Atomic structure

• Atomic number


  Number of protons in nucleus of an atom
• Atomic mass


  Number of protons and neutrons
• Isotopes


  Are atoms of the same element with a different number of neutrons
• Why do isotopes have the same chemical properties?


  Same electron configuration
• What is time of flight Mass Spectrometer used for?

  To calculate the relative atomic mass of an element or the relative molecular mass of a compound
• What are the 4 steps of TOF?

  • Ionisation
  • Acceleration
  • separation of ions
  • detection
• What happens during Electron impact ionisation?


  Sample is injected into spectrometer and vaporised. High energy electrons from an electron gun are fired at the sample

  they knock off an electron from each particle to form positive ions
• Why is electron impact ionisation used?

  It’s for elements and small molecules
• Ionisation equation for ei ionisation

  M (g) -> M+(g) + e-

  state symbols must be included
• What happens during electro spray ionisation?

  Sample is dissolved into a volatile solvent

  and injected through a fine hypodermic needle to give a fine mist

  the tip of the needle is attached to the positive terminal of a high voltage supply

  particles gain a proton (h+ ion) from the solvent to form positive ions
• Why is esi used?

  For bigger molecules
• Esi equation


  M + H+ -> MH+
• Why is a volatile solvent used in esi?

  To ensure all solvent has evaporated before detection.
• Electro spray ionisation: mass of element/compound +1
• What happens during acceleration?

  An electric field is applied to accelerate the positive ions, this gives all the ions with the same charge the same kinetic energy
• Kinetic energy equation


  K.E = 1/2 x m x v^2
• What happens during separation?

  The positive ions travel through the mass spectrometer to the detector. Time of flight of each particle depends on its velocity
• Time of flight = distance (m) / velocity (m/s)
• Lighter ions travel


  Faster and so reach detector first
• Heavier ions travel


  Slower and reach detector last
• What happens during detection?

  A detector records the different ions as they arrive at the detector. At the detector, each positive ion gains an electron which generates an electrical current . M+ + e- -> M. The size of the current is proportional to the abundance of each ion
• Why must the sample be ionised?

  So it can be accelerated and detected
• Why must the whole mass spectrometer be kept under vacuum?

  Prevent collisions with air
• Calculate the mass of an ion in kg

  the mass of one ion = mass number x 10^-3/ 6.022 x 10^23
• Calculate the time of the flight of an ion

  If the time of flight of one ion with mass is known then time of flight of a different ion with mass can be Calculated

  ion 1. Ion 2

  m1/(t1)2. = m2/(t2)2
• Relative atomic mass can be used to calculate time of flight of another ion
• Mass spectrum is a plot of relative abundance against mass to charge ratio (m/z)
• Relative isotopic mass


  Mass of an isotope relative to 1/12th the mass of an atom of carbon-12
• relative atomic mass


  The weighted average mass of all the isotopes relative to 1/12th the mass of an atom of carbon-12
• Calculate relative atomic mass

  Sum of (mass x abundance of each isotope) / total abundance
• Mass spectrometry of molecular elements (diatomic)


  Br2 is diatomic so in mass spectrum, peaks will be seen for both ions of the diatomic molecule and the Br atoms themselves.
• As bromine can exist as 2 isotopes 79Br and 81Br in the ratio 1:1, multiple peaks will be seen in the mass spectrum. So 4 possible ions will be: [79Br - 81Br]+, [81Br-81Br]+, [79Br-79Br]+, [81Br-79Br]+
• Electron impact causes fragmentation causing peaks below m/z value
• Electrospray ionisation is a softer technique and does not cause fragmentation so has fewer peaks
• The electrons in atoms are arranged in shells. Each main shell can hold a max number of electrons . Within in each main shell, there a su shells called s,p,d,f
• Main shell 1 contains sub shell S, max number of electrons?

  2 (2n2)
• Main shell 2, contains sub shell sp, how many electrons?

  8
• Main electron shell 3, sub shells spd, how many e-?

  18
• Main electron shell 4, sub shells spdf, how many e-?

  32
• each sub shell consists of orbitals, each orbital is a region which can hold a maximum of 2 electrons with opposite spins