Atomic Structure

• The atomic number is the number of protons in the nucleus of an atom
• The mass number is the total number of protons and neutrons in the nucleus of an atom
• Isotopes are atoms of the same element with a different number of neutrons
• The relative atomic mass is the average mass of an atom of an element compared to 1/12th the mass of a carbon 12 atom
• Relative isotopic mass is the mass of an atom of an isotope relative to 1/12th of the mass of an atom of carbon-12
• First Ionisation energy is the energy required to remove 1 mole of electrons from 1 mole of atoms in the gaseous state
• RAM = Σ ( mass x abundance ) / total abundance
• Mass spectrometry is used to find the abundance and mass of each isotope in an element to determine the Ar and to find the relative molecular mass of molecules
• In Time of Flight mass spectrometry, particles of a substance are ionised to for positive ions which are accelerated so they have the same kinetic energy. The time taken to travel a fixed distance is used to find the mass of each ion in the sample
• Ionisation is needed so the ion can be accelerated by an electrical field and so the ion can be detected by the negative detector plate (only ions will create a current)
• Electron impact is when high energy electrons from a hot electron are fired at the sample, which knocks of an electron forming a 1+ ion
• Electrospray is where the sample is dissolved in a volatile solvent and injected through a fine hypodermic needle with a high voltage. The particle gains a proton forming MH+
• Electron impact is used for smaller Mr molecules, whereas electrospray is used for larger Mr molecules
• Fragmentation is likely when using larger molecules with electron impact, whereas it is rare in electrospray
• The ions are accelerated using an electrical field so they have the same kinetic energy
• A negatively charged plate is used so the positive ions are attracted to the negative plate by electrostatic attraction
• The velocity of the particles depends on the mass
• KE = 1/2 mv^2
• Lighter particles are faster ( they have a greater velocity )
• time to reach the detector ∝ √ mass of particle
• size of the current ∝ abundance
• The positive ion hits a negatively charged electric plate. When it hits the plate the positive ion is discharged by gaining an electron. This generates a movement of electrons and so a current is produced
• the m/z value of the molecular ion = Mr + 1
• First ionisation decreases down a group because

  • atomic radius increases
  • shielding increases
  • There is a weaker attraction between the nucleus and outer electron
  • So less energy is needed to remove it
• The first ionisation energy increases across a period because

  • nuclear charge increases
  • Shielding stays the same
  • atomic radius increases
  • There is a stronger attraction between the outer electron and the nucleus
  • more energy is needed to remove it
• There is a dip in 1st ionisation energy between group 2 and 3 elements

  This is because the electron is being removed from a higher energy sub shell, which is further from the nucleus and there is more shielding
• There is a dip between group 5 and 6 elements, this is because in group 6, there is a shared pair of electrons which repel, making it easier to remove
• There is a drop in 1st IE at the end of on period and start of another as the electron that's being removed is on the next shell , so there's a large increase in atomic radius, shielding increases , so there's a weaker attraction and less energy is needed to remove it
• Successive ionisation energies gradually increase as electrons are removed as the electron is being removed from a more positive species