1.1 atomic structure

• Atomic number (Z)


  Number of protons in the nucleus
• Number of neutrons


  A - Z
• Isotopes


  Atoms with the same number of protons, but different numbers of neutrons
• Isotopes have similar chemical properties because they have the same electronic structure
• Isotopes may have slightly varying physical properties because they have different masses
• Steps in a mass spectrometer

  1. Ionisation

  2. Acceleration

  3. Flight Tube

  4. Detection
• Electron impact ionisation


  A vaporised sample is injected at low pressure, an electron gun fires high energy electrons at the sample, this knocks out an outer electron, forming positive ions with different charges
• Electrospray ionisation


  The sample is dissolved in a volatile, polar solvent, injected through a fine needle giving a fine mist or aerosol, the tip of needle has high voltage, at the tip the sample molecule, M, gains a proton, H+, from the solvent forming MH+
• Electron impact is used for elements and substances with low formula mass, can cause larger organic molecules to fragment
• Electrospray ionisation is used preferably for larger organic molecules, the 'softer' conditions of this technique mean fragmentation does not occur
• Acceleration


  Positive ions are accelerated by an electric field to a constant kinetic energy
• Flight Tube


  The positive ions with smaller m/z values will have the same kinetic energy as those with larger m/z and will move faster, the heavier particles take longer to move through the drift area, the ions are distinguished by different flight times
• Detection


  The ions reach the detector and generate a small current, which is fed to a computer for analysis, the current is produced by electrons transferring from the detector to the positive ions, the size of the current is proportional to the abundance of the species
• Relative atomic mass (RAM)

  Weighted average of all the isotopes, calculated as (isotopic mass x % abundance)/100
• Molecular ion


  The peak with the largest m/z in a mass spectrum, equal to the relative molecular mass (Mr) of the molecule
• Molecular ion (electrospray ionisation)


  The peak equals the mass of the MH+ ion, so Mr = peak mass - 1
• Bohr model of the atom

  • Electrons in spherical orbits, atoms and ions with noble gas electron arrangements should be stable
• 1. level model of the atom

  • Electrons arranged in principle energy levels and sub-energy levels (s, p, d, f), each sub-level has a specific shape and can hold a maximum number of electrons
• Positive ion formation


  Electrons are lost from the outermost shell
• Components of electronic configuration

  • Main energy level number
  • Sub-level name
  • Number of electrons in sub-level
• Fluorine electronic configuration


  • Spin diagrams show electrons in orbitals with arrows representing spin
  • s sub-levels are spherical
  • p sub-levels are shaped like dumbbells
• Filling sub-levels


  Fill each orbital singly before starting to pair up the electrons
• Forming positive ions

  Electrons are lost from the outermost shell
• Positive ion examples

  • Mg2+ is 1s2 2s2 2p6
  • O2- is 1s2 2s2 2p6
• Periodic table blocks

  • s block - outer electron filling s sub-shell
  • p block - outer electron filling p sub-shell
  • d block - outer electron filling d sub-shell
• There is disagreement in the scientific community about the order of filling of 4s and 3d orbitals in d-block elements
• First ionisation energy

  The enthalpy change when one mole of gaseous atoms forms one mole of gaseous ions with a single positive charge
• Second ionisation energy

  The enthalpy change when one mole of gaseous ions with a single positive charge forms one mole of gaseous ions with a double positive charge
• Factors affecting ionisation energy

  • Attraction of the nucleus
  • Distance of electrons from nucleus
  • Shielding of nuclear attraction
• Successive ionisation energies

  Always increase as more electrons are removed
• Reason for increase in successive ionisation energies

  Positive charge on ion increases, attracting remaining electrons more strongly
• Helium has the largest first ionisation energy
• Sodium has a much lower first ionisation energy than neon
• The small drop in first ionisation energy from Mg to Al is due to the outer electrons filling the 3p sub-shell, which is slightly higher in energy and more shielded than the 3s sub-shell
• The small drop in first ionisation energy from P to S is due to the second electron filling the 3p sub-shell, causing repulsion between the electrons
• Lithium has the second largest second ionisation energy, as the second electron is removed from the 1s shell closest to the nucleus with no shielding