Atomic Structure

• An atom is a positively charged nucleus made up of both protons and neutrons surrounded by negatively charged electrons.
• Most of the mass of an atom is concentrated in the nucleus
• The electrons are arranged at different distances from the nucleus
• The electron arrangements may change with:

  • the absorption of electromagnetic radiation
  • by the emission of electromagnetic radiation
• In an atom the number of electrons is equal to the number of protons in the nucleus.
• Atoms have no overall electrical charge.
• The total number of protons and neutrons in an atom is called its mass number
• isotopes are atoms of the same element that have a different number of neutrons
• Atoms turn into positive ions if they lose one or more outer electron
• Before the discovery of the electron, atoms were thought to be tiny spheres that could not be divided.
• The plum pudding model suggested that the atom is a ball of positive charge with negative electrons embedded in it
• The results from the alpha particle scattering experiment led to the conclusion that the mass of an atom was concentrated at the nucleus and that the nucleus was charged
• Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances.
• James Chadwick provided the evidence to show the existence of neutrons within the nucleus
• Some atomic nuclei are unstable. The nucleus gives out radiation as it changes to become more stable. This is a random process called radioactive decay
• Activity is the rate at which a source of unstable nuclei decays.
• Activity is measured in becquerel (Bq)
• Count-rate is the number of decays recorded each second by a detector (eg Geiger-Muller tube)
• an alpha particle consists of two neutrons and two protons, it is the same as a helium nucleus
• • a beta particle is a high speed electron ejected from the nucleus as a neutron turns into a proton
• • a gamma ray is electromagnetic radiation from the nucleus
• Nuclear equations are used to represent radioactive decay.
• alpha decay causes both the mass and charge of the nucleus to decrease
• beta decay does not cause the mass of the nucleus to change but does cause the charge of the nucleus to increase.
• Radioactive decay is random.
• the half life of a radioactive isotope is the time taken for the activity to halve
• Radioactive contamination is the unwanted presence of materials containing radioactive atoms on other materials.
• The hazard from contamination is due to the decay of the contaminating atoms.
• Irradiation is the process of exposing an object to nuclear radiation. The irradiated object does not become radioactive.