Group 7

• Group 7:

  • The elements in Group 7 are known as the halogens
  • These are fluorine, chlorine, bromine, iodine and astatine
  • These elements are non-metals that are poisonous
• Group 7:

  • All halogens have similar reactions as they each have seven electrons in their outermost shell
  • Halogens are diatomic, meaning they form molecules made of pairs of atoms sharing electrons (forming a single covalent bond between the two halogen atoms)
  • When halogen atoms gain an electron during reactions, they form -1 ions called halide ions
• The atoms of the elements of Group 7 all have 7 electrons in their outer shell:

  
• • At room temperature, the halogens exist in different states and colours, with different characteristics.
• The Appearance, Characteristics and Colour in Solution of the Halogens:

  
• • The melting and boiling points of the halogens increase as you go down the group
  • This is due to increasing intermolecular forces as the atoms become larger, so more energy is required to overcome these forces
• This graph shows the melting and boiling points of the halogens:

  
• • At room temperature (20 °C), the physical state of the halogens changes as you go down the group
  • Fluorine and chlorine are gases, bromine is a liquid and iodine is crumbly solid
  • The colours of the halogens also change as you descend the group - they become darker
• The physical states and colours of chlorine, bromine and iodine at room temperature :

  
• Halogen Reactivity:

  • Reactivity of Group 7 non-metals decreases as you go down the group
  • As you go down Group 7, the number of shells of electrons increases, the same as with all other groups
• • However, halogen atoms form negative ions when they gain an electron to obtain a full outer shell
  • Fluorine is the smallest halogen, which means its outermost shell is the closest to the positive nucleus of all the halogen
  • Therefore, the ability to attract an electron is strongest in fluorine making it the most reactive
  • As you move down the group, the forces of attraction between the nucleus and the outermost shell decreases
  • This makes it harder for the atoms to gain electrons as you go down the group making them less reactive
• Displacement Reactions:

  • A halogen displacement reaction occurs when a more reactive halogen displaces a less reactive halogen from an aqueous solution of its halide
  • The reactivity of Group 7 elements decreases as you move down the group
  • You only need to learn the displacement reactions with chlorine, bromine and iodine
  • Chlorine is the most reactive and iodine is the least reactive
• Chlorine with Bromine & Iodine:

  • If you add chlorine solution to colourless potassium bromide or potassium iodide solution a displacement reaction occurs:
  • The solution becomes orange as bromine is formed or
  • The solution becomes brown as iodine is formed
• Chlorine with Bromine & Iodine:

  • Chlorine is above bromine and iodine in Group 7 so it is more reactive
  • Chlorine will displace bromine or iodine from an aqueous solution of the metal halide

  chlorine + potassium bromide →  potassium chloride + bromine

  Cl2 + 2KBr → 2KCl + Br2

  chlorine + potassium iodide →  potassium chloride + iodine

  Cl2 + 2KI → 2KCl + I2
• Bromine with Iodine:

  • Bromine is above iodine in Group 7 so it is more reactive
  • Bromine will displace iodine from an aqueous solution of the metal iodide

  bromine + potassium iodide →  potassium bromide + iodine

  Br2 + 2KI → 2KBr + I2
• Summary of the displacement reactions of the halogens (chlorine, bromine and iodine):

  
• • You could be asked at Higher Tier to provide ionic equations to show what is happening during displacement reactions of the halogens
  • These are:

  Cl2 + 2Br- → 2Cl- + Br2

  Cl2 + 2I- → 2Cl- + I2

  Br2 + 2I- → 2Br- + I2
• Reactions of Halides:

  • Chlorine, bromine and iodine react with metals and non-metals to form compounds
• Metal Halides:

  • The halogens react with some metals to form ionic compounds which are metal halide salts
  • The halide ion carries a -1 charge so the ionic compound formed will have different numbers of halogen atoms, depending on the valency of the metal
  • E.g., sodium is a Group 1 metal:

  2 Na + Cl2 → 2 NaCl
• Metal Halides:

  • calcium is a Group 2 metal:

  Ca + Br2 → CaBr2
• • The halogens decrease in reactivity moving down the group, but they still form halide salts with some metals including iron
  • The rate of reaction is slower for halogens which are further down the group such as bromine and iodine
• Sodium donates its single outer electron to a chlorine atom and an ionic bond is formed between the positive sodium ion and the negative chloride ion: part 1-

  
• part 2-

  
• part 3-

  
• Non-metal Halides:

  • The halogens react with nonmetals to form simple molecular covalent structures
  • For example, the halogens react with hydrogen to form hydrogen halides (e.g., hydrogen chloride)
  • Reactivity decreases down the group, so iodine reacts less vigorously with hydrogen than chlorine (which requires light or a high temperature to react with hydrogen)
  • Fluorine is the most reactive (reacting with hydrogen at low temperatures in the absence of light)
  • Other compounds formed include CCl4, HF and PCl5