OCR 21st Century Science - Chemistry C456

C4 - Chemical Patterns

  • Line spectra
    • When heated, the electrons in atoms vibrate, releasing energy as light
    • The wavelengths emitted can be recorded as line spectra - different due to different electron arrangement
    • Some elements emit distinctive colours when heated
  • History of the periodic table
    • Dobereiner (1828) - sorted into triads, middle element = the mean of the other two (atomic mass)
    • Newlands (1865) - rows of seven in order of atomic mass, coloumns similar; groups in Newlands' method combined dissimilar elemnts, mixed metals + non-metals, no spaces for undiscovered elements
    • Mendeleev (1869) - order of atomic mass, left gaps to keep groups similar, method confirmed when new elements were found and fitted the trend
  • Electron shells
    • 1st shell, 2 electrons; every shell after, 8 electrons
  • Ionic bonding
    • Made between two ions (charged atoms) - one positive, one negative
    • Happens when electrons needed to fulfill complete outer shell equals the amount needed to be dropped - eg. G1 and G7 elements ionically bond
    • Ionic compounds form a regular lattice (crystal); only conduct electricity when molten
  • Group 1 - Alkali metals
    • All very reactive due to one electron in outermost shell
    • All shiny when freshly cut; tarnish in air as they react with oxygen
    • Going down, becomes more reactive, higher density, lower boiling point 
    • Reacts with cold water to give fizzing from hydrogen, makes solution alkali, creates hydroxide of the metal - eg. 2Na(s) + 2H2O(l) --> 2NaOH(aq) + H2(g)
    • Reacts with chlorine to give vigorous reaction producing colourless crystalline salts
  • Group 7 - Halogens
    • Seven outer electrons, making them very reactive
    • Going down, becomes less reactive, higher boiling points
    • Flourine - yellow vapour; chlorine - green; bromine - orange; iodine - dark grey solid, purple vapour
    • Reacts with alkali metals to make salts called metal halides
    • Reacts with iron to form colourless solids called iron halides

C5 - Chemicals of the Natural Enviornment

  • Chemicals in the atmosphere
    • 78% nitrogen, 21% oxygen, 1% other (inc. 0.04% carbon dioxide)
    • Molecular substances are small, meaning weak forces of attraction, making boiling points low
    • Pure molecular substances don't conduct electricty as there are no free electrons
  • Covalent bonding
    • Atoms share an electron to acheieve a full outer shell - held to together by electrostatic attraction between the positive nuclei and negative electrons
  • Chemicals in the hydrosphere
    • Consists of all water in oceans, seas, lakes…



thanks for the notes

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