atomic structure

• sub shells

  
• Proton

  • mass 1
  • charge +1
  • nucleus
• Neutron

  • mass 1
  • No charge
  • nucleus
• Electron

  • mass negligible
  • charge -1
  • Orbits the nucleus
• Atomic Number

  The number of protons in the nucleus
• Mass Number

  The number of protons plus the number of neutrons in the nucleus
• Isotope

  Atoms of the same element that contain different numbers of neutrons
• Chemical Reactivity of Isotopes

  The chemical reactivity of different isotopes of an element is the same because they have the same electronic structure
• Stages of TOF mass Spec

  1. Ionisation

  2. Acceleration

  3. Separation

  4. Detection
• Ionisation Electron Impact

  • Sample bombarded with high energy electrons
  • High Fragmentation (therefore use for low formula mass samples /elements)
• Ionisation Electron Impact

  X(g) → X+(g) + e–
• Ionisation Electrospray

  • Sample dissolved in polar solvent, injected through fine syringe, to make fine mist
  • The syringe tip is connected to high voltage power supply
  • Sample gains H+
  • Low fragmentation: use for large biological molecules
• Ionisation Electrospray


  X(g) + H+ → XH+(g)
• Acceleration

  • Positive ions attracted towards negative electric plate and all ions accelerated to the same kinetic energy
• Separation/ Drift Zone/ flight tube

  • Particles are separated according to their 'time of flight' through the flight tube -which depends on their mass
  • Lighter ions travel fastest and reach the detector in least time
• Detection

  • Positive ions hit the detector plate and gain an electron
  • This generates a movement of electrons (an electric current)
  • The size of the electric current is proportional of the number of ions hitting the detector. Therefore, current is proportional to Relative Abundance
• Units of Time of Flight

  • Time – seconds (s)
  • Kinetic Energy - Joules (J)
  • Mass of one ion– kilograms (kg)
  • Distance/length of flight tube /drift region – metres (m)
  • Velocity – metres per second (ms-1)
• Rearrange KE = mv^2/2

  1. m = 2KE/v^2

  2. v^2 = 2KE/m

  3. v = √(2KE/m)
• Rearrange v = d/t


  1. t = d/v

  2. d = vt
• Ar (calculation)

  The average mass of the atoms of an element relative to 1/12 the mass of one atom of carbon-12

  
• Principle energy levels

  The main energy levels (ie 1,2,3 etc)
• Orbitals

  Regions of space that electrons are most likely to be in
• Electrons in an orbital

  Each orbital contains a maximum of 2 electrons
• Filling of orbitals within a sublevel

  Each orbital within a sub-level is SINGLY OCCUPIED before PAIRING UP
• 4s electron rules for transition metal atoms and ions

  • 4s - first in before 3d (filling for atoms)
  • 4s - first out before 3d (removing for ions)
• Elements that do not follow the 4s rule

  • Copper and Chromium
  • There is added stability of a half full 3d sub level (Chromium)
  • There is added stability of a full 3d sub level (Copper)
• First ionisation energy

  The energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous ions. X(g) -> X+(g) + e-
• Why is the second ionisation energy, and all successive ionisation energies greater than the first ionisation energy

  We are removing a negative electron from an ion that is now positive, so more energy is needed due to the greater attraction between the electron and positive ion
• Factors that first ionisation energy depends on

  • The energy of the electron being removed (average distance from the nucleus)
  • The size of the nuclear charge
  • The shielding effect by the electrons in filled inner shells
• Successive ionisation energies

  Large jumps in successive ionisation energies reveal where electrons are being removed from a lower principal energy level (a new shell closer to the nucleus that has less shielding)
• First ionisation energy generally increases across a period

  Nuclear charge increases, shielding is constant (the electrons are being removed from the same shell)
• Two groups that do not follow the general trend of first ionisation energy increasing across a period are group 3 (e.g. aluminium) and group 6 (e.g. sulfur)
• Why aluminium (group 3) does not follow the trend

  The electron removed in aluminium is from a higher energy 3p sub-level (SLIGHTLY FURTHER FROM THE NUCLEUS), as such the electron is lost easier than the 3s electron that is lost by Magnesium
• Why sulfur (group 6) does not follow the trend

  The lower first ionisation energy for sulfur is because it has a pair of electrons in one of the 3p orbitals. Repulsion between these two electrons makes it easier to remove one of the electrons.
• Trend in first ionisation energy down a group

  First ionisation energy decreases down the group because nuclear charge increases down the group, the electrons are being removed from higher energy levels/electron shells which are further from the nucleus, and there is more shielding by inner electron shells. Outer electrons are less attracted to the nucleus and need less energy to remove.
• Explain why the value you have calculated is slightly different from the relative atomic mass given in the Periodic Table.

  The Ar in the Periodic table takes account of the other isotopes /different amounts of isotopes (
• Sometimes the mass spectrum of has a very small peak/ why why particles with the same mass and velocity can be deflected by different amounts in the same magnetic field.

  Has 2 electrons knocked out / gets a 2+ charge. size of charge is different
• Explain how ions are accelerated, detected and have their abundance determined in a time of flight (TOF) mass spectrometer

  Ions accelerated by attraction to negatively charged plate. Ions are detected by gaining electrons. Abundance is proportional the size of current flowing (or amount of electrons gained) in the detector