phy atomic structure

• Radius of an atom

  Around 10^-10m
• Basic structure of an atom

  • A positively charged nucleus composed of both protons and neutrons surrounded by negatively charged electrons
• Most of the mass of an atom is concentrated in the nucleus
• How electron arrangements can change in an atom

  1. Absorption of electromagnetic radiation - move further from nucleus; higher energy level

  2. Emission of electromagnetic radiation - closer to nucleus; lower energy level
• The number of protons in an atom is equal to the number of electrons in the atom
• Mass number

  The total number of protons and neutrons
• How atoms can be represented

  Mass number on top, atomic number under
• What happens if atoms lose one or more outer electrons

  They turn into positive ions
• Isotope

  An atom of the same element with a different number of neutrons
• New scientific evidence may lead to a scientific model being changed or replaced
• How the atomic model has developed

  • Tiny undivisible spheres
  • Plum pudding model
  • Nuclear model
  • Bohr's nuclear model
  • Discovery of protons
  • Discovery of neutrons
• Plum pudding model

  The atom is a ball of positive charge with negative electrons embedded in it
• Nuclear model (alpha scattering experiment)

  • Most particles went straight through - mostly empty space, nucleus small compared to whole atom
  • Some particles deflected several degrees - nucleus positively charged as positive particles were repelled
  • A few particles deflected almost 180 degrees - nucleus contains most of the mass of the atom
• Niels Bohr's nuclear model

  Electrons orbit the nucleus at specific distances - his theoretical calculations agreed with experimental observations
• Proton discovered
• Neutrons discovered
• How long after the nucleus became an accepted scientific idea were neutrons discovered

  20 years
• Radioactive decay

  A random process in which an unstable atomic nucleus give out radiation as it changes to become more stable
• Activity

  The rate at which a source of unstable nuclei decays, measured in bequerel (Bq)
• Count-rate


  The number of decays recorded each second by a detector (eg Geiger-Muller tube)
• Alpha particle

  Two neutrons and two protons, the same as a helium nucleus
• Beta particle


  A high speed electron ejected from the nucleus as a neutron turns into a proton
• Gamma ray


  Electromagentic radiation from the nucleus
• Types of nuclear radiation

  • Alpha particle
  • Beta particle
  • Gamma ray
  • Neutron
• Alpha radiation

  • Absorber materials, range in air, ionising power
• Beta radiation

  • Absorber materials, range in air, ionising power
• Gamma radiation

  • Absorber materials, range in air, ionising power
• Why alpha radiation is used in smoke alarms

  Ionises the air so there is a current; smoke causes a drop in current, triggering the alarm
• Why beta radiation is used in metal foil production

  Detector measure radiation passing through foil; if the foil is too thick, the detector reading drops
• The emission of the different types of nuclear radiation may cause a change in mass and/or the charge of the nucleus
• Half-life

  1. The time it takes for the number of nuclei in a sample for halve

  2. The time it takes for the count rate/activity from a sample containing the isotope to fall to half its initial level
• Equation for the final count rate (remaining number of unstable nuclei) = initial count rate (number of unstable nuclei)/2^n
• Radioactive contamination

  The unwanted presence of materials containing radioactive atoms on other materials
• Hazard of contamination

  The decay of the contaminating atoms
• What affects the level of hazard

  The type of radiation emitted
• Irradiation

  The process of exposing an object to nuclear radiation; the irradiated object does not become radioactive
• Why radiation is dangerous

  Radiation causes ionisation which can damage or kill the cell, causing mutations and possibly cancer
• How workers can reduce their exposure to ionising radiation

  • Keep as far away from source of radiation

  Spend as little time as possible in at-risk areas

  Shield themselves by staying behind thick concrete barriers and/or using thick lead plates
• Why it is important for the findings of studies into the effects of radiation on humans to be published and shared with other scientists
• Background radiation


  Around us all the time