Atomic Structure

• Isotopes are atoms with different numbers of neutrons but the same number of protons, resulting in different masses.
• Mass of one ion = (relative atomic mass x 10^3)/L
• KE = 1/2mv^2
• When 1+ isotopes are accelerated, all isotopes are given the same kinetic energy.
• KEa = KEb
• Stages of TOF Mass Spectrometry:

  1. Ionisation
  2. Acceleration
  3. Ion drift
  4. Ion detection
  5. Data analysis
• Electrospray ionisation:

  • Sample dissolved in volatile polar solvent.
  • Injected through fine hypodermic needle.
  • Tip of needle attached to positive terminal of high voltage supply.
  • Particles gain a proton as they leave the needle, forming 1+ ions.
  • All particles have a mass of Mr+1.

  Used for compounds with high Mr as it prevents fragmentation.
• Electron Impact:

  • Sample vaporised.
  • High energy electrons fired at sample from an electron gun.
  • One electron knocked off of each particle, forming 1+ ions.
  • Ion formed is known as a molecular ion.

  Used for molecules with low Mr, fragmentation occurs.
• Acceleration:

  • Positive ions accelerated with an electric field.
  • All have same kinetic energy.
  • Velocity depends on the ions mass.
• Ion Drift:

  • Positive ions travel through a hole in the negative plate into the flight tube.
  • Flight tube is a fixed distance so particles separated by mass.
  • Time taken for a particle to travel down the flight tube is proportional to the ions mass.
• Detection:

  • Positive ions hit negatively charged plate.
  • Gain an electron to become atoms.
  • This causes a flow of electrons as the ions further behind move forward.
  • Size of current proportional to ion abundance.
• Data Analysis:

  • Computer uses data to form a mass spectrum.
  • Shows m/z (mass to charge ratio) and abundance of each ion that reaches the detector.
  • Electron impact: signal with greatest m/z ratio is the molecular ion and gives the relative molecular mass.
  • Smaller peaks = isotopes/fragmentation.
• Relative Atomic Mass

  The average mass of an atom of an element relative to 1/12 the mass of an atom of Carbon-12.
• Relative Molecular Mass

  The weighted mean mass of a molecule relative to 1/12 the mass of an atom of Carbon-12.
• Relative Isotopic Mass

  The mass of a single isotope of an element relative to 1/12 the mass of an atom of Carbon-12.
• Models of the Atom:

  1. Dalton (hard spheres)
  2. Thompson Plum Pudding (added electron)
  3. Rutherford nuclear model (gold foil, pos. nucleus, empty space)
  4. Bohr planetary model (fixed orbitals)
  5. Pauli exclusion principle (orbitals can have two e- max and opposing spins)
  6. Schrodinger quantum model (clouds of possibility)
  7. Chadwick Modern (neutron)
• When there are two electrons in an orbital, they spin in opposite directions to minimise repulsion.
• Why is the 4s subshell filled before 3d?

  The 3rd and 4th energy levels are so close that they overlap. This is because as the orbitals get further from the nucleus, the distance between them decreases. The 4s subshell therefore sits closer to the nucleus. Hence, it takes less energy to fill the 4s subshell so it is filled first and then electrons are added to the 3d subshell.
• If there are 2 or 3 electrons in a p subshell, they will go into different orbitals to minimise repulsion without filling another shell.
• What are the exceptions to the 4s/3d subshell trend?

  Chromium (has half filled 4s and 3d subshells)

  Copper (3d subshell fills up first)
• Most of the time, the 3d subshell empties before the 4s subshell. The exception to this is d block; in which the 4s subshell empties first.
• Why is it necessary for isotopes to be ionised before being analysed in a TOF mass spectrometer?

  1. ions will interact with and be accelerated by an electric field
  2. ions create a current when hitting the detector
• Ionisation definition

  The process of removing electrons from atoms.