Atomic Structure

• relative atomic mass?


  average mass of 1 atom / 1/12 mass of 1 atom of C12
• relative molecular mass? 


  average mass of molecules / 1/12 mass of 1 atom 12C
• what is a time of flight?


  a mass spectrometer that forms ions from the sample and then separating ions according to the ratio of their charge to their mass
• time of flight stages:


  • vacuum
  • ionisation
  • acceleration
  • ion drift
  • detection
  • data analysis
• time of flight stages:


  • vacuum
  • ionisation
  • acceleration
  • ion drift
  • detection
  • data analysis
  • vacuum
  • ionisation
  • acceleration
  • ion drift
  • detection
  • data analysis
• what happens in the vacuum stage in the TOF?


  • apparatus is kept under a high vacuum to prevent ions that are produced colliding with molecules from the air
• what happens in the ionisation stage in TOF?


  • sample is dissolved in a volatile solvent and forced through a fine hallow needle that is connected to the positive terminal of a high volatile supply.
  • this produces tiny positively charged droplets that have gained a proton from the solvent.
  • solvent evaporated from droplets into the vacuum and the droplets get smaller and smaller until they may contain no more than a single positively charged ion.
• what happens in electron impact in the TOF:


  • sample is vaporised and high energy electrons are fired at it from an electron gun, which is a hot wire filament with a current running through it that emits beam of high energy electrons. this knocks off one electron from each particle forming a 1+ ion
• what happens in acceleration in the TOF:


  • positive ions are attracted towards a negative charged plate, and accelerate towards it. higher ions and highly charged ions achieve a higher speed
• what happens in the ion drift in the TOF?


  • ions pass through a hole in a negatively charged plate, forming a beam and travel along a tube, called the flight tube , to a detector
• what happens in the detection stage in TOF?


  • when ions with same charge arrive at the detector, the lighter ones, are first as they have higher velocities. the flight times are recorded. positive ions pick up an electron from the detector, which causes current to flow
• what happens in data analysis stage in TOF?


  • the signal rom the detector is passed to a computer, which generates a mass spectrum
• high resolution mass?


  can measure atomic masses to five decimal places of an atomic mass unit
• low resolution mass?


  can measure atomic masses to one decimal places
• how to work out relative atomic mass?


  (abundance 1 x mass 1 ) + (abundance 2 x mass 2) / 100
• why does atomic orbital of the same energy fill singly before pairing?


  because they repel each other
• ionisation energy?

  energy required to remove electrons from atoms leaving atoms as positive ions
• successive ionisation energy?

  more energy required the more times an electron is required
• why does ionisation energy increase?


  increases across a period as the nuclear charge is increasing and this makes it more difficult to remove an electron
• why does first ionisation energy decrease down the group?


  because outer electrons is in a main level that gets further from nucleus
• why does nuclear charge increase down the group?


  because of the inner electron shielding the positive charge 'felt' by an electron in outer shell is less than the full nuclear charge