P4 - Atomic Structure

• Size of an atom

  the radius of an atom is about 0.1 nm (1 × 10-10 m)
• Size of a nucleus

  • the radius of a nucleus (1 × 10-14 m) is less than 1/10,000 of the radius of an atom
• Parts of an atom and their masses

  Nucleus: Protons - Positive charge - Mass of 1

  Nucleus: Neutrons - Neutral charge - Mass of 1

  Shells: Electrons - Negative charge - Mass of almost 0
• Why are electrons found on different shells (Energy levels)?

  The electron arrangements may change with the absorption of electromagnetic radiation (move further from the nucleus; a higher energy level) of by the emission of electromagnetic radiation (move closer to the nucleus; a lower energy level).
• Changes to the atomic model over time

  1897 - Thomson - The discovery of electrons. - Atoms can be broken down into smaller parts. An atom is made of tiny negatively charged electrons dotted about a positively charged sphere like a plum pudding.

  
• Changes to the atomic model over time 2

  1909-1911 - Rutherford (and Geiger and Marsden) - Some positively charged particles fired at gold foil bounced back when they were expected to pass straight through. - Atoms have a central positive nucleus. Most of the mass of an atom is found in the nucleus.

  
• Changes to the particle model over time 3

  1913 -Bohr - In-depth work on Rutherford's model showed it had limitations. The electrons should just spiral in towards the positive nucleus. - Electrons move in fixed orbits, called electron shells, around the nucleus.

  
• Isotopes and their uses

  They are the unstable form of an element. They emit different levels of radiation, which makes them useful in medicine, industry, agriculture, radiopharmaceutical sciences, industrial applications, environmental tracing and biological studies.
• Alpha Particles

  Alpha particles consist of two protons and two neutrons and have a travel range of up to a few centimetres in the air. The process of producing alpha particles is called alpha decay.

  Although these particles can be absorbed by metal foils and tissue paper, they are highly ionising (i.e. they have sufficient energy to interact with electrons and detach them from atoms). Among the three types of radiation, alpha radiation is not only the least penetrating with the shortest range but is also the most ionising form of radiation.

  
• Alpha Decay

  During alpha decay, the nucleon number (sum of the number of protons and neutrons, also called mass number) decreases by four, and the proton number decreases by two. This is the general form of an alpha decay equation, which also shows how alpha particles are represented in isotope notation.
• Beta Radiation

  Beta radiation consists of beta particles, which are fast-moving electrons or positrons ejected from the nucleus during beta decays.

  Beta particles are relatively ionising compared to gamma photons but not as ionising as alpha particles. Beta particles are also moderately penetrating and can pass through paper and very thin metal foils. However, beta particles cannot go through a few millimetres of aluminium.

  
• Beta Decay

  When an electron is emitted, the process is called beta minus decay. It is caused by the disintegration of a neutron into a proton (which stays in the nucleus), an electron, and an antineutrino. As a result, the proton number increases by one, and the nucleon number does not change.

  When a positron is emitted, the process is called beta plus decay. It is caused by the disintegration of a proton into a neutron (which stays in the nucleus), a positron, and a neutrino. As a result, the proton number decreases by one, and the nucleon number does not change.
• Gamma Radiation

  Gamma radiation is a form of high energy (high frequency/short wavelength) electromagnetic radiation.

  Because gamma radiation consists of photons that have no charge, gamma radiation is not very ionising. It also means that gamma radiation beams are not deflected by magnetic fields. Nevertheless, its penetration is much higher than the penetration of alpha and beta radiation. However, thick concrete or a few centimetres of lead can impede gamma rays.

  Gamma radiation contains no massive particles, but, the composition of the atom is bound to change after emitting photons.

  
• Discovery of nuclear radiation

  Marie Curie studied radioactivity (nuclear radiation emission) shortly after another famous scientist named Henri Becquerel discovered spontaneous radioactivity. Curie discovered that uranium and thorium were radioactive through the use of an electrometer that revealed the air around radioactive samples had become charged and conductive.

  Marie Curie also coined the term “radioactivity” after discovering polonium and radium. Her contributions in 1903 and 1911 would receive two Nobel prizes.
• Differences between irradiation and contamination - hazards

  • Irradiation is linked to beta and gamma sources, whilst contamination is linked to alpha sources.
  • Beta and gamma sources can get into the body. Through irradiation, they can use their ability to travel long distances and get inside the body.
  • Alpha sources work in small areas. If an alpha source is in a closed area, then it can cause a lot of harm by contamination. It is very strongly ionising, so it can contaminate a lot of particles in a small space. However, it can't travel that far, so can't use irradiation to enter the body.