Chemistry Unit 1

Chemistry Unit 1

1. History of the atom

   • Democritus
     • 450BC
       • The name of the atom
   • Dalton
     • 1803
       • All matter is composed of atoms
   • Sir William Crookes
     • 1879
       • Discovered cathode rays are negatively charged
   • Stoney
     • 1894
       • Suggested the term 'electron'
   • JJ Thomoson
     • 1897
       • Isolated and measured the charge to mass ratio of electrons.
       • Showed cathode rays were really beams of electron.
       • Proposed the idea that electron were fundamental sub-atomic particles
       • Produced the idea of the plum pudding model of an atom
   • James Chadwick
     • 1932
       • Discovered the existance of the neutron which had the properties Rutherford proposed
   • Jöns Berzelius
     • 1818
       • Chemical language of symbols for each atom of each element
   • Neils Bohr
     • 1913
       • Showed the electrons travelled around the nucleus in shells
   • Rutherford
     • 1911
       • proposed the idea of a nuclear model,with a small positive nucleus surrounded by electrons
     • 1919
       • Showed that every nucleus contained a proton and that the number of protons was equal to the atomic number
       • Discovered that a proton didn't make up all the mass in a nucleus
   • Millikan
     • 1897
       • Showed electrons had a mass of 1/2000 of a hydrogen atom
   • Henry Moseley
     • 1913
       • Showed the positive charge of an atom was equal to the atomic number

2. Carbon Dating

   • Half life
     • The time it takes for half of the nuclei to decay

3. Mass Spectrometry

   • Gives accurate information about relative isotopic mass and also about the relative abundance of isotopes
   • Can be used to identify elements
   • Used to determine relative atomic mass
   • Stage 1- Ionisation
     • 1) The sample is dissolved in a volatile solvent and injected through a fine hypodermic needle as a fine spray into a vacuum in the ionisation chamber
     • 2) A very high voltage is applied to the end of the needle where the spray emerges (positively charged) -this prduces tiny positivley charged droplets (due to the loss of electrons in the needle)
     • 3) The solvent evaporates leaving 1+ ions
   • Stage 2- Acceleration of ions
     • 4)The ions are accelerated using an eletric filed so that all the ions have the same kinetic energy
   • Stage 3- Separation of charged ions
     • 5) Ion drift -the ions then enter the flight tube
     • 6) Ions with different masses have different fight times
     • 7) The lighter ions travel faster and take less time to reach the detector
   • Stage 4- Detection
     • 8) The detector is a negatively charged plate -a current is produced when the ions hit the plate -the more ions that hit the plate, the bigger the current
     • 9) The mass of the ions hitting the detector can be calculated from the time it takes to reach th edetector
     • 10) The mass spectrum shows the number of particles (abundance) of each mass that hit the detector

4. Electron Arrangement

   • Electrons are arranged in electron shells (energy levels) which then have sub-shells (sub-levels)
   • 1s, 2s, 2p, 3s,3p,4s,3d,4p,4d,4f
   • Each sub-level consists of electron orbitals (region of space in which th electron spends most of its time
   • Each orbital can hold 2 electron with opposite spins (one clockwise, the other anti-clockwise)

5. Ionisation Energy

   • The energy required to remove one electron from each atom in a mole of gaseous atoms producing one mole of 1+ gaseous ions
   • Ionisation changes when going down a group because; atoms get bigger, more shielding, a weaker attraction from the nucleus to the electron in the outer shell
   • Ionisation changes when going across a period because; increased nuclear charge, atoms get smaller(radius decreases as charge increases) and there is stronger attraction from nucleus to the electron in the outer shell
   • When going across a period, some first ionisation energies are lower than other elements because their outer electron maybe in a orbital on its own so will have a lower ativation energy