Atomic structure

• Atomic Structure


  • Atoms are very small with a radius of around 1 x 10 metres
  • Atoms contain a positively charged nucleus made up of protons and neutrons, surrounded by negatively charged electrons
  • Protons have an electrical charge of +1 and electrons have a charge of -1
  • Atoms have equal numbers of electrons and protons, so they have no overall electrical charge
  • The nucleus of the atom contains most of the mass, but its radius is less than 1/10000 of the radius of the atom
  • Electrons are arranged at different distances from the nucleus in different energy levels
  • The energy level of an electron may change when the atom emits or absorbs electromagnetic radiation
  • An atom that loses one of its outer electrons becomes a positive ion, if it gains an extra electron, it becomes a negative ion
• Isotopes


  • All atoms of a particular element have the same number of protons
  • The number of protons in an atom of an element is called its atomic number
  • The total number of protons and neutrons in an atom is called its mass number
  • Atoms of the same element can have different numbers of neutrons, these atoms are called isotopes
  • Isotopes are represented with different mass numbers but the same atomic number
• Plum Pudding Model


  • Depicts the atom as a ball of positive charge with electrons embedded in it like plums in a pudding
  • Challenged by the discovery of the electron and the results of the Rutherford, Geiger, and Marsden experiment
• Rutherford, Geiger, and Marsden Experiment

  1. Bombarded gold foil with alpha particles

  2. Expected results based on the Plum Pudding Model, but observed deflections and reflections of alpha particles led to the conclusion of a central positively charged nucleus surrounded by electrons
• Further Developments in Atomic Structure

  1. Niels Bohr suggested electrons orbit the nucleus at specific distances

  2. Discovery of the proton by experiments led by James Chadwick in 1932

  3. Introduction of the neutron as a neutral particle within the nucleus, refining the nuclear model
• The structure of the atom is an example of a theory that has changed and developed over time through experiment and peer review
• Some atomic nuclei are unstable and emit radiation to become more stable
• Nuclear Decay and Radiation


  • The type of radiation emitted depends on the instability of the nucleus
  • Activity of a radioactive source is measured in becquerels
  • Count-rate is the number of decays recorded each second by a detector
  • Alpha, beta, and gamma radiation are the main types of nuclear radiation
• Alpha radiation is absorbed by a few centimetres of air or a thin sheet of paper
• Beta radiation passes through air and paper but is absorbed by a few millimetres of aluminium
• Alpha is absorbed by a few centimetres of air or a thin sheet of paper
• Beta passes through air and paper but is absorbed by a few millimetres of aluminium
• Each type of radiation consists of different particles or electromagnetic radiation
• All three types of nuclear radiation are ionising - they are capable of knocking electrons from atoms
• Alpha is the most ionising and gamma is the least ionising
• Gamma is very penetrating and needs many centimetres of lead or many metres of concrete to absorb most of it
• Neutron is the fourth type of nuclear radiation, which can be emitted during radioactive decay
• Radioactive contamination is the unwanted presence of materials containing radioactive atoms on other materials
• Decay of contaminating atoms leads to radioactive contamination
• Radioactive contamination does not cause the object to become radioactive
• When using radioactive sources, it is important to protect against unwanted irradiation by using sources of the lowest activity possible for the shortest amount of time possible and using appropriate protective clothing such as a lead apron and not handling sources with bare hands
• Food contaminated with an alpha source would be more hazardous than food contaminated with a gamma source because alpha radiation is more strongly ionising
• An area contaminated with an alpha source would not be dangerous unless it was entered, due to the low penetration of alpha radiation. However, if it was contaminated with a source that emitted gamma radiation, this would irradiate people nearby
• Alpha radiation is more ionising than beta radiation. However, it could be argued that beta radiation is more dangerous
• Radioactive contamination continues to give out radiation until decontaminated. An irradiated object does not become radioactive
• The half-life of a radioactive isotope is the average time it takes for half of the nuclei to decay or the time it takes for the count rate, or activity, of a sample containing the isotope to fall to 50% of its original value
• If the half-life is known, then the activity of a source after a certain amount of time can be calculated
• The half-life of a radioactive isotope is 2 years and the initial activity is 800 Bq. The activity after 6 years would be 100 Bq
• When an alpha particle is emitted, the mass number of the element is reduced by 4 and the atomic number is reduced by 2
• With beta decay, the mass number does not change, but the atomic number is increased by 1
• The emission of a gamma ray does not cause a change in the mass or the charge of the nucleus
• The mass numbers on the right-hand side must add up to the same number as those on the left. The atomic numbers on the right must have the same total as those on the left
• An alpha particle is represented by the symbol He. A beta particle is represented by the symbol je
• Radioactive decay is a random event, meaning it cannot be predicted which nucleus will decay next
• The count rate of a radioactive source takes 4 months to fall from 1000 to 250 Bq. The half-life is 2 months. After 15 years, 1/128 of the original isotope would remain