Atomic structure

• Define relative atomic mass:

  Average mass of all isotopes of an element compared to 1/12 of the mass of an atom of Carbon 12
• Relative atomic mass = (mass of isotope x abundance) / sum of masses x abundances
• 4 stage of Mass spectrometry

  1. Ionisation
  2. Acceleration
  3. Ion drift
  4. Detection
• Two types of ionisation in mass spectrometry:

  Electrospray

  Electron impact
• What is electrospray ionisation?


  Sample is dissolved in a solvent and high voltage is added. The high voltage rips of a proton and then the sample become positive. Gentler and prevent fragmentation. This is often used for polymers and biological molecules such as DNA
• What is electron impact ionisation?


  The sample is vapourised and then hit with electrons from an electron gun. This knocks off electrons and forms a positive molecule. Often causing fragmentation
• What happens in acceleration of mass spectrometry?


  Molecules are accelerated so they all have the same kinetic energy. Therefore the energy is the same and the mass dictates the speed. Lighter particles move faster and are detected before heavier particles
• What happens in the detection process of ionisation?


  Ionisation hits a negatively charged plate and this causes a current. The size of the current allows the number of molecules that hit the plate to be measured. This gives abundance of molecules
• Once a sample has passed through a mass spectrometer a spectrum is produced and has these features:

  x axis = mass/charge ratio

  y axis = % abundance

  The largest spike = molecular ion peak which is the m/z charge of the molecule

  Tiny spikes are caused by fragments

  Two spikes close together are isotopes
• First ionisation energy = energy required to remove 1 mole of electrons from 1 mole of gaseous atoms
• Second ionisation energy = energy required to remove an electron from every ion in a mole of unipositive gaseous ion
• Ionisation is a reaction where 1 electron is removed. Meaning there will be one electron in the product of the equation which is on the right hand side
• Ionisation energy are defined as being in the gas state
• S sub-shell = 1 s orbital

  P sub shell = 3 p orbitals

  D sub shell = 5 d orbitals
• Ionisation energy equation

  Where n is the ionisation energy

  x ^ (n-1) (g) --> e- + X ^ n+ (g)
• Factors affecting ionisation:

  Distance to nucleus

  Nuclear charge

  Shielding
• Factors affecting ionisation: Distance to nucleus

  Electrostatic attraction decreases with distance. Meaning less energy is needed as it gets further away
• Factors affecting ionisation: Nuclear charge

  The greater number of protons = greater the attraction of the electrons to the nucleus meaning ionisation energy is higher
• Factors affecting ionisation: Shielding

  Greater number of electron between the nucleus and outer shells and the greater number of electron = lower the ionisation energy
• First ionisation energy increases across period:

  • Atomic radius decreases and the proton number increases across the period so more electrons are attracted to the nucleus
  • Nuclear charge increases across the period
  • Electron are closer to the nucleus so it experiences as greater attraction
• First ionisation energy decrease down the group:

  • Atomic radius increase so the electron is further away from the nucleus so experiences lower attraction
  • Shielding as more electrons between the nucleus and outer electrons
• Successive ionisation energies of an element

  • The successive ionisation energies of an element increase
  • This is because once you have removed the outer electron from an atom, you have formed a positive ion
  • Removing an electron from a positive ion is more difficult than from a neutral atom
  • As more electrons are removed, the attractive forces increase due to decreasing shielding and an increase in the proton to electron ratio
• • The first electron removed has a low IE1 as it is easily removed from the atom due to the spin-pair repulsion of the electrons in the 4s orbital
• • The second electron is more difficult to remove than the first electron as there is no spin-pair repulsion}
• • The third electron is much more difficult to remove than the second one corresponding to the fact that the third electron is in a principal quantum shell which is closer to the nucleus (3p)
• • Removal of the fourth electron is more difficult as the orbital is no longer full, and there is less spin-pair repulsion
  • The graph shows there is a large increase in successive ionisation energy as the electrons are being removed from an increasingly positive ion
  • The big jumps on the graph show the change of shell and the small jumps are the change of subshell 
• • Electrons are arranged around the nucleus in principal energy levels or principal quantum shells
  • Principal quantum numbers (n) are used to number the energy levels or quantum shells
  • The lower the principal quantum number, the closer the shell is to the nucleus
  • So, the first shell which is the closest to the nucleus is n = 1
  • The higher the principal quantum number, the greater the energy of the shell and the further away from the nucleus
• • Each principal quantum number has a fixed number of electrons it can hold
  • n = 1 : up to 2 electrons
  • n = 2 : up to 8 electrons
  • n = 3 : up to 18 electrons
  • n = 4 : up to 32 electrons
• • The s orbitals are spherical
  • The size of the s orbitals increases with increasing shell number
  • E.g. the s orbital of the third quantum shell (n = 3) is bigger than the s orbital of the first quantum shell (n = 1)
• p orbital shape

  • The p orbitals have a dumbbell shape
  • Every shell has three p orbitals except for the first one (n = 1)
  • The p orbitals occupy the x, y and z axes and point at right angles to each other, so are oriented perpendicular to one another
  • The lobes of the p orbitals become larger and longer with increasing shell number
• p orbital shape

  • The p orbitals have a dumbbell shape
  • Every shell has three p orbitals except for the first one (n = 1)
  • The p orbitals occupy the x, y and z axes and point at right angles to each other, so are oriented perpendicular to one another
  • The lobes of the p orbitals become larger and longer with increasing shell number
• Mass number has the symbol A
• Atomic number has the symbol Z
• Isotopes have the same proton number but different mass numbers as they have different of neutrons
• It's the number and arrangements of electrons that decides it chemical properties and therefore isotopes have the same configuration of electron so they have the same chemical properties
• Relative isotopic mass is the mass of an atom of an isotope of an element on a scaler where and atom of carbon 12 is a 12
• The relative molecular mass = the average mass of a molecule on a scale where an atom of carbon 12 is a 12
• Transition metals donate one of their 4s electrons to the 3d sub shell because it makes them more stable
• When metals become ions they lose there 4s electrons before there 3d electrons
• Reason why there is a dip in ionisation energy between group 2 and 3?


  E.g Al outer electron is in the 3p orbital rather than the 3s. The 3p has a slightly higher energy than the 3s orbital so the electron is found further from the nucleus. The 3p orbital has additional shielding provided by the 3s2 electrons. These factors override the increased nuclear charge and cause a dip in ionisation energy