chemistry

• the group 7 elements are called halogens because they all contain chlorine, bromine, iodine, fluorine and astatine.
• The reactivity of halogens increases as one goes from fluorine to iodine due to decreasing electron affinity and increasing size of halogen atoms.
• Halogens react with alkali metal hydroxides to produce alkali halides and water.
• Halogens react vigorously with most metals, releasing hydrogen gas.
• chlorine has seven electrons in its outer shell so it needs one more electron to be stable.
• fluorine has one electron in its outer shell so it needs one more electron to fill up its outer shell
• halogens have the ability to gain an electron to form negative ions
• chlorine is used to purify water by adding bleach which contains sodium hypochlorite (NaOCl) which kills bacteria and viruses
• fluorine is the most reactive element known it can displace other non-metals from their compounds
• Fluorine is the most reactive element known and can only be stored under liquid nitrogen at -189°C
• Iodine is a dark grey solid which sublimes when heated.
• Bromine is a reddish-brown liquid that evaporates easily into a brown vapour.
• halogens have high melting points and boiling points due to strong intermolecular forces between the molecules.
• halogens can also accept an electron to become positive ions
• chlorine is used to make plastics such as polyvinyl chloride (PVC)
• iodine can be used as a disinfectant because it destroys germs on contact
• bromine is used as a preservative in cosmetics and medicines
• halogens are very reactive non-metal elements that exist as diatomic molecules at room temperature
• bromine is used as a preservative in medicine because it prevents bacterial growth
• iodine is also used as a contrast medium in x rays
• iodine is used as a mild antiseptic because it stops microorganisms growing on wounds
• Chlorine gas is poisonous, irritating and corrosive
• Bromine is a reddish brown liquid that evaporates easily into vapour
• Chlorine is a green gas with a pungent smell, which turns yellow on exposure to light.
• iodine is used to test for starch because it turns blue black with starch
• The reactivity of halogens decreases down group 7 because the atomic radius increases down the group making it harder for them to lose electrons.
• Halogen atoms are highly electronegative so they attract bonding pairs of electrons towards themselves, leaving the central atom with a partial positive charge.
• Halide ions are good reducing agents because they donate an electron pair to form a covalent bond with another species.
• the larger the radius of the halide ion, the weaker its attraction towards the metal cation, resulting in lower melting point and higher solubility in water
• fluorine is used to harden tooth enamel and prevent decay
• fluorine is used to make fluorocarbons, which are used in refrigerators and air conditioning units
• Constituents of air


  • Nitrogen
  • Oxygen
  • Noble gases
  • Carbon (iv) oxide
  • Water vapour
  • Other gases
• Nitrogen


  Accounts for about 4/5 of the atmosphere by volume
• Oxygen


  Occupies the remaining 1/5 of the atmosphere by volume
• Percentage composition of air constituents

  • Nitrogen (78.09%)
  • Oxygen (20.95%)
  • Noble gases (0.93%)
  • Carbon (iv) oxide (0.03%)
  • Water vapour (Variable 0-1.2%)
  • Other gases (0.003%)
• Nitrogen


  • Acts as important diluent of air to slow down combustion and corrosion
• Oxygen


  • For cellular respiration and burning
• Noble gases


  • Generally unreactive, used to produce colourful light for advertisement
• Carbon (iv) oxide


  • For photosynthesis
• Water vapour in air

  Due to evaporation of water from oceans, rivers, lakes, sea and water reservoirs. Amount varies with temperature, weather and location