3.1.1 Atomic Structure

• Mass spectrometry is a method of analysis which helps us to determine relative atomic masses of unknown substances. It also helps us to determine the abundances of specific isotopes.
• Outline the 4 steps in TOF mass spectrometry.

  1. Ionisation
  2. Acceleration
  3. Ion Drift
  4. Detection
• What are the two types of ionisation that can happen in TOF mass spec?

  • Electrospray Ionisation
  • Electron Impact (preferred method)
• Describe the steps involved in ElectroSpray Ionisation:

  • Dissolve the sample in a polar solvent
  • Push it through a tiny nozzle at high pressure
  • Apply a high voltage
  • Each particle will gain a proton
  • The sample will be a gaseous sample of positive ions
• Describe Electron Impact Ionisation:

  1. Vaporise the sample
  2. Sample is bombarded by high energy electrons
  3. An electron will be removed from each particle.
  4. This will create +1 ions
• Describe the Acceleration phase of TOF Mass Spec:

  • The positive ions move through a negative electric field accelerating toward the negative plate.
  • Lighter ions and ions with a higher charge achieve a higher speed here.
• Describe the Ion Drift phase of Mass Spec:

  • During ion drift the different ions will move across the flight tube.
  • Positive ions with smaller m/z values will have the same KE as those with a large m/z values and will move faster. 
  • The ions are distinguished by different flight times.
• When they hit the detector plate, the positive ions are discharged by gaining electrons from the plate. This generates a movement of electrons and hence an electric current that is measured. The size of the current gives a measure of the number of ions hitting the plate.
• Use electrospray ionisation over electron impact for larger organic molecules  
• If a molecule is put through a mass spectrometer it will often break up and give a series of peaks caused by the fragments
• State the order in which sub shells fill up:

  1s - 2s - 2p - 3s - 3p - 4s - 3d - 4p - 5s - 4d - 5p
• According to the first rule, electrons always enter an empty orbital before they pair up.
• p-orbitals are dumbbell shaped and four of the five d orbitals are cloverleaf shaped.
• An s-orbital is spherical with the nucleus at its centre
• In time of flight Mass Spec, the apparatus is kept under a high vacuum to prevent any ions that are produced from colliding with molecules in the air
• First ionisation energy: the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms
• Second ionsation energy: The energy required to remove 1 mol of electrons from 1 mole of gaseous ions with a  single positive charge
• Dalton's model (1803)

  John Dalton thought that all matter was made of tiny particles called atoms, which he imagined as tiny solid balls.
• Niels Bohr developed Rutherford's nuclear model by proposing that electrons exist in shells. These were orbits with fixed energy When the electrons moved between shells electromagnetic radiation with a fixed energy was emitted or absorbed.
• Why is the Bohr model not fully correct?

  • The lecterns in each shell have slightly different energies
  • The model now includes sub-shells
  • These explain experimental ionisation energy trends
• First ionisation energy: The energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions
• Relative atomic mass of an element: The average mass of an atom relative to the mass of 1/12th of the mass of an atom of Carbon-12
• Mass number of an isotope: The total number of protons and neutrons in the nucleus of an atom
• What does the atomic number tell us about an element? Atomic number = number of protons in an atom
• Which letter is used to represent the atomic number of an atom? Z
• What letter represents mass number? A
• What are isotopes of an element? Different forms of the same element, containing the same number of protons but different numbers of neutrons. They still have the same chemical properties
• Orbital: A region around the nucleus that can hold up to two electrons
• How many orbitals and electrons do these shells contain? a) 1s- 1 orbital, 2 electrons b) 2p- 3 orbitals, 6 electronsc) 3s- 1 orbital, 2 electrons d) 3d- 5 orbitals, 10 electrons e) 4s- 1 orbital, 2 electrons