LE

C9

  • Development of the periodic table
    • used to predict how elements react
    • method: categorise elements according to properties
    • made:
      • 1869
      • Dmitri Mendeleev
        • Professor of Chemistry at st Petersburg University based on chemical + physical properties
        • only 63 elements at the time
    • others tried categorising elements
    • Mendeleev = most successful
      • arranged elements in order of increasing atomic weight
      • elements with similar properties in same column
      • columns: groups
      • rows: periods
      • predicted undiscovered elements: fit in empty gaps
        • new elements discovered -> fit in gaps
        • ex. 'eha-silikon'-> predicted:
          • color
          • density
          • melting point
          • atomic weight
      • predicted atomic weight of 10 elements (7 discovered)
      • proved: idea = correct
      • was accepted
      • Rutherford& Moseley modified it
      • discoveries about sub-atomic particles -> elements should be arranged by proton number
    • 18 discovered elements
    • elements with similar properties: same groups
      • 8 groups
      • Group 1 = Group I, group 2 = GroupII... Group VIl
      • final group: Group VIII or Group O
      • some have names
        • Group I = alkali metals
        • Group Il = alkaline earth metals
        • Group VIl = halogens
        • Group VIIl = inert gases/noble gases
    • rows = periods
      • 1-7
      • between Groups II-III: block of elements -> transition elements
    • left of metalloids: metals
    • right: non-metals
    • metalloids: similar features metals & non-metals
  • Periodic trends
    • trends in periods
      • gradual change: metal -> non-metal
      • increase number of valance electrons
      • change in element structure: giant metallic (metals) through giant covalent → simple molecular
  • Electronic structure & periodic table
    • number of valance electrons correspond with group in periodic table
      • ex. Group I = 1 valance electron
      • Group O: exception + 2 or 8 valance electrons
    • valance electron: responsible for chemical properties
    • elements in same group -similar chemical properties
    • metallic character of element in group: increases → right electrons = easier to lose when further from nucleus less attraction between nucleus + valance electron
  • Group I - the alkali metals
    • Metals: Lithium, sodium, potassium, rubidium, caesium
    • Radioactive: francium
    • lithium, sodium, potassium:
      • kept under oil
      • very reactive
      • properties:
      • good heat + electricity conductors
      • soft metals (lithium: hardest - potassium: softest)
      • low density
      • shiny surface when freshly cut
      • low melting point
      • burn in oxygen/air with characteristic flames
      • react vigorously with water → gives alkaline solution of metal hydroxide + hydrogen gas
      • potassium: most reactive towards water
        • sodium, lithium
    • further down -> more reactive
  • Group VII: Halogens
    • fluorine, chlorine, bromine, iodine,
    • Radioactive: asatine
    • chlorine, bromine, iodine:
      • down group → darker
      • exist as diatomic molecules
      • show gradual change from gas( Cl2) → liquid (Br2)-> solid (I2) as density increases
  • Displacement reactions
    • ex.
      • Chlorine bubbled into potassium iodine solution (less reactive halogen)-> iodine = displaced by chlorine (more reactive)
      • potassium iodide + chlorine -> potassium chloride + iodine
      • 2Kl(aq) + Cl2 (g) → 2KCl (aq) + I2 (aq)
      • decreasing reactivity
        • chlorine
        • bromine
        • iodine
      • Order of reactivity decreases going down group
      • ex.
        • chlorine & bromine: 7 valance electrons
        • need 1 electron
        • chlorine more reactive bromine
        • incoming electron: more attracted
        • outer energy level = closer to nucleus
  • Transition elements
    • ex. copper, iron, nickel, zinc
    • harder + stronger > metals in Group I & II
    • higher density > metals in Group I & II
    • high melting points (except mercury a room temp: liquid)
    • less reactive
    • colorful compounds
    • good heat & electricity conductors
    • show catalytic activity as elements& compounds
    • react (corrode) slow with oxygen/water
  • Group VIII: the noble gases
    • helium, neon, argon, krypton, xenon& radon
    • discovered after Mendeleev published periodic table
    • discovery: 1894-1900, mainly by Sir William Ramsay & Lord John William Strutt Rayleigh
    • characteristics
      • colorless gases
      • monatomic gases (exist as individual atoms)
      • very unreactive
      • alternative names: Group O, Group VIII
    • No helium, neon + argon compounds found
    • Xenon, Krypton, fluorine "oxygen produced
    • Stable -> unreactive
    • uses:
      • Argon: fill light bulbs
      • Neon: advertising signs + lasers
    • Helium separated by other gases through their liquefaction
      • others: fractional distillation of liquid air