Atomic Structure and The Periodic Table

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Mass Spectrometry

A mass spectrometer measures the mass of positive ions formed from atoms:

  • The sample is vapourised.
  • The vapour is ionised by bombardment with high energy electrons. Electrons are knocked out of the atoms. 
  • The positive ions are accelerated by an electric field.
  • The velocitites of the ions are equalised by a velocity selector.
  • The ions are deflected in a magnetic field.
  • The feild is steadily increased so that only ions of a particular mass/charge ratio pass through and reach the detector at any one time.
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Mass Spectrometry (cont)

  • Most of the ions formed have a +1 charge, so the charge/mass ratio corresponds to the mass of the ion
  • A mass spectrum (graph) shows us the masses of the +1 ions detected and their relative abundances.
  • This is the isotopic composition of an element, which can be used to calculate the relative atomic mass
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Mass Spectrometry (cont)

  • The mass spectrum of a compound can be used to work out the relative molecular mass of that compound.
  • When the vapourised sample is bombarded with electrons, as well as being ionised the molecule (compound) is fragmented
  • The fragments produced all have different masses, and each fragment corresponds to a peak in the mass spectrum.
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Mass Spectrometry (cont)

  • The peak with the largest charge/mass value (NOT tallest) is due to the molecular ion (parent ion).
  • The molecular ion is formed when the original molecule loses just one electron and is not fragmented. Therefore its m/z value is the relative molecular mass of that compound. 
  • Mass spectrometry can be used for radioactive dating, space research, detection of illigal drugs in urine (sports), and identifying potential molecules that can be used in the pharmaceutical industry. 
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Ionization Energy and Electron Shells

  • Ionization energy indicates the amount of energy required to remove an electron from an atom. 
  • It is endothermic.
  • It is measured as the energy required to remove one mole of electrons from one mole of gaseous atoms.
  • The first ionization energy is the removal of an electron to form a +1 ion.
  • The total energy required to form a +2 ion from an atom is the sum of the first and second ionization energies. 
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Ionization Energy and Electron Shells (cont)

  • The series of succesful ionization energies for an element provides evidence for the existance of electron shells or energy levels.
  • An electron furthest from the nucleus in the outer shell is removed first. It becomes steadily more difficult to remove electrons, as they are closer to the nucleus.
  • When there is a big jump in ionization energy, the electron removed has required significantly more energy- it is in a closer shell.
  • From this we can see what group the element is in.  
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Ionization Energy and Electron Shells (cont)

  • The first ionization energy of succesive elements provides elements for electron sub-shells.
  • If there were no sub-shells, ionization energy across the 1st period would just slightly increase.
  • However, ionization energy slightly decreases from beryllium to boron and from nitrogen to oxygen.
  • There are 3 types of sub shell: s, p and d, all which can take a maximum of 1, 3 and 5 electron pairs respectively. This is also the number of orbitals in each sub-shell.
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Ionization Energy and Electron Shells (cont)

  • All the orbitals in a given sub-shell are at the same energy level, but the 2s subshell is at a lower energy than the 2p subshell.
  • Electrons populate orbitals singly before pairing up (Hund's rule). This is why it is more difficult to remove an electron from beryillium than boron. The outer electron in boron is in a p subshell, which is at a higher energy level than the s subshell, so less energy is required to remove an electron from it. 
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Periodic Properties

  • Going across the period, melting point varies hugely.
  • The metals in groups 1,2 and 3 are all metallic structures, and it takes a lot of energy to break the bonds that hold the atoms/ions together.
  • Carbon and Silicon are examples of giant molecular structures, held together by strong covalent bonds. It is very difficult to melt these elements. 
  • Sulphur and chlorine are simple molecular structures, which are held together by weak intermolecular forces. Their melting temperatures are low. 
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