group 17 : the halogens

• properties of the halogens:

  • low melting and boiling point
  • exist as diatomic molecules
• electronic configuration : ns2 np5
• Trend in boiling points

  • boiling point increases as you go down the group and physical state changes from gas, to liquid, to solid

  - this is because each successive element has an extra shell of electrons- leading to a higher level of London forces
• reactivity of the halogens

  • halogens are very reactive and highly electronegative
  • they are strong oxidising agents as they attract and capture electrons
  • in reactions , each atom gains an electron to form 1- ions
  • reactivity decreases going down the group:

  - atomic radius increases

  - electron shielding increases

  - ability to gain an electron in the p-sub shell and 1- ions decreases

  - the decrease in reactivity can be shown in reactions that halogens undergo with other halide ions
• Redox reactions

  Chlorine - in water, pale green - in cyclohexane, pale green

  Bromine - in water orange - in cyclohexane, orange

  Iodine - in water, brown - in cyclohexane, purple
• Define disporportionation

  where the same element is both oxidised and reduced in the same reaction