Atomic structure

• First ionisation energy 


  enthalpy change when 1 mole of electrons are removed from 1 mole of gaseous atoms to form 1 mole of gaseous ions with positive charge
• Why is second ionisation energy larger than first ionisation energy?

  Because more energy is needed to overcome the electrostatic energy between the positive ion and negative electron
• Why has helium the largest ionisation energy?

  Its first electron is the closest to the nucleus, has no shielding effect. Larger IE than Hydrogen because it has 1 more proton
• Why do ionisation energy decrease down a group?

  Atom becomes larger. More shielding between the outer electron and nucleus, so weaker attraction
• Why is there a general increase in first ionisation in a period?

  Same shielding because electron added to the same shell each time. But, number of protons increases, increasing the attraction between the nucleus and outer electrons
• Why has Na have a lower first ionisation energy than neon?

  outer electron in 3s1, higher energy level, further away from the nucleus, increase shielding. So easier to remove.
• Why is there a small drop from Mg to Al?

  outer electron in Al in 3p1 and in 3s2 in Mg. higher energy level so further away, more shielding so easier to remove.
• Why is there a small drop in ionisation energy from P to S?

  electrons pair up causes slight repulsion. Make it easier to be removed
• Electron impact


  • Vaporised sample injected at low pressure
  • electron gun fires high energy electron at the sample and knocks out outer electron
  • Forming positive ions
• Ar


  average mass of 1 atom divided by average mass of 1/12th of a C12 atom
• Mr 


  average mass of 1 molecule compared to average mass of 1/12th of a C12 atom
• Electron impact is used for elements and substances with low molecular mass. Electron impact can cause larger organic molecules to fragment
• electro spray ionisation 


  • sample dissolved in volatile, polar solvent
  • injected through a fine needle at high voltage
  • sample gains a proton forming MH+
  • Solvent evaporates away while MH+ ion move towards negative plate
• Electro spray is used for larger organic molecules, so fragmentation doesn't occur
• Acceleration 


  Positive ion accelerated by electric field to constant kinetic energy
• All particles have have same kinetic energy, velocity depends on its mass. Lighter particles have faster velocity
• smaller m/z will have same kinetic energy as larger m/z and will move faster.

  Heavier particles will take longer. Ions are distinguished by different flight times
• Detection


  Ions reach detector and generate small current. Size of current is proportional to the abundance of the species
• How is current generated?

  Electrons transferring from detector to positive ions
• How are ions detected?

  Ions hits the detector and gain electron. Generating a current as electrons transferred from detector to positive ion. Current is proportional to abundance
• Formula for mass (kg) with Avogadro's constant

  Ar/1000 divide 6.022x10^23
• How can mass spec be used to separate 2 species?

  -Positive ions accelerated by electric field to constant kinetic energy

  -both ions will have the same kinetic energy but the ion with lower m/z, less heavy, move faster

  -ion with lower m/z will reach detector first
• Why is it that species are ions in mass spec?

  So they can interact and be accelerated by electric field and produce a flow of charge
• Chlorine diatomic molecules

  C35 (75%)

  C37 (25%)
• Bromine diatomic molecule

  Br79 (50%)

  Br81 (50%)
• What do peaks represent on mass spec graph?

  -isotopes and its abundance
• How to figure out Mr on mass spec graph?

  -peak furthest to the right = heaviest molecular peak = Mr
• What needs to be done when doing calculations with a mass spectra done with electron spray?

  Mr-1 because have an extra proton