Main points

• The relative atom mass of a proton is 1
• The relative atom mass of a neutron is 1
• The relative atomic mass of an electron is 1/1840
• The charge of a proton is +1
• The charge of a neutron is 0
• The charge of an electron is -1
• Atoms have a radius of 1 x 10^-10 m
• The radius of a nucleus is 1 x 10^-14 m
• • Energy levels which are further from the nucleus are at a higher energy than those which are closer to the nucleus
  • Electrons can change energy levels if they gain or lose energy
  • If atoms absorb electromagnetic radiation, electrons can jump to higher energy levels
  • When these atoms emit electromagnetic radiation, electrons will fall back to the lower energy level
• Atomic number = The number of protons in the nucleus of an atom
• Mass number = the total number of protons and neutrons in the nucleus of an atom
• Isotopes = forms of an element that have the same number of protons but different numbers of neutrons
• Some isotopes are unstable because their nucleus contains too many neutrons so they will decay by emitting radiation in order to become stable. These are called radioisotopes.
• Ions = when atoms lose or gain electrons from the outer shell to become charged particles
• When atoms lose electrons they become positively charged ions
• When atoms gain electrons they become negatively charged ions
• Order of development of the Model of the Atom -

  1.Democritius vs Aristotle

  2.John Dalton

  3.JJ Thomson

  4.Plum pudding model

  5.Geiger and Marsden

  6.Ernest Rutherford

  7.Neils Bohr

  8.James Chadwick
• Democritus vs Aristotle happened in 440BC which was Ancient Greece
• Democritius believed that everything was made up of tiny spheres and Aristotle believed that everything was made up of earth, wind, water and fire.
• John Dalton believed:

  • Everything was made up of tiny spheres
  • Every element is from the same atom
  • Different elements have different atoms
• JJ Thomson believed:

  • discovered the electron (negative charge)
  • not just spheres but actually have an internal structure
• Plum pudding model:

  • Solid sphere of positive charge with negative electrons embedded
• The evidence for the plum pudding model was:

  • solids cannot be compressed = atoms must be solid
  • Rubbing two solids often produces a static charge = something (electrons) can be transferred as atoms collide
• Geiger and Marsden did the alpha particle scattering experiment
• Alpha particle scattering experiment:

  • alpha particles were fired at a thin sheet of gold (one atom thick)
  • If the plum pudding model was correct, the alpha particles should pass straight through
• Alpha particle scattering experiment results:

  • most went straight through
  • 1 in 10,000 were deflected by large angles
  • 1 in 20,000 were deflected straight back
• Ernest Rutherford:

  • most went straight through = an atom is mostly empty space
  • 1 in 10,000 were deflected by large angles = there is a concentration of positive charge in the atom as like charges repel
  • 1 in 20,000 were deflected straight back = positive charge and mass is concentrated in a small volume in the atom (the nucleus)
• Ernest Rutherford made the nuclear model
• The nuclear model:

  • negative electrons orbiting a positive nucleus but it is mostly empty space
• Niels Bohr:

  • revised Rutherford's nuclear model by stating the electrons orbit the nucleus at specific distances (in different energy levels)
• Niels Bohr made the planetory model
• James Chadwick made the atomic model
• James Chadwick:

  • discovered the neutron because the atomic number is different to its atomic mass
• Present day:

  • Scientific explanations are provisional and become more convincing when predictions based on them are conficed by experiments
• Radioisotope = an isotope that is unstable because its nucleus contains too many neutrons so it will decay by emitting radiation in order to become stable
• An alpha particle is made up of 2 protons and 2 neutrons (helium nucleus)
• A beta particle is a high energy electron from the nucleus
• Gamma radiation is a wave of radiation
• The mass of an alpha particle is 4
• The mass of a beta particles is 1/1840