3.1.1 Atomic Structure

Cards (29)

  • 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