Atom

• Particulate nature of matter

  The idea that materials are made up of small particles
• Diffusion

  1. Spreading of gases

  2. Spreading of particles in a liquid
• Atom


  The tiny particles of which all matter is composed, indivisible
• Dalton's Atomic Theory

  • All matter is made up of very small particles called atoms
  • All atoms are indivisible. They cannot be broken down into simpler particles
• Cathode rays

  Rays coming from the negative electrode (cathode) in a vacuum tube
• Crookes' experiments with cathode rays

  1. Passing electric current through air at low pressure

  2. Cathode rays cast a shadow of a Maltese cross

  3. Cathode rays cause a paddle wheel to turn and move down the tube
• Properties of cathode rays

  • Travel in straight lines
  • Cause glass to fluoresce
  • Possess enough energy to move a paddle wheel
• Electron

  Negatively charged particles that make up cathode rays
• J.J. Thomson is credited with the discovery of the electron
• J.J. Thomson's experiments with cathode rays

  1. Passing cathode rays through a small hole in the positive electrode

  2. Deflecting the beam of electrons with electric and magnetic fields
• Millikan's Oil Drop experiment

  1. Spraying tiny droplets of oil between two charged metal plates

  2. Using X-rays to ionise the air and give the oil droplets a negative charge

  3. Adjusting the charge on the plate to suspend the negatively charged oil droplet in mid-air
• Thomson's Plum Pudding model of the atom

  An atom is a sphere of positive charge with negative electrons embedded in it in random placement
• In 1909 Ernest Rutherford studied the scattering of alpha particles by a very thin sheet of gold
• The electron has a very small mass
• By 1909, the electron had been identified as a negatively charged particle of very small mass
• Thomson's 'Plum Pudding' model of the atom

  An atom is like a sphere of positive charge with electrons embedded in it at random
• Thomson's model did not have any experimental evidence to back it up
• Rutherford's experiment

  Alpha particles were scattered by a thin gold foil
• Rutherford deduced the nucleus had a diameter about 10,000 times smaller than the atom
• Rutherford discovered that positive particles called protons were located in the nucleus
• Rutherford's new model of the atom

  It consisted of a nucleus containing protons, with electrons in an 'electron cloud' surrounding the nucleus
• Chadwick's experiment

  He bombarded beryllium with alpha particles and discovered neutral particles called neutrons
• Neutrons have about the same mass as protons but no charge
• Neutrons help cement the positively charged protons together in the nucleus
• Sub-atomic particles

  • Protons
  • Neutrons
  • Electrons
• Charge of sub-atomic particles

  Proton: +1

  Neutron: 0

  Electron: -1
• Relative mass of sub-atomic particles

  Proton: 1

  Neutron: 1

  Electron: 1/1838
• Protons and neutrons are located in the nucleus, electrons are located outside the nucleus