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…