A-Level Chemistry

• Lithium (Li) atom


  Atomic number: 3, Mass number: 7, Atomic symbol: Li
• Atomic number (Z)


  Number of protons in the nucleus
• Mass number (A)


  Total number of protons and neutrons in the atom
• Number of neutrons


  A - Z
• Isotopes


  Atoms with the same number of protons, but different numbers of neutrons
• Isotopes have similar chemical properties because they have the same electronic structure
• Isotopes may have slightly varying physical properties because they have different masses
• Mass spectrometer


  • Can determine all the isotopes present in a sample of an element and to therefore identify elements
• Mass spectrometer steps


  1. Ionisation

  2. Acceleration

  3. Flight Tube

  4. Detection
• Electron impact ionisation


  A vaporised sample is injected at low pressure, an electron gun fires high energy electrons at the sample, this knocks out an outer electron, forming positive ions with different charges
• Electrospray ionisation


  The sample is dissolved in a volatile, polar solvent, injected through a fine needle giving a fine mist or aerosol, the tip of needle has high voltage, at the tip the sample molecule, M, gains a proton, H+, from the solvent forming MH+
• Electron impact is used for elements and substances with low formula mass, can cause larger organic molecules to fragment
• Electrospray ionisation is used preferably for larger organic molecules, the 'softer' conditions of this technique mean fragmentation does not occur
• Acceleration


  Positive ions are accelerated by an electric field to a constant kinetic energy
• Flight Tube


  The positive ions with smaller m/z values will have the same kinetic energy as those with larger m/z and will move faster, the heavier particles take longer to move through the drift area, the ions are distinguished by different flight times

  
• Detection


  The ions reach the detector and generate a small current, which is fed to a computer for analysis, the current is produced by electrons transferring from the detector to the positive ions, the size of the current is proportional to the abundance of the species
• Given that all the particles have the same kinetic energy, the velocity of each particle depends on its mass, lighter particles have a faster velocity, and heavier particles have a slower velocity

  
• m/z (mass/charge ratio)


  The mass spectrometer can measure this for each isotope, along with the abundance
• Sometimes two electrons may be removed from a particle forming a 2+ ion, 24Mg2+ with a 2+ charge would have a m/z of 12
• Relative atomic mass (R.A.M.)


  A weighted average of all the isotopes, calculated as (isotopic mass x % abundance)/100
• If asked to give the species for a peak in a mass spectrum, give charge and mass number e.g. 24Mg+
• Cl has two isotopes Cl35 (75%) and Cl37(25%), Br has two isotopes Br79 (50%) and Br81(50%), this leads to characteristic mass spectra for diatomic molecules
• Mass spectrometers have been included in planetary space probes so that elements on other planets can be identified, elements on other planets can have a different composition of isotopes
• Molecular ion


  The peak with the largest m/z in a mass spectrum, due to the complete molecule, equal to the relative molecular mass, Mr, of the molecule
• Electrospray ionisation


  For molecules, the peak will be for the MH+ ion, so 1 must be subtracted to get the Mr
• Bohr model of the atom

  • Electrons in spherical orbits, atoms and ions with noble gas electron arrangements should be stable
• 1. level model of the atom

  • Electrons arranged in principle energy levels, sub-energy levels (s, p, d, f), and orbitals which hold up to 2 electrons of opposite spin, orbitals have specific shapes
• Filling up electronic structure

  Electrons fill up sub-levels in order of increasing energy, using spin diagrams to show electron configuration
• Positive ion formation


  Electrons lost from the outermost shell, e.g. Mg2+ is 1s2 2s2 2p6
• Negative ion formation


  Electrons gained, e.g. O2- is 1s2 2s2 2p6
• Electronic configurations of first row transition metals

  • Sc 1s22s22p63s23p6 4s23d1
  • Ti 1s22s22p63s23p6 4s23d2
  • V 1s22s22p63s23p6 4s23d3
  • Cr 1s22s22p63s23p6 4s13d5
  • Mn 1s22s22p63s23p6 4s23d5
  • Fe 1s22s22p63s23p6 4s23d6
  • Co 1s22s22p63s23p6 4s23d7
• Calcium electronic configuration


  1s2 2s2 2p6 3s2 3p6 4s2
• Parts of electronic configuration

  • Main energy level
  • Type of sub-level
  • Number of electrons in sub-level
• Fluorine electronic structure


  • Arrow represents one electron
  • Arrows going in opposite direction represent different spins of electrons in orbital
  • Box represents one orbital
  • s sub-levels are spherical
  • p sub-levels are shaped like dumbbells
• Filling up sub-levels with several orbitals


  Fill each orbital singly before starting to pair up the electrons
• Oxygen electronic configuration


  1s2 2s2 2p4
• Positive ion formation


  Electrons are lost from the outermost shell
• Positive ion examples


  • Mg2+ is 1s2 2s2 2p6
  • O2- is 1s2 2s2 2p6
• Electronic configurations of transition metals

  • Sc 1s22s22p63s23p6 4s23d1
  • Ti 1s22s22p63s23p6 4s23d2
  • V 1s22s22p63s23p6 4s23d3
  • Cr 1s22s22p63s23p6 4s13d5
  • Mn 1s22s22p63s23p6 4s23d5
  • Fe 1s22s22p63s23p6 4s23d6
  • Co 1s22s22p63s23p6 4s23d7
  • Ni 1s22s22p63s23p6 4s23d8
  • Cu 1s22s22p63s23p6 4s13d10
  • Zn 1s22s22p63s23p6 4s23d10
• Transition metal ion configurations

  • Sc3+ [Ar] 4s03d0
  • Ti3+ [Ar] 4s03d1
  • V3+ [Ar] 4s03d2
  • Cr3+ [Ar] 4s03d3
  • Mn2+ [Ar] 4s03d5
  • Fe3+ [Ar] 4s03d5
  • Co2+ [Ar] 4s03d7
  • Ni2+ [Ar] 4s03d8
  • Cu2+ [Ar] 4s03d9
  • Zn2+ [Ar] 4s03d10