Chemistry - C3.1 - The Periodic Table

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C3.1.1 - The Early Periodic Table

  • 19th century - many elements discovered, but didn't know about structure - tried to classify based on atomic weight
  • 1863 - Newlands proposed law of octaves: similar properties repeated every 8th element - put the 62 known elements in table according to atomic weights - Ca didn't fit: idea wasn't accepted
  • 1869 - Mendeleev produced table - left gaps for undiscovered elements so known elements were in groups of similar properties - missing elements found and fitted table: ideas accepted - became basis for modern periodic table
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C3.1.2 - The Modern Periodic Table

  • Start of 20th century - found out about protons and electrons - soon developed models of electron arrangement - elements were arranged in periodic table in order of atomic numbers and lined in vertical groups
  • Groups of elements have similar chemical properties: same no of electrons on outer shell - same as group number
  • Within group, reactivity depends on total no of electrons - further down group = more shells: atoms get larger: electrons in outer shell less attracted to nucleus
  • Metals react: lose electron - reactivity increases going down group
  • Non-metals react: gain electron - reactivity decreases going down group
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C3.1.3 - Group 1 - The Alkali Metals

  • Group 1: alkali metals - react with air and water
  • Soft solids at room temperature, low melting/boiling point - decreases going down group, low densities - Li, Na and K float on water
  • React with water to produce hydrogen and a metal oxide (alkali) - 2Na(s) + 2H2O(l) -> 2NaOH(aq) + H2(g)
  • One electron in outer shell - lose it in reactions: ions have single positive charge
  • React with halogens in group 7, forming white/colourless salts crystals - 2Na(s) + Cl2(g) -> 2NaCl(s)
  • Compounds of alkali metals dissolve in water to form colourless solutions
  • Reactivity increases going down group: outer electron less strongly attracted to nucleus as no of shells increase and atom gets larger
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C3.1.4 - The Transition Elements

  • Transition elements found between group 2 and 3 - all metals
  • Have higher melting/boiling point than group 1 (except mercury)
  • Malleable and ductile, good conductors of heat and electricity
  • React slowly or not at all with oxygen and water at ordinary temperatures
  • Most are strong and dense: useful building materials, often as alloys
  • Form positive ions with various charges, eg. Fe2+ and Fe3+
  • Transition metal compounds often brightly coloured
  • Many transition metals and their compounds used as catalysts
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C3.1.5 - Group 7 - The Halogens

  • Halogens: non-metallic elements in group 7, 7 electrons in outer shell
  • Molecules made from pairs of atoms
  • Low melting/boiling points, increase going down
  • Room temperature: F - pale yellow gas, Cl - green gas, Br - red-brown liquid, I - grey solid - vapourises easily to violet gas
  • Form ionic compounds with metals, where the halide ions have a 1- charge - also bond covalently with non-metals to from molecules
  • Reactivity decreases going down group - more reactive halogen able to displace less reactive halogen from aqueous solution of halide compound
  • Reactivity decreases as attraction of outer electrons with nucleus decreases as no of shells increases
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