atomic structure

• radius of an atom?

  • 0.1nm
  • 1x10*-10m
• what is the basic structure of an atom

  • positively charged nucleus
  • composed of protons and neutrons
  • surrounded by negatively charged electrons
• In an atom the number of electrons is equal to the number of protons in the nucleus.
•  Atoms have no overall electrical charge.
• number of protons = atomic number
• total number of protons and neutrons = mass number
• Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element
• plum pudding model+ who?

  • Thompson
  • sphere of positive charge
  • randomly arranged electrons within it
• results of alpha particle scattering experiment explained

  • atom is mostly empty space so they passed through
  • atoms contains nucleus where positive charge and mass is concentrated so can be deflected
• what did Bohr contribute to the history of an atom

  • electrons are located at different distances from the nucleus
  • electromagnetic radiation would be emitted if the electrons moved down an energy level
  • energy would be absorbed when an electron moves up an energy level
• what did Chadwick contribute to the history of an atom


  • discovered neutron
  • said it is located inside nucleus
• nuclear model of an atom

  • shows mass is concentrated at nucleus
  • electrons orbit nucleus at different distances
  • nucleus is positively charged
• what is radioactive decay


  • where an unstable atomic nucleus gives out radiation to become more stable
  • it is a random process
• what is activity 


  • rate at which a source of unstable nuclei decays
  • measured in Bq
• Count-rate is the number of decays recorded each second by a detector (geiger muller tube)
• The nuclear radiation emitted may be

  • an alpha particle (α)
  • a beta particle (β)
  • a gamma ray (γ)
  • a neutron (n)
• what is beta radiation (what, charge, stops it, ionising)


  • electron
  • -1
  • aluminium foil
  • moderately ionising
• what is gamma radiation (what, charge, stops it, ionising)


  • electromagnetic wave
  • 0
  • thick lead
  • weakly ionising
• inversely proportional

  2 variables can be inversely proportional if when one variable doubles the other one halves
• what symbol is alpha radiation represented by


  4

  He

  2
• what symbol is beta radiation represented by

  0

  e

  -1
• what is half life 


  • time it takes for the number of nuclei of the isotope in a sample to halve
  • time it takes for the count rate (or activity) from a sample containing the isotope to fall to half its initial level
• contamination


  • unwanted presence of materials containing radioactive atoms on other materials
• irradiation


  • process of exposing an object to nuclear radiation
  • the irradiated object does not become radioactive.
• sources of background radiation

  • rocks
  • radon gas
  • cosmic radiation
  • nuclear weapon testing
  • nuclear accidents
• uses of nuclear radiation

  • exploration of internal organs
  • control or destruction of unwanted tissue
• nuclear fission


  • nucleus absorbs neutron
  • splitting of a large and unstable nucleus
  •  into two smaller nuclei
  • emits two or three neutrons plus gamma rays
  • energy is released
  • chain reaction occurs
  • nuclear accidents occur when chain reaction is not controlled
• nuclear fusion 


  • joining of two light nuclei to form a heavier nucleus
  • some of the mass may be converted into the energy of radiation