge of an atom is concentrated in the centre of the atom (the nucleus)
     • Negative electrons orbit te ncleus.

5. Other changes to the atom

   • Niels Bohr altered Rutherford's model so electrons only followed certain paths and would not spiral into the nucleus.
   • Henry Mosely discovered a ink between X-ray frequencies and an elements atomic number.
   • Louis de Broglie suggested that particles have the properties of waves and particles.
   • Erwin Schrodinger suggested that an electron as wave properties in an atom. He also introduced the idea of electron orbitals.
   • James Chadwick discovered that neutrons were emitted whan light elements were bombarded with α particles.
   • Further theories have been proposed and it is now thought that protons and neutrons are made up of even smaller particles called quarks.