P1.1 the particle model

• The three states of matter are solid, liquid and gas.
• Dalton's theory of atoms: all matter is made of atoms. all atoms of the same element are identical. a chemical reaction rearranges the atoms. atoms are spheres that are tiny, hard and indivisible.
• Thomsons cathode ray experiment:

  To test the properties of the particles, Thomson placed two oppositely-charged electric plates around the cathode ray. The cathode ray was deflected away from the negatively-charged electric plate and towards the positively-charged plate. This indicated that the cathode ray was composed of negatively-charged particles.
• • Thompson proposed the Plum Pudding model
  • The atom was thought to consist of negatively charged electrons (the ‘plums’) in a positively charged ‘dough’ or ‘pudding’
• Rutherford's Alpha Particle Scattering Experiment

  • They were directing a beam of alpha particles (He2+ ions) at a thin gold foil
  • They expected the alpha particles to travel through the gold foil, and maybe change direction a small amount
• Rutherford observations:

  • Most of the alpha particles passed straight through the foil
  • Some of the alpha particles changed direction but continued through the foil
  • A few of the alpha particles bounced back off the gold foil
• Rutherford conclusions:

  • atoms are mostly empty space.
  • the nucleus of an atom has a strong positive charge.
  • the atom contains a small, heavy nucleus.
• Rutherford proposed the nuclear model.

  • nearly all the mass is concentrated in the centre of the atom.
  • The nucleus is positively charged
  • Negatively charged electrons orbit the nucleus at a distance

  
• the Bohr model of the atom:

  • Electrons orbit the nucleus at different distances
  • The different orbit distances are called energy levels
• the Bohr model:

  • Up to 2 electrons orbit in the first energy level
  • Up to 8 electrons can orbit in the second energy level
  • Up to 8 electrons can orbit in the third energy level
• protons are positively charged particles with a relative atomic mass of 1
• • Neutrons – no charge, and also with a relative atomic mass of 1
• • Electrons – negative charge with almost no mass (1/2000 the mass of a proton or neutron)
• • Atoms are incredibly small, with a radius of only 1 × 10-10 m
• A group of atoms can bond together to make molecules, such as water
• density = mass/volume
• Gases are less dense than solids because the molecules are more spread out (same mass, over a larger volume)
• • If a material is more dense than water (1000 kg/m3), then it will sink
• • In solids and liquids, the molecules are tightly packed together
  • The difference is, in a liquid, the molecules have enough energy to push past each other
  • As a result of this, the density of solids and liquids are roughly the same
• • In a gas, the molecules are widely separated
  • As a result of this, gases have significantly lower densities than solids or liquids